Roofshield is a unique, three - layer, nonwoven, spunbonded, polypropylene breather membrane with a patented melt - blown core. It is intended for use as a pitched roof underlay (breathable roofing felt) and is fixed beneath tiles and slates.
Roofshield provides a secondary barrier to the ingress of rain, wind and snow. It has a low vapour resistance and is air - permeable; additionally, it eliminates the incidence of interstitial condensation in pitched roofs.
The product provides the most cost - effective solution to controlling interstitial condensation in a pitched roof.
- Water & UV Resistant
- Low Vapour Resistance
- Air Permeable
- IAB and BBA Certified
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Datasheets
Installation Guides
Material Properties
BIM Objects
UK Availability
Certification
CAD Details
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Material Properties
Roofshield
| Property | Test Method | Result |
|---|---|---|
| Mass Per Unit Area | EN 1849-2 | 185 g/m2 |
| Reaction to Fire | EN 11925-2 | E Class |
| Water Vapour Resistance Sd | EN 12572 | 0.013 m |
| Vapour Resistance | EN 12572 | 0.065 MNs/g |
| Air Permeability | EN 12114 | 34.4 m3/m2/hr @ 50Pa |
| Water Penetration - Before Ageing | EN 1928 | W1 Class |
| Water Penetration - After Ageing | EN 1928 | W1 Class |
| Tensile Strength - Before Ageing - MD | EN 12311-1 | 390 N |
| Tensile Strength - Before Ageing - CD | EN 12311-1 | 230 N |
| Tensile Strength - After Ageing - MD | EN 12311-1 | 330 N |
| Tensile Strength - After Ageing - CD | EN 12311-1 | 190 N |
| Elongation - MD | EN 12311-1 | 55 % |
| Elongation - CD | EN 12311-1 | 75 % |
| Flexibility at Low Temperature | EN 1109 | No cracking at minus 60C |
| Thickness | 0.75 mm |
BIM Objects
| File | Format |
|---|---|
| Roofshield - BIM Object - Cold Roof Slate Softwood Sarking | IFC |
| Roofshield - BIM Object - Cold Roof Tiles No Sarking | IFC |
| Roofshield - BIM Object - Cold Roof Tiles Ply Sarking | IFC |
| Roofshield - BIM Object - Warm Roof Slate Softwood Sarking | IFC |
| Roofshield - BIM Object - Warm Roof Tiles No Sarking | IFC |
| Roofshield - BIM Object - Warm Roof Tiles Ply Sarking | IFC |
| Roofshield - BIM Object - Cold Roof Slate Softwood Sarking | RVT |
| Roofshield - BIM Object - Cold Roof Tiles No Sarking | RVT |
| Roofshield - BIM Object - Cold Roof Tiles Ply Sarking | RVT |
| Roofshield - BIM Object - Warm Roof Slate Softwood Sarking | RVT |
| Roofshield - BIM Object - Warm Roof Tiles No Sarking | RVT |
| Roofshield - BIM Object - Warm Roof Tiles Ply Sarking | RVT |
CAD Details
- Roofshield - CAD Detail - Cold Roof Tiles
- Roofshield - CAD Detail - Warm Roof Slates And Softwood Sarking
- Roofshield - CAD Detail - Triso Super 10
- Roofshield - CAD Detail - Triso Super 10
- Roofshield - CAD Detail - Warm Roof Tiles
- Roofshield - CAD Detail - Cold Roof Slates And Softwood Sarking
- Roofshield - CAD Detail - Warm Roof OSB Or Plywood Sarking
CAD Detail Review
Use this service to check that your proposed detailing is correct. Upload your details, and a member of our technical support team will review and comment on them, offering amendments where required. Click below to get started.
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Our team are available to take Zoom meetings at a date & time that suits you. We are happy to discuss project specific questions, review drawings, assist with specifications, documentation, calculations or anything else you need.
Book MeetingThermal & Condensation Calculation
Demonstrating the thermal performance of a building element, or that the specified construction does not carry an interstitial condensation risk, are important steps in the design and approval process. Get project specific calculations with this service.
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Roofshield FAQs
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How “Breathable” is Roofshield?
While “Breathability” is a commonly used term, it is more technically accurate to refer to a materials “vapour permeability”. As Roofshield is air permeable as well as vapour permeable, it can certainly be argued that it does breathe, as it allows air movement, but this does not hold true for all “breathable” materials. In terms of vapour permeability, Roofshield, with a vapour resistance of 0.065MNs/g (Sd-value 0.013m) is one of the most vapour permeable membranes on the market, as well as benefitting from the additional advantage of air permeability.
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What difference does air permeability make?
