The inaugural AT Technical Seminar, in partnership with SIG Design & Technology, explored roofing design and specification best practice.
On Thursday 18th January at The Building Centre in London, Architecture Today hosted a technical seminar with SIG Design & Technology focusing on roofing design and specification best practice. Specialists Ross Finnie, Ian Dryden, Terry Collins, Sam Finnie, Chris Collins, Jamie Dunsmore, Michael Lace and Joe McLatchie from SIG were on hand to answer questions and provide guidance. The day was divided in two, starting off with quick-fire presentations followed by a ‘roofing clinic’ where delegates came to the specialists with queries about their projects and more.
Flat roofing solutions
To kick things off, SIG put forward the roofing systems commonly available on the market, including membranes, single ply systems, liquid waterproofing, hot melt bituminous roofing, green roofing, blue roofs, metal roofs, and natural slate.
But how to specify each one? To do so requires careful management the roof design process, product selection and site processes – all of which, when employed in conjunction, can enable providers to supply genuine guarantees.
What affects specification?
Product choice, roof design and installation are all key factors when specifying roofing systems. While specific products often take the blame for the failure of a roof, typically, the issue will be linked to misspecification. To better understand this, a more holistic project approach is required to work out what solution can be provided and how. Building on this, SIG outlined the key components for consideration in getting the specification right.
Factors for establishing the client’s brief.
Specifying and installing flat roof insulation
Insulation can account for up to 50 per cent or more of the cost of any flat roofing scheme. SIG stressed the importance of adhering to Approved Document L, conducting condensation control and wind load calculations, as well as considering factors like foil-faced, tissue-faced, or bituminous-faced insulation boards.
Fire safety
Ensuring adequate fire safety in flat roof insulation involves BROOF(T4) testing, consideration of materials like cellular glass or mineral wool, early-stage understanding of the fire strategy, and attention to acoustic performance through the use of mineral wool or additional matting. Specifiers should also be aware of other issues, including adherence to BS 6229:2018 for cut to falls design, the use of thermally broken fixings for optimised performance in mechanical fixing, weighted U-value, and specifying the correct fully sealed vapour control layer (VCL) dependent on building use.
“It’s never too early to get your fire testing done,” stressed Ross Finnie, who highlighted the current three-to-five-month fire test waiting list, with some providers even going to Belgium to bypass this.
Flat roof waterproofing solutions for educational buildings
Presenters addressed specific considerations for educational buildings, including high humidity areas like swimming pools, gyms and changing room roofs. Common issues such as fragile decks, saturated build-up, life expectancy, and maintenance challenges were discussed as important factors for consideration, as well as understanding the wider context of the project. Case in point: for a school, consider the fact that children will often climb onto roofs to fetch balls, among other things. As a result, a robust roof that can withstand such traffic should be specified.
Interstitial condensation
Interstitial condensation is challenging to detect without thorough investigation, occurs within the fabric of a building, and is not an obvious fault (commonly mistaken for a roof leak), explained Ian Dryden. Such condensation can lead to the failure of the roof deck, causing internal leaks within the building. If not addressed, interstitial condensation may result in unnecessary refurbishment costs and can adversely impact thermal performance.
In addition to a U-value calculation, specifiers will require interstitial condensation risk analysis to ensure that the proposed design remains free of condensate build-up throughout the year.
Bio-Solar roofs
Specifying Bio-Solar roofing (where PVs are combined with green roofing) involves early design involvement with a self-ballasted system, including wind mapping and the production of system depths of growing mediums for dry ballasting, among others. When specified well, Dryden pointed out, the ‘bio’ and ‘solar’ aspects of such a roof can work in tandem, whereby condensation from a PV array can ‘feed’ the green roof surface below.
Upstands best practice
Upstands play a crucial role in preventing water ingress during adverse weather conditions. Flat roof upstands, as mandated by BS 6229:2018, are necessary to address rainwater build-up during heavy storms. The code specifies that at all abutments the waterproof layer should extend up to a level not less than 150 mm above the adjacent finished roof system. This requirement applies to protected roofing systems, such as paving slabs or gravel, where the waterproof layer should be 150mm above the upper surface of the protective layer.
Read a more in-depth article covering this specific subject here.
After a series of short presentations, attendees of the seminar were able to ask SIG’s technical experts various questions on roofing solutions, organised as an array of mini roundtable collective discussions.
Working with slate
The final section of the day’s presentations concerned slate roofing specification. Terry Collins and Chris Collins delved into natural roofing slate design, addressing environmental exposure, roof pitch considerations, slate ratings, and the SIG natural slate range.
A project’s geographical location plays a big role in determining the specification for a slate roof, pertaining specifically to the ‘lap’ or ‘headlap’. In slate roofing, the lap refers to the amount by which one slate tile overlaps the slate immediately beneath it. The lap is an essential aspect of proper slate installation, providing a water-resistant barrier and preventing water from penetrating the roof.
The degree of exposure of a building to driving rain determines the minimum lap specified, as does the pitch of roof. In general, the lower the pitch of the roof, the greater should be the lap. The map below shows the four exposure zones which relate to expected weather conditions across the UK.
Map showing weather exposure across the UK. SIG has an introductory guide to designing slate roofs on its website, available here.
Manufacturers of natural slates are required to provide a branded Declaration of Performance (DoP), including information on essential tests. The latter are conducted annually or after 25,000 tonnes of material is extracted from one quarry or vein. Test results, ranking slates based on water absorption (W), thermal cycle (T), and sulphur dioxide resistance (S), should be printed on crate labels. Buyers should exercise caution, as some natural slates may still be sold with questionable test certifications. It’s crucial to note that BS EN12326 is not a pass/fail test but rather a ranking system for various slate properties.
‘T2’ rated slate in use in the UK.
‘T2’ rated slate, for example, isn’t suitable for use in the UK as it rusts easily. Other second or third ‘class’ slates can result in ‘bird mouthing’ – gaps between courses, and delamination. The lower quality the slate is, the greater amount of sorting and grading will be required, leading to more labour and more waste, so can often be a false economy.
More to come
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