Good morning everyone, my name is Keira Proctor, and welcome to our fourth webinar of 2022. Our series of webinars has been running now since 2020, and if you’ve missed any you can go back and review them all on-demand right here on our YouTube channel.
You can also catch up on our learning hub at www.proctorgroup.com, where you can also book in for follow-ups with our team of experts around the country, access up to date information on our full range of products, or order product samples.
Today’s webinar is the second in our series of toolbox talks focussing on installation and site practice, and we’ll be looking at our Roofshield air and vapour permeable underlay membrane.
We’ll being with an overview of Roofshield's purpose and performance and some of the legislation and regulations that apply to roof underlays.
We’ll then run though some frequently asked installation questions and common situations found on site.
Lastly we’ll finish up with our regular Q&A session, today featuring special guest Keith Soulsby, Operations director at Northern Bear.Roofshield Product Intro
The key to the high performance of Roofshield relative to most other vapour permeable membranes is it’s meltblown core. This layer of microscopic fibres, many times narrower than a human hair, gives a micropourous structure through which air and water vapour can pass easily, but liquid water cannot. This high performance core layer is sandwiched between two robust and durable outer layers of spunbond polypropylene, making the entire composite material lighter and more flexible than traditional roofing underlay felts, which still providing tough and durable secondary protection. The three layer structure is open to the passage of both water vapour and air. So where most vapour permeable underlays rely purely on diffusion to release moisture, a Roofshield roof also has substantial airflow through the roof voids, similar to a conventionally ventilated roof. This additional airflow helps prevent condensation by turbocharging the transfer of moisture vapour greatly reducing the likelihood of moisture problems arising even under the most extreme conditions. As the same time the hydrophobic additives present in all three layers work to actively repel liquid water from the surface of the membrane. In all but the most extreme weather conditions, where an additional tarpaulin is recommended, this provides effective temporary protection to buildings during construction until the outer covering is in place. So what is condensation and what causes it? Most everyday human activities such as cooking, cleaning and even just breathing generate moisture vapour, and this, combined with warmer indoor environmental conditions, causes what is known as “vapour pressure” pushing this moisture outwards and into the building fabric. As we move through the fabric of the building, the temperature drops until it reaches the outside temperature. The warmer air is, the more water vapour it can carry, so as this temperature drops, at a certain point the air will become saturated and dump this vapour out as liquid water, this is called the dew point.
This is the same effect that causes bathroom mirrors to “steam up” or water droplets to form on a cold drinks can and if it occurs within the building fabric it can cause damage or mould growth. To make sure we keep this under control, we have to balance the design of the building and the materials we use, with both the purpose of the building, and the weather conditions it is expected to face. A warmer and more humid building like a swimming pool, in a cold climate represents a far higher risk of moisture problems than a largely unoccupied warehouse but somewhere warmer, so we have to account for these factors when we considering the materials and products used. At its simplest, we need to consider how heat, air and moisture move and interact within the building, and make sure whatever is designed and built can cope with these conditions. The effects of these are all interlinked, so it’s important to take an integrated or “holistic” view of these factors, rather than just looking an each one in isolation.
How we do this is explained in BS5250, the code of practice for the management of moisture in buildings. BS5250 defines the various types of construction membranes used in the management of moisture, and explains how they are used. It also covers other methods of moisture control, for example ventilation, and gives detailed prescriptive guidance that covers most situations. BS5250 is directly referenced in the building regulations for all areas of the UK and Ireland, so it’s guidance must be followed to meet regulation requirements. When it comes to roofing applications, BS5250 details the ventilation requirements for two types of underlay.
High resistance or HR underlays, are impermeable membranes, which rely on ventilation alone to prevent condensation. Traditional roofing felts and it’s more modern plastic equivalents fall into this category. Low resistance or LR underlays allow the passage of moisture vapour therefore the level of ventilation required to avoid problems can be reduced somewhat. It’s important to remember that BS5250 only provides generic guidance, and outlines the minimum performance standards that are applicable. For roofing applications this mean that it does not include guidance for situation where all ventilation is removed.