Roofshield, in addition to having one of the lowest vapour resistance available, is also air permeable. Industry research concluded that air permeability, combined with very low vapour resistance, inhibits the formation of condensation in a pitched roof to the point where it’s virtually impossible for condensation to occur under normal conditions.
Studies conducted by the BRE and Glasgow Caledonian University have concluded that not only does an air permeable roofing underlay outperform conventional airtight underlays, but may provide a higher air change rate than a
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Does Roofshield suffer from “tenting”?
As anyone who’s slept in a cheap tent can tell you, some vapour permeable fabrics can lose their water resistance if anything happens to touch the underside. Developed in Scotland, where the use of underlays fully supported on timber sarking board is standard practice, ensuring Roofshield does not suffer from this effect was always an important consideration for the A. Proctor Group. In fact while the first generation of VPUs suffered from this problem, most modern roof underlays are unaffected by this phenomenon.
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So vents aren't required?
Over the course more than a 20-year lifespan, Roofshield has undergone extensive testing to prove that ventilation is not required to the underside of the underlay in both warm and cold roof applications, and is BBA and NSAI certified to that effect. Roof features successfully assessed include duo-pitched, monopitched, hipped, mansard, gable-end, valleys, room in the roof, dormers and timber sarking. In more complex roof configurations, the use of Roofshield will provide a far more robust solution than a complex layout of ventilation openings.
The sole remaining situation where ventilation to the roofspace is required is in a cold roof with Plywood or OSB sarking. If in doubt, our team of technical experts can assist specifiers in achieving the most appropriate solution for their specific project.
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How about high level vents?
Although non-ventilated roofs have been specified successfully for many years, recently BS5250, the NHBC technical standards and NFRC Technical Bulletin 6 have recommended that ridge only ventilation equivalent to 5mm per metre is used when vapour permeable underlays are specified. In both cases, the exception to this is where the underlay specified is both vapour AND air permeable. As Roofshield meets this requirement, this additional high level ventilation is not required when using Roofshield.
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Does Roofshield “chatter” in the wind?
Wind blowing up into the eaves of a roof can cause a ‘chatter’ type noise with some types of underlay. Roofshield is silent in such situations. As Roofshield does not suffer from this problem, the membrane does not have to be pulled taut and does not have any special fixing instructions compared to that of some underlays. Counterbattens can be provided to increase the air movement when used with close-fitting slates or tiles, or to provide drainage below the tile battens when used fully supported, but otherwise Roofshield may simply be draped between the rafters as normal.
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Can I use Roofshield with timber treatments?
Timber treatments containing fungicides, insecticides and wood preservatives are extensively used in the building trade to protect rafters, sarking boards and tile battens. As such, a number of tests have been carried out to see if these timber treatments will affect the water hold out properties of Roofshield.
Four timber treatments were investigated - two water based micro-emulsions, a solvent-based treatment, and a CCA. Treatments were applied to the fabric and allowed to dry, then the water resistance of the material was tested. The water resistance of the Roofshield was not affected by these timber treatments.
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What is the "drying out period"?
This is the period immediately after the building is completed, during which, there are significantly higher amounts of moisture within the building. These include moisture in damp timber, from wet trades (concrete, plaster etc) and moisture that may have found its way in, prior to the building shell being wind and watertight. Although this moisture will eventually dry out, condensation is more likely to occur as it does so. This will usually be most apparent in the first winter when the building is heated. Roofshield roofs are far less prone to this effect.
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Is Roofshield expensive?
In terms of the cost per roll, Roofshield is more expensive than a traditional non-breathable felt, however if we consider the costs associated with ventilation hardware then using Roofshield will save you money. Independent assessments carried out by Hardies Property and Construction Consultants of installed costs have shown that Roofshield can offer savings of between 4% and 6% when compared with either impermeable felt and full ventilation, or a lower specification VPU with high level ventilation only.
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What about severe weather conditions?
Although Roofshield is highly water resistant, the BBA, in its Site Practice Bulletin Number 2, states: “An underlay is not a total waterproof barrier and if used as a temporary waterproof covering, some rain penetration may occur. In certain conditions, particularly if there is persistent heavy rainfall combined with subsequent severe freeze/thaw conditions, an underlay should not be exposed for more than a few days.” If such conditions are expected, the temporary use of a tarpaulin covering is recommended.
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Where can Roofshield be used under BS5534:2014 + A2:2018
Roofshield can be used in all zones 1 - 5.
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What accessories are required to comply?
Most importantly Roofshield Does Not Require Taping to Comply!
No extra considerations need to be made when using Roofshield in Zones 1-3 – installation is as usual. If the building is in Zone 4 or 5, with sarking board use, with slates fixed directly through, then install as usual.
If the building is in Zone 4 or 5, and there is an open rafter construction, then a minimum 11mm counter batten fixed over Roofshield is required.
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Underlay Overlaps – battened, taped or increased?
There are a number of manufacturers view points and arguments regarding the correct detailing of membrane overlaps. BS5534:2014 states the following.
"Unless counter battens are used, underlay laps should be covered by a batten and, where necessary, the lap of the underlay adjusted to coincide with the nearest slating or tiling batten. Laps may also be restrained using proprietary means in accordance with manufacturers' instructions (see also 5.9.3.2). Where a proprietary sealant is used, its durability should meet the same recommendations as the underlay.""
It has always been good practice to lay out the membrane so the lap falls below a tile batten. If this is not possible, due to the tile gauge, then either the lap should be increased to ensure it is below a batten or an additional batten should be used between the tile batten gauge spacing.
The use of this additional batten, a “fly batten”, is deemed by most roofers to be a trip hazard and as such we have always recommended this is not done and that the lap is increased. This is based on a fundamental health and safety precaution.
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What is a fly batten – as referenced by some of our competitors?
As regional slang for building components varies a fly batten may mean a different thing to different people. Some of the possible descriptions include counter batten, batten to membrane overlap or a temporary batten, often used when you need to get the underlay on quickly to protect the insides of the building (also referred to as strap battening). This practice can also come in useful when the roof is going to have counter battens ‘on-top’ of the underlay instead of, or as well as underneath, because you can get all the underlay on and then install full length counter battens, rather than small sections.
There may be other terms for this therefore the use of fly batten as a term could be/is confusing to the market. Our use of an 11mm counter batten, and not an additional batten at the overlap, is clear and transparent.
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Do the wind zones cover all eventualities?
NO - the table small print says it is only applicable where there is a well-sealed ceiling, ridge height is not greater than 15m, pitch is between 12.5 and 70 degrees and there is no significant topography present. For areas outwith this you may need a stronger membrane or have to seek professional advice.
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Does that mean you will need to install a VCL to ensure you have a well sealed ceiling?
No, BS 9250 (the Code of Practice for Design for the Airtightness of Ceilings in Pitched Roofs) discusses how to achieve a well-sealed ceiling. It does not stipulate that you must use a VCL (although it shows it as an example as it does showing the plasterboard being used as an air tight layer) to achieve a well-sealed ceiling. Indeed it focusses more on detailing and workmanship. The following are extracts from the relevant British Standards
BS5250 Defines a well-sealed ceiling as: 3.37 well-sealed ceiling ceiling incorporating seals which prevent the transfer of warm, moist air into the loft or roof sp
BS9250 gives this definition: 3.7 well-sealed ceiling ceiling that satisfies the following criteria:
a. The design avoids constructional gaps, especially at the wall/ ceiling junction and holes in the ceiling.
b. No access door or hatch should be located in rooms where large amounts of moisture are produced, including kitchens or bathrooms
c. The air leakage rate through an access hatch, including its frame, when tested to BS EN 13141-1:2004 4.3 is less than 1 m3 /h at a pressure difference of 2 Pa
d. Penetrations, such as those for services and rooflights, are permanently sealed with suitable proprietary products.
e. The ceiling is sealed to the external walls to limit any leakage through cracks
f. The total leakage through all recessed light fittings should not exceed 0.06 m3 /h•m2 of ceiling at 2 Pa pressure difference across the ceiling.
g. The head of any cavity in any wall or partition should be sealed to prevent transfer of warm moist air into the loft [Based on BS 5250]
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Does the BBA certificate still say no VCL required?
Yes and the existing benefits of Roofshield remain the same
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We use Roofshield as our default membrane for use on all of our quotes confident in the knowledge that no matter which build up is specified it will perform to meet all necessary requirements. Craig Hopkinson. MIoR - Senior Estimator, Elliotts
We have used Roofshield now for some 20 years, recommending it for major housebuilders such as Bellway Homes and have never received one complaint regarding condensation. Keith Soulsby, Wensley Roofing Limited
Roofshield provides a cost effective solution, allowing our clients to provide more new roof coverings for the same budget. Roofshield is an exceptional product we recommend to all clients . G Hirons - Commercial Manager, O'brien Roofing & Leadwork
Condensation In The Roofspace
This presentation discusses the requirements for breather membranes, air barriers and vapour control layers, looks at the principals of condensation, the effects of condensation, issues of detailing cold pitched roofs i.e. insulation, ventilation and underlay. It also looks at condensation controlling measures and solutions.
Topics covered in this CPD:
- What is condensation?