If a roof is to have no ventilation, BS5250 refers to the use of 3rd Party Certification, such as a BBA certificate. Each certificate is specific to a particular product used in a specific application, and while a lot of certification outlines similar requirements, this cannot be guaranteed.
If a product is changed, it’s critical to ensure that the certification covering the replacement does not require different guidance to be followed, for example minimum requirements for vapour control membranes or how well sealed the ceiling should be. Failure to install a product in accordance with its 3rd party certificate can lead not just to a failure to comply with BS5250, and hence the building regulations, but also to noticeable moisture problems, resulting in call backs to sites and costly remediation works. These variations are particularly important when using air permeable membranes like Roofshield in cold pitched roofs. If these membranes allow enough airflow and are installed correctly, the additional benefits air permeability provides can greatly simplify the design and construction. Roofs with Roofshield do not require ventilation openings at the eaves, but also do not require openings at the ridge, regardless of the size, shape or layout of the roof. This is particularly important for complex roof shapes or room-in-the-roof constructions, where getting airflow evenly distributed throughout all the roof voids can be a problem.
Omitting low level ventilation also means loft insulation can be placed further into the eaves. This makes it far simpler to detail the junction between the roof and wall insulation, minimising heat loss in this area.
As Building regulations look for further lower U values 4-500 mm of insulation may be needed at ceiling level. This makes ensuring a ventilation path at eaves even harder.
The temptation is to pull back the insulation but this could lead to what is known as a “cold bridge” at the ceiling level at eaves which could lead to condensation and or black mould on the ceiling. Ridge ventilation is also not required when used on NHBC projects, which typically require 5mm ridge ventilation, even when a vapour permeable underlay is specified. This additional ventilation is required to ensure higher initial moisture loads from wet trades can dry out, but the higher moisture transfer Roofshields air permeable structure means this isn’t needed. As well as being simpler and cheaper, omitting this opening means the ridge of the roof is never open to the weather during construction, which combined with a “W1” weathertightness rating, allows Roofshield to provide temporary weather protection for up to three months. If particularly extreme weather is anticipated though, it’s good practice to add a tarpaulin for additional protection during such conditions, and the time the membrane is left exposed should kept to a minimum.
Another benefit of air permeable membranes is in the ceiling specification. The 3rd party certification for Roofshield allows it to be used without a vapour control layer at ceiling level, which may not be the case for other membranes. As we discussed earlier, every LR-Type membrane allows vapour to escape the roof by diffusion, where the warmer internal conditions “push” the vapour out through the roof. If the underlay is both AIR and VAPOUR permeable though, the airflow through the roof boosts this effect substantially, which in turn lessens the additional measure required to avoid condensation. This means that using Roofshield on a project is subject to a lot less restrictions on applications, or specific additional requirements than other types of vapour permeable underlay. So while using Roofshield means no vents, no vcl and no problems, swapping it for another membrane without fully considering the implications on the design can lead to a failure to comply with building regulations, and damage to the roof. So that concludes our intro to the Roofshield membrane, but if any further assistance is required, we have a dedicated technical team available to answer any queries, as well as network of expert regional managers around the UK. You can also visit our online learning hub at www.proctorgroup.com for more information, or use our on-site assistance app to access any information needed from wherever you are.Roofshield Installation Section
We'll now move on to take a look at the practicalities of working with Roofshield on site. Our team have worked on projects of all sizes all over the world, so whatever your requirements are, it’s unlikely to be something we haven't come across before.
As we saw earlier in today webinar, Roofshield is a three layer membrane with a unique spunbond polypropylene core layer. This gives the material it's vapour and air permeable structure.
Because of the additional mechanism for vapour transfer provided by the airflow through the roof, condensation risks in the roof are greatly reduced meaning ventilation and vapour control requirements are far less stringent than with other types of roofing underlay which do not allow the passage of air as well as vapour.
Many years of experience, testing and certification have shown this type of air and vapour permeable solution to be the simplest and most robust means of controlling condensation in roofing.
While Roofshield was designed to be installed similarly to a traditional felt underlay, just without ventilation, it's still important to ensure Roofshield is detailed and installed correctly. So lets now run through some of the best practice procedures on site.
Before starting and installation its important to make sure the rolls are correctly stored to prevent damage prior to installation.