- Pitched roof design considerations
- BS5250 and BS5534
- Ventilated and non-ventilated pitched roof constructions
- Vapour permeable underlay certification
- Types of vapour permeable membranes and their properties
While the technical and practical benefits of the Roofshield membrane are clear, at first glance the perceived higher cost compared to conventional felt and vents may appear to limit it'’'s suitability to projects where specific non-standard project requirements must be met.
Considered more closely however, the various omissions and specification changes that can be made mean that this gap, based purely on material costs, narrows significantly.
This closer gap in material cost serves to emphasise the potential savings that can then be realised in terms of simplified design and installation, and reduced likelihood of condensation related problems and customer related call backs.
To illustrate this, this document breaks down material costings across twelve typical 2-5 bedroom houses, of a size and general layout typical of national and regional housebuilders in the UK.
Why Roofshield
As Building Regulations demand ever higher thermal efficiency in both the commercial and domestic sectors, today’s building envelopes are becoming increasingly airtight. While this is undoubtedly beneficial for building energy performance, it also makes careful consideration and management of moisture more critical than ever
Since their introduction in the late 1980s, “breather membranes” have become an important part of the construction industry landscape, however the term itself is widely misunderstood. “Breather membrane” relates to membranes used in timber frame walls, with a vapour resistance of 0.6MNs/g or less, while for those membranes used on roofs, the term “vapour permeable underlay” is more appropriate, these membranes must have a vapour resistance of 0.25MNs/g or lower. It can be argued that higher performance, air permeable membranes such as Roofshield, are “breathable” in the truest sense.
All activities within a building, from initial construction and wet trades, to cooking and washing, generate substantial amounts of water vapour, which must be managed successfully to ensure the longevity of the building fabric and a healthy indoor environment. The vapour permeable structure used in these membranes allow this to be achieved without compromising temporary weather protection during construction, or requiring complex and expensive passive or active ventilation systems and accessories.
While such systems may once have seemed somewhat alien to the industry, non-ventilated roof constructions, with their speed, efficiency and lower cost are fast becoming the norm. The once radical building physics behind them now well understood and widely accepted across all sectors of the industry.
Roofshield is a vapour- and air-permeable underlay (VPU) for pitched roof applications. First launched in 1996, and with an unchanged specification since, it’s unique blend of physical properties has allowed it to consistently outperform not only competing vapour permeable underlays, but also traditionally ventilated roofs
While the majority of VPUs in use today utilise an airtight VP film layer to achieve their performance, Roofshield’s patented SMS (Spunbond Meltblown Spunbond) structure allows high levels of airflow in addition to the transport of moisture vapour, making the formation of condensation virtually impossible. It was this outstanding air-permeability powered performance in BRE trials that led to the granting of one of the first BBA certificates for non-vented cold roofs in 1999, and which today allows the NHBC to accept its use without high level vents, a position further reinforced by NFRC Technical Bulletin 6 in 2012. Independent studies of the effect of air permeability have confirmed that lofts using Roofshield have a more consistent air flow through the roof than those found in traditionally ventilated lofts, according to BS5250, so whatever side of the vents/no vents debate you sit on, Roofshield has it covered.
Developed and manufactured in Scotland, Roofshield has been widely used in some of the harshest climates on earth, from northern Canada to the Antarctic, and it’s superb resistance to wind loadings allow it to be used without restrictions on batten spacing in any UK exposure conditions, giving specifiers the flexibility to choose whatever configuration of outer roofcovering meets their requirements. Its heavyweight 185gsm three-layer structure is also fully hydrophobic, giving a Class W1 rating under the latest EN13859-1 specifications, and at Euroclass D offers the highest available fire rating in its class.
Over the last 20 years, we have undertaken projects from domestic housing to the refurbishment of historic castles. The performance and design flexibility of Roofshield provides a winning combination time and again. It has become recognised as one of the most dependable solutions for specifiers and contractors available globally
Key Features
Roofshield has a vapour resistance of 0.065 MNs/g and an Sd-value of 0.013m, making Roofshield one of the highest-performing vapour-permeable membranes on the market.
FULLY AIR PERMEABLEAir permeable membranes allow air movement through the roof, as well as allowing moisture to escape by diffusion. This means that condensation is far less likely to form on the membrane itself, and also allows the membrane to deal with much higher moisture levels within the building, for example during the drying out period.
MORE UNIFORM AIRFLOW THAN VENTSThe air permeability of Roofshield means a non-ventilated roof fitted with Roofshield allows a more consistent air flow through the roof than a roof ventilated as per BS5250, without expensive and time consuming ventilation hardware fitted to the roof.
HIGHLY WATER RESISTANTRoofshield is rated W1 under EN13859-1, and can support a water column of over a metre without leakage. The membrane can be left exposed to provide temporary weather protection to the building envelope for up to 3 months.
Roofshield is fully certified for use in non-ventilated warm or cold roof applications, and has been since 1996. In 1999, Roofshield became the first membrane certified for use in cold non-vented roofs. While the construction industry has changed considerably over the last 20 years, Roofshield offers the same benefits as it always has.
NHBC ACCEPTANCEWith a certified air permeability of 34.4m3 /m2 h.50Pa, Roofshield does not require additional high level ventilation when used on NHBC-approved projects. It also allows the same specification to be used across all your projects, regardless of the regulatory regime applied.
NO VCL REQUIREDRoofshield is the only vapour-permeable underlay available which the BBA puts enough trust in to explicitly state in their certificate that a vapour control layer is not required for non-ventilated cold pitched roof constructions.
BS5534 COMPLIANCEBased on fully independent 3rd party testing, Roofshield can continue to be used across the UK (see table below). This, in addition to no requirement for high level ventilation or the use of a vapour control layer, ensures Roofshield remains the simplest and most cost effective method of achieving regulation compliance.
Typical Roof Constructions
Cold Roof Slate Sarking Detail
- 1 - Slate
- 2 - Roofshield
- 3 - Timber sarking / board
- 4 - Rafter
Cold Roof Tile Detail
- 1 - Tile
- 2 - Batten
- 3 - Roofshield (draped)
- 4 - Rafter
Metal Roof Profile Detail
- 1 - Metal Cladding
- 2 - Ventilation Air Space
- 3 - Roofshield
- 4 - Insulation
- 5 - Vapour Control Layer
- 6 - Metal Lining
Warm Roof Slate Sarking Detail
- 1 - Slate
- 2 - Roofshield
- 3 - Timber sarking / board
- 4 - Insulation
- 5 - Rafter
Warm Roof Tile Detail
- 1 - Tile
- 2 - Batten
- 3 - Counter Batten
- 4 - Roofshield
- 5 - Insulation
- 6 - Rafter
Warm Roof Tile Alternate Detail
- 1 - Tile
- 2 - Batten
- 3 - Roofshield (draped)
- 4 - Insulation
- 5 - Rafter
Warm Roof Tile with OSB Detail
- 1 - Tile
- 2 - Batten
- 3 - Counter batten
- 4 - Roofshield
- 5 - OSB
- 6 - Insulation
- 7 - Rafter
Warm Roof Tile with OSB Alternate Detail
- 1 - Tile
- 2 - Batten
- 3 - Roofshield (draped)
- 4 - Insulation
- 5 - Rafter
Wind Uplift Resistance
| Batten Gauge | Declared Wind Uplift Resistance Pa(N/m2 | Accessories | Zone Suitability |
|---|---|---|---|
| 1252 | NONE | 1 - 3 | |
| ≤345mm | 2192 | ≥11mm* counter batten | 1 - 5 |
| 2615 | Wraptite Tape | 1 - 5 | |
| ≤250mm | 2574 | NONE | 1 - 5 |
| Softwood sarking with slates** | 2974 | n/a | 1 - 5 |
*Alternatively, a 38mm tile batten can be used instead of a 25mm tile batten which would alleviate the need for an 11mm counter batten.
***The slates were set with a headlap of 54mm; which is the minimum allowed in BS5534. The nail diameter of 2.65mm is also the minimum allowed in BS5534. This roof configuration as tested therefore represents the weakest (with respect to wind uplift) configuration allowed in BS5534 for these slates.
Warm & Cold Roof
Non-ventilated Warm Roof ConstructionA “warm roof” is a roof construction where the insulation layer is placed either over, or between the rafters, and follows the pitch of the roof from eaves to ridge. This configuration keeps the roof structure within the heated envelope of the building, and allows spaces within the roof to be used as habitable spaces, or easily converted at a later date. Warm roofs are typically insulated using rigid boards, and the underlay may be installed either fully supported, or draped, depending on the location of the insulation. Full details of warm roof design and site practice are given in BBA certificate No.96/3220.
In a cold pitched roof construction, the insulation is placed horizontally at ceiling level, running from eaves to eaves, leaving the loft and roof structure above the heated envelope of the building. Traditionally, this cold loft space would require ventilation, but this can be impractical for some roof configurations, and avoiding such ventilation has long been desirable.
Long term studies carried out by the BRE between 1997 and 2006 concluded that the moisture content found in non-ventilated Roofshield roofs were comparable with the moisture content found in a conventionally ventilated roof space, and following this research Roofshield has been certified for this use since 1999. The relevant BBA certificate for cold pitched roof and room-in-roof constructions is 99/3648.