Rolls of Roofshield are supplied individually wrapped and with an enclosed installation reference guide. The rolls should be carefully stored under cover and on a clean, level surface.
Care should be taken not to place rolls on top of anything that could potentially cause damage to the membrane, such as discarded nails or screws, as these may cause small punctures or scuff damage which can be difficult to see during installation and potentially lead to water ingress.
Roofshield should also be protected from direct exposure to sunlight prior to being installed on the roof.
Its also important to consider the weather forecast prior to installation, and if necessary take appropriate precautions. High winds can make any work at height more difficult, and wet conditions can make the membrane surface more slippery.
If particularly extreme conditions are expected prior to the installation of the outer roof covering, the use of additional temporary protection such as a tarpaulin is recommended.
This recommendation is given in BBA Information Bulletin 2, the Permeable Roof Tile Underlay Guide to Good Site Practice. This document is available from the BBA and is a useful resource for anyone involved in the design and installation of roofing underlays.
The BBA certification for Roofshield gives a maximum exposure time in the UK of 3 months, however as with all temporary weather protection membranes, the period of exposure prior to the competition of the roof should be kept to a minimum, especially during periods of expected bad weather.
The outer covering specified for use on the roof can also affect the installation in a number of ways.
The air openness of the specified slate or tile probably has the largest effect, as certain types of roof finish may require additional ventilation to be provided into the batten cavity of the roof. Depending on the overall roof configuration this may mean additional counterbattens are needed.
The detailed requirements for when and how much ventilation is needed are given in the British Standard BS5250 and 5534, the codes of practice for moisture management and slating and tiling respectively.
What's important to remember though is that the entire roof system is considered as a whole at the design stage, so changes to any of the materials or requirements specified may have unintended effects on other parts of the system.
If it’s necessary to make changes to the design on site, the factors should be fully considered and any changes approved by the design team prior to commencing with the installation.
Finally the fixings used should be carefully considered. While this may not seem a major consideration, round profile clout nails are typically a more robust fixing that narrower staples.
Clout nails tend to stretch the fibres in the membrane, rather than cutting them, leading to improved water penetration resistance and less likelihood of damage occurring in high winds.
The resistance of the roof to wind uplift forces is another critical consideration, and the process for determining this is detailed in the BS5534 standard.
The standard mainly details fixing requirements for ridges and tile assemblies, with methods given to calculate how many nails or clips are required to fix the outer tiles in place. If these roof fixings are not correctly designed to resist wind uplift forces then in extreme cases the tiles or slates of the outer covering can be damaged.
This damage will be mainly caused by uplift forces directly removing the slates or tiles by sucking them up and off the roof, but in theory it could also occur due to the underlay membrane “ballooning” and pushing them off.
Although there is little if any conclusive evidence of this occurring, the 5534 standard sets criteria for underlays to resist wind forces, and divides the UK into five wind zones.
This table gives the uplift resistance force for the Roofshield membrane for each installation configuration, along with the measures that must be taken when used in a given wind zone.
These measure can include taping joints or using additional counterbattens, however for NHBC projects, counterbattens are not acceptable for this purpose and batten gauge should be reduced instead.
Scottish practice of slating directly into softwood sarking boards is acceptable in any wind zone, and in fact wind resistance is precisely the reason this detail is traditionally used in the harsher scots climate.
Its worth highlighti9ng though that the simplified approach given in this table is only suitable for roofs less than 15m in height, and located on sites at less than 100m above sea level “where topography is not a significant factor”.
Outwith this the 5534 standard document should be consulted for more detailed guidance on how to address the wind uplift resistance.
The minimum horizontal laps required between runs of membrane vary slightly according to the pitch of the roof. This is to ensure that in addition to water running down off the roof, wind driven rain cannot be blown back up through the horizontal laps, and these lap dimension are the industry norms.
In practice the majority of roofs are between 15 and 34 degrees, so the 150mm minimum lap size is marked on the Roofshield membrane. If however the roof is outwith that pitch range, it important to ensure installers are aware the lap width must be altered to accommodate this, particularly if the pitch is lower.
The minimum roof pitch is dictated by the outer covering rather than the underlay, so if the outer covering can be used below 15 degrees, then so can Roofshield.