Building Regulations & NHBC
Condensation control is covered by Approved Document C in England & Wales, Section 3 in Scotland, and Technical Booklet C in Northern Ireland. All of these documents refer to BS5250: “Code Of Practice for Control Of Condensation in Buildings” for detailed guidance on how best to mitigate condensation risk in roof and wall constructions.
BS5250 defines two types of underlay membrane - high resistance type HR membranes (which includes traditional impermeable roofing felts and modern plastic equivalents) - and low resistance type LR membranes (vapour permeable underlays), and gives examples of appropriate ventilation strategies for each. BS5250 does not however deal directly with non-ventilated roof constructions, but does allow for their use, provided the proposed solution is covered by 3rd party certification.
Roofshield is covered by BBA certificates for both warm (no.96/3220) and cold (no.99/3648) pitched roof constructions. Roofs constructed in accordance with the certificates’ conditions will therefore comply with the requirements of BS5250 and hence the building regulations.
The NHBC operates its own parallel technical standards which differ from national Building Regulations. Although reference is still made to the BS5250 Code of Practice, Standard 7.2 Clause S11 has, since 2011, required high level ventilation equivalent to 5mm/m to be used with type LR underlays regardless of any recommendations given in 3rd party certification. Although not explicitly stated in the technical standards, an exception to this requirement is made where the underlay specified has third party certification of both vapour and air permeability. Having this certification, Roofshield is therefore exempt from this requirement, and can be specified as outlined in the BBA certification, without high or low level roof ventilation. As a result of this, some national housebuilders specify Roofshield on all their developments.
Released by the National Federation of Roofing Contractors in 2012, Technical Bulletin 6 outlines best practice for roof system installers. TB6 aligns itself with the NHBC technical standards by recommending high level ventilation where airtight type LR underlays are used. As with the NHBC standard however, it is recognised that this provision is unnecessary where the underlay is both vapour and air permeable, therefore Roofshield is once again exempt from this recommendation.
Roofshield Details
Duopitch Ridge Detail
Monopitch Ridge Detail
Eaves Detail
Verge Abutment Detail
Valley Detail
Pipe Detail
FAQs
How “Breathable” is Roofshield?
While “Breathability” is a commonly used term, it is more technically accurate to refer to a material’s “vapour permeability”. As Roofshield is air permeable as well as vapour permeable, it can certainly be argued that it does breathe, as it allows air movement, but this does not hold true for all “breathable” materials. In terms of vapour permeability, Roofshield, with a vapour resistance of 0.065MNs/g (Sd-value 0.013m) is one of the most vapour permeable membranes on the market, as well as benefitting from the additional advantage of air permeability.
What difference does air permeability make?
Roofshield, in addition to having the lowest vapour resistance available, is also air permeable. Industry research concluded that air permeability, combined with very low vapour resistance, inhibits the formation of condensation in a pitched roof to the point where it’s virtually impossible for condensation to occur under normal conditions.
Studies conducted by the BRE and Glasgow Caledonian University have concluded that not only does an air permeable roofing underlay outperform conventional airtight underlays, but also provides a higher air change rate (50-60ACH vs 10-12ACH) than a roof ventilated according to the recommendations in BS5250.
Does Roofshield suffer from “tenting”?
As anyone who’s slept in a cheap tent can tell you, some vapour permeable fabrics can lose their water resistance if anything happens to touch the underside. Developed in Scotland, where the use of underlays fully supported on timber sarking board is standard practice, ensuring Roofshield does not suffer from this effect was always an important consideration for the A. Proctor Group. In fact while the first generation of VPUs suffered from this problem, most modern roof underlays are unaffected by this phenomenon.
So vents aren’t required?
Over the course of its more than 20-year lifespan, Roofshield has undergone extensive testing to prove that ventilation is not required to the underside of the underlay in both warm and cold roof applications, and is BBA certified to that effect. Roof features successfully assessed include duo-pitched, mono-pitched, hipped, mansard, gable-end, valleys, room in the roof, dormers and timber sarking. In more complex roof configurations, the use of Roofshield will provide a far more robust solution than a complex layout of ventilation openings.
The sole remaining situation where ventilation below the underlay is required is in a cold roof with Plywood or OSB sarking. If in doubt, our team of technical experts can assist specifiers in achieving the most appropriate solution for their specific project.
How about high level vents?
Although non-ventilated roofs have been specified successfully for many years, recently BS5250, the NHBC technical standards and NFRC Technical Bulletin 6 have recommended that ridge only ventilation equivalent to 5mm per metre is used when vapour permeable underlays are specified. In both cases, the exception to this is where the underlay specified is both vapour AND air permeable. As Roofshield meets this requirement, this additional high level ventilation is not required when using Roofshield.
Does Roofshield “chatter” in the wind?
Wind blowing up into the eaves of a roof can cause a ‘chatter’ type noise with some types of underlay. Roofshield is silent in such situations. As Roofshield does not suffer from this problem, the membrane does not have to be pulled taut and does not have any special fixing instructions compared to that of some underlays. Counterbattens can be provided to increase the air movement when used with close-fitting slates or tiles, or to provide drainage below the tile battens when used fully supported, but otherwise Roofshield may simply be draped between the rafters as normal
Can I use Roofshield with timber treatments?
All three layers of the Roofshield underlay have additives to increase the water hold out of the membrane. Timber treatments containing fungicides, insecticides and wood preservatives are extensively used in the building trade to protect rafters, sarking boards and tile battens. As such, a number of tests have been carried out to see if these timber treatments will affect the water hold out properties of Roofshield.
Four timber treatments were investigated - two water based micro-emulsions, a solvent-based treatment, and a CCA. Treatments were applied to the fabric and allowed to dry, then the water resistance of the material was tested. The water resistance of the Roofshield was not affected by these timber treatments.
What is the “drying out period”?
This is the period immediately after the building is completed, during which there are significantly higher amounts of moisture within the building. These include moisture in damp timber, from wet trades (concrete, plaster etc) and moisture that may have found its way in, prior to the building shell being wind and watertight. Although this moisture will eventually dry out, condensation is more likely to occur as it does so. This will usually be most apparent in the first winter when the building is heated. Roofshield roofs are far less prone to this effect.
Is Roofshield expensive?
In terms of the cost per roll, Roofshield is more expensive than a traditional non-breathable felt, however if we consider the costs associated with ventilation hardware then using Roofshield will save you money. Independent assessments carried out by Hardies Property and Construction Consultants of installed costs have shown that Roofshield can offer savings of between 4% and 6% when compared with either impermeable felt and full ventilation, or a lower specification VPU with high level ventilation only
What about severe weather conditions?
Although Roofshield is highly water resistant, the BBA, in its Site Practice Bulletin Number 2, states: “An underlay is not a total waterproof barrier and if used as a temporary waterproof covering, some rain penetration may occur. In certain conditions, particularly if there is persistent heavy rainfall combined with subsequent severe freeze/thaw conditions, an underlay should not be exposed for more than a few days.” If such conditions are expected, the temporary use of a tarpaulin covering is recommended.
Articles
Covering All Bases
As an effective method of addressing condensation in roofs that needs no ventilation or tapes, the Roofshield air and vapour permeable membrane from the A.Proctor Group presents a compelling case.
Vapour permeable membranes have been gaining favour in recent years for preventing condensation in cold pitched roof applications, providing a combination of water resistance and a high degree of breathability, alleviating the need for traditional eaves and ridge ventilation. This ability of the A. Proctor Group’s vapour permeable membrane product Roofshield was recognised by the NHBC in 2011 following an amendment to their Technical Standards resulting from complaints of condensation occurring in some non-ventilated roofs. This change required ridge ventilation to be installed in all cold roof constructions where a vapour permeable underlay was used. The Partners in Innovation (PiI) study, carried out in 2004, showed that a vapour and air permeable underlay, Roofshield, could be used to prevent condensation without the need for additional ventilation.
The potential cost savings on a construction project of being able to install an effective membrane without the need for ventilation, despite the high quality nature of the Roofshield membrane itself, have been shown in a 2014 report undertaken by property consultants Hardies. More recently, the case for Roofshield has been further bolstered with amendments to official guidance on roof construction due to its ability to resist wind uplift without requiring tape, as is detailed below.
Background
Roofshield was developed 20 years ago in response to problems caused by the double-whammy of the UK’s fondness for cold pitched roof construction, and increasing thermal performance requirements which led to a tendency to place additional insulation above the ceiling joists leading to an increased risk of condensation in the roof space. With vapour condensing on the cold surfaces in the roof, particularly in winter months, a variety of efforts were made across the industry to develop membranes which would allow the vapour to escape. These breathable membranes were designed to be installed over the rafters as the roof underlay as an alternative to traditional 1F felt.
The majority of those membranes were vapour permeable but air tight, rather like a Gore-Tex jacket. While water resistant, offering additional benefits during construction similarly to Roofshield, they did not completely prevent condensation within the roof space, meaning some low and/or high level ventilation had to be introduced in order to allow air to circulate and fix this problem.