Before moving on, we’ll take a minute here to disucss the various types of pitched roof configuration commonly used in the UK.
Firstly we have cold pitched roofs, where the insulation is placed at ceiling level with a large cold loft space above. In this type of roof the insulation used is usually mineral fibre as space is rarely an issue.
Traditionally these loft spaces are ventilated, with opening at the eaves and or ridge to allow airflow, but modern LR membrane may allow this ventilation to be reduced or eliminated, as is the case with Roofshield.
With roof insulation requirements increase with each revision of the building regulations, ventilation paths may become an issue at the eaves, or the insulation could be pulled back leaving cold bridging spots.
As we saw earlier, air permeable membranes like Roofshield are the least restricted in terms of conditions attached to non-ventilated use.
A warm pitch roof place the insulation along the slope of the roof, keeping the spaces with the roof warm. Typically rigid insulation boards are used in this type of application as space between rafters is more limited.
There are less restrictions and condition applied to membrane types in this roof configuration, and ventilation is less common provided a type LR underlay is used. This is because the lack of large voids where moisture laden air can accumulate reduces the condensation risk somewhat when compared to a cold roof construction.
The third configuration of roof is the “room in the roof”, common to many properties where an existing loft has been converted into habitable space.
In this type of roof there are a mix of cold and warm spaces, and this can make providing adequate ventilation across the whole roof difficult.
Roofshield works very well in this type of roof as it is used in a similar way across both cold and warm areas and usually requires no ventilation or other special measure in either type of roof.
This means it also works well when a roof is built as a cold roof, but with provision made for a loft conversion in future.
The type of roof configuration used has a bearing on how the membrane is installed, and specifically on how a drainage path under the tiling battens is ensured.
In a cold roof where the membrane is unsupported this is most commonly achieved by simply draping the membrane between the roof rafters, thus providing a gap for water to run off the roof.
Too large a drape can cause problems with the membrane flapping or chattering in the wind, and while Roofshield doesn’t tend to suffer form this problem, BS5534 suggests 15mm is the maximum underlays should be draped. This also helps reduce the risk of the membrane touching the underside of the tiles.
At the other end of the scale, the minimum is not explicitly stated, but it can safely be assumed that a drape of between 5 and 15mm is sufficient to permit drainage.
This can easily be verified on site if necessary by simply pouring a bottle of water down the roof and ensuring it can run under the tiling battens.
For warm and roof in roof constructions, draping between the rafters can be still be achieved if there is sufficient space available, and indeed this can be a good option in refurbishment where raising an existing roofline is not possible.
In this type of build up it’s perfectly fine to lay Roofshield directly onto the insulation, and no gap is required. Earlier generations of vapour permeable membrane could suffer from an effect known as tenting, where water would leak through if the underlay came into contact with insulation beneath, but this is more or less unheard of these days.
Warm roofs do however require vapour diffusion to be controlled, for example by using insulation boards with a high vapour resistance. The rigid foams commonly used in warm roof constructions will usually meet this requirement.
If the thickness of insulation specified does not allow the membrane to be draped sufficiently between the rafters, then counterbattens should be used to elevate the tile battens. The BBA certification for Roofshield specifies a minimum counterbatten depth of 12mm.
The membrane can be positioned either directly onto the rafter with counterbattens over the top, or the counterbattens can be fitted first and the membrane draped over them.
This approach is the same if a sheet sarking material such as OSB or plywood, or if a continuous layer of over rafter insulation is used, and again the membrane can go either under or over counterbattens. If this type of sarking is used used on a cold roof however, it’s important to remember that ventilation to the loft space is required, as sheet sarking is not sufficiently permeable. In fact this is one of the very few circumstances where ventilation is required below the underlay when using Roofshield. If the sarking used is softwood boarding, as is traditional Scottish slating practice, then ventilation is once again not required. The BBA certification defines this as being softwood planks nominally 150mm wide with 2mm gaps between. In practice, the sarking boards are usually fitted relatively damp, then as they dry out a degree of shrinkage occurs, giving plenty of spacing between adjacent boards. This shrinkage therefore provides the 2mm spacing referenced in the definition.