Therefore two types of technological solutions have been presented to specifiers; vapour permeable but air tight solutions, based on film laminated polypropylene technology which has been favoured by several manufacturers, or the vapour and air permeable version alleviating the need for ventilation, such as Roofshield. A debate continues to run in the industry around whether roofs fitted with the air tight membranes will still require ventilating due to their being vapour permeable, although testing undertaken within the 2004 Partners in Innovation study confirmed this was the case. The continued discussion has however led to a recommendation for a vapour control layer to be considered at ceiling level to reduce the moisture load into the roof space when film based laminates are used as the underlay.
Iain Fairnington, Technical Director of the A. Proctor Group, explains the building physics around why air tight membranes’ limited vapour permeability means that condensation can still occur: “If you have a big cold roof space, and you have a sudden drop in temperature, you want to have air movement. People assumed that because they were installing a vapour permeable membrane you didn’t need to ventilate your roof, but in certain circumstances moisture levels were too high or temperatures too cold to allow the vapour to permeate without condensing.”
By contrast, Roofshield has a far higher degree of vapour permeability, as well as air permeability, so will still perform in conditions in which air tight alternatives will not. The 2004 study was conducted on a cross-industry basis with Glasgow Caledonian University because there was a view that a consensus was desirable on whether it was necessary to ventilate buildings using vapour permeable membranes. It had the result of “dispelling a lot of theories around ventilation” says Iain Fairnington.
In fact the study contained the crucial finding that when a roof was unventilated, and used an air and vapour permeable underlay, such as Roofshield, this would further reduce and inhibit the formation of condensation on the underlay. This contributed to the NHBC making a statement in their Technical Extra bulletin Issue 6 that independently certified air and vapour permeable underlays, such as Roofshield, could be used without additional ridge ventilation in cold roofs. This has seen a groundswell of interest in the product recently, aided by its hydrophobic and UV resistant qualities. Another key benefit for installers is that the product is manufactured in lightweight 1 m wide rolls, the same size as traditional IF felt underlay, and can therefore be fixed and laid on open rafters in the same way, reducing health and safety risks of overreaching with wider membranes on open rafters.
Meeting the new Code of Practice on wind uplift
While there has been widespread acceptance of the abilities of air permeable membranes to offer a range of benefits for contractors and developers, the 2014 revision of BS 5534 Slating and tiling for pitched roofs and vertical cladding - Code of Practice caused a stir in the world of roof construction and again put the spotlight on their differences. This was particularly with regard to the technical specification of underlays, in particular lightweight membranes, and how they behave under wind.
The revision of the standard includes a number of recommendations which affect the way all slated and tiled roofs will be constructed in future. Advice contained in Annex A has been introduced to avoid incorrect specification which has been perceived, in extreme cases, to potentially lead to underlays being lifted by wind, ballooning and dislodging slates or tiles. The annex includes a new wind uplift resistance test and classification system for underlays in relation to the batten gauge being used and the location of the particular project. It requires manufacturers to provide a Zonal Classification label to enable specifiers to easily identify a suitable underlay.
The change has led to some membranes manufacturers introducing tapes, or integral adhesive strips, which will enable them to be used, sealed at the overlaps. However as Iain Fairnington, who sat on the standards committee, explains, this was not an requirement for Roofshield being a heavier membrane, a factor which has been welcomed by installers: “I was told by the roofing industry that they do not want tapes, they may work well in a lab, but out on site, the situation with wind, dirt and dust does not lend itself to tapes.”
The A. Proctor Group has undertaken independent, third party testing, in accordance with Annex A, to establish that Roofshield is fully compliant and are providing specifications to interested parties on that basis. This enables Roofshield to be used in three of the five specified zones with no additional requirements such as time-intensive taping, and in all zones if an 11mm counter batten or 38mm tiling batten is used.
This new requirement for compliance with wind uplift issues brings the conclusions of the Partners in Innovation research back into the foreground again, as the findings were based on roofs with open overlaps (ie not taped). In that study, even in the case of air tight membranes there was a degree of vapour escape. Now however, if laps must be taped, that mechanism for escape has gone. Iain concludes: “While there does not seem to be much appetite to confront this issue, there is a very real risk that we will see moisture occurring again in such specifications.”
The recent Code of Practice revision, plus the previous statements from the NHBC and the PiI Research, adds up to a compelling case for air and vapour permeable membranes as a robust and installer-friendly solution for healthier buildings. In the case of Roofshield the manufacturer is able to make a credible case that the product will comply with all standards and avoid the need for additional measures such as ventilation and tapes. Before the PiI study and research into implications of omitting ventilation, there had been a lot of resistance to change in the industry; however specifiers now have more and more reasons to believe that Roofshield offers the simple answers.