So we’ll now move on to consider some common details, eaves, ridges, valleys, penetrations and verges.
At the eaves, the most critical concern is ensuring the flow of water into the gutters is unimpeded. Although the Roofshield is a secondary weather protection, during construction or in the event of damage to the slates or tiles, drainage is critical.
To ensure this works effectively, an eaves carrier is used, either of 5U roofing felt, or a proprietary plastic system. The Roofshield should lapped over this carrier by a minimum of 150mm and the carrier dressed into the guttering.
This carrier not only ensure a smooth flow into the gutters for water running off, but also helps ensure insulation fitted into the eaves does not press the underlay up against the battens, which would also impeded drainage.
It’s important to take extra care to ensure insulation at the eaves is fitted tightly, but not so tightly as to restrict drainage.
As well as checking insulation, any changes in level and positioning of the lower tiling battens could also block the drainage pathways and as such should be considered carefully prior to installation, and double checked throughout and afterwards.
At verges, the membrane should be turned sufficiently up the abutting wall to ensure the flashing fully covers it to prevent water ingress. The outer flashing itself should extend above the tiles to ensure rainwater is kept on top of the outer roofcovering.
When rain water lands on a surface, the majority of the splashes do not reach above 150mm, or 6 inches, so this is usually considered the minimum spacing above a flat surface to which weather reprotection is required. That said, it’s worth double checking this requirement for the specific outer covering specified as there may well be variations between different products and systems
On a duo-pitch roof, the Roofshield is simply return 150mm over each side, creating a robust weathertight ridge to the roof with no openings required at any point.
This lack of openings for ridge vents means the roof is wind and watertight sooner in the build process, and with no possibility of problems arising from failure to cut the underlay to allow airflow.
This lack of ridge ventilation is also true for new build NHBC projects, where Roofshield’s suitability for use without the usual 5mm ridge only ventilation is now written directly into the BBA certificate.
In a mono-pitched roof, the membrane is returned over the ridge board a minimum of 150mm.
In this type of roof, a fully permeable underlay makes for a far simpler solution than providing ventilation as it ensure moisture can escape form the entire roof with less concern about dead spots where there is inadequate airflow.
Where such roofs abut vertical walls, these areas are detailed in the same way as the verges discussed previously.
Where it is necessary for a pipe or flue to penetrate through the underlay, an appropriately sized hole should be cut in the membrane, and a pipe of flue collar used.
The collar should be sealed to the membrane surface using wraptite tape, bearing in mind it may be necessary to provide additional support below the membrane to ensure a proper seal can be made.
Where boiler flues pass through the underlay, additional separation may be needed. The maximum service There is in increased risk of softening and deformation of Roofshield where temperatures are above 70 degrees centigrade, so if the expected service temperature of a flue is above this, advice from the supplier should be sought as to the spacing required.
The final detail we’ll consider is a valley. The first stage here is to lay addiotnal battens either side of the low point, to form the outline of a channel. The Roofshield is then dressed up and over these battens.
This base of the channel is then formed using the Roofshield detailing strip, taken up over the battens either side, so there is an unbroken strip of membrane down the low point of the valley.
Finally the permanent flashing strip is added, giving the valley it’s fully weatherproof outer covering.
Lastly, let’s review some general consideration to be aware of when working with Roofshield.
Although air permeable underlays are better able to deal with excessive moisture loads, for example from construction moisture during the drying out period, it’s still important to ensure ventilation and extract systems are installed and functioning correctly.
Of particular importance is making sure any ductwork running form fans through the roof spaces are properly connected and sealed. Similarly any downlighters, particularly in wet rooms, should be sealed and convection tight to prevent excess moisture ingress.
Ensuring the habitable spaces are sufficiently ventilated with correctly sized and installed trickle vents is another important part of ensuring good indoor air quality is maintained, as well as limiting moisture ingress to roofs.
Lastly, loft hatches should be properly sealed and insulated, this not only limits moisture ingress, but plays an important role in reduce the heat loss from the building.
So that brings us to the end of todays presentation, and we’ll now move onto the Q&A session
This Webinar Includes
- Roofshield Product Overview
- Benefits of Air Permeable Underlays
- Installation Best Practice
- Site Conditions and Preparation