Get a single-ply roof membrane right and no one notices. But get it wrong, and it’s another, very costly, story. Steve Cleminson, Technical Manager at SIG Design & Technology gives his top tips on the art of specifying a successful single-ply roof membrane.
1. Take a holistic approach
Choosing the actual product is only part of the story. You can investigate the difference between the products themselves all you like, but it’s the whole, holistic story that matters from the start to finish, including the availability of trained contractors and suitable support on site. That’s 10 times more important than simply choosing the single-ply roof membrane system itself.
2. Interrogate the brief
Be clear about what you’re trying to achieve with the product in terms of fire rating, u-values, warranty periods, third-party accreditations, environmental considerations and aesthetics. Ensure you address these key issues in your specification, and that the implications for not meeting any of them are understood. Investigate site conditions such as the nature of the deck and whether you can have a loose-laid, bonded or mechanically fixed installation, and produce clear and concise NBS specification for main contractors to go out to tender with.
3. Educate yourself on compliances
I never fail to be amazed by how some architects seem more interested in the colour of the toilet walls than complying with best practices. The BS 6229 code of practice for flat roofs states that no part of the finished roof should have a fall of less than 1:80. Getting the falls right is a common pitfall, especially when there are third parties involved. Even if non-compliance isn’t picked up by building control or third-party insurers, poor design regarding falls can lead to increased maintenance requirements, accelerated ageing of roof finishes and dangerous conditions for roof access routes.
4. Understand the products
Single-ply roof membranes are flat, synthetic polymer-based roofing materials manufactured in a single waterproof sheet. All may look very similar but their base compounds and chemistry are very different. They are often wrongly considered a drab solution, rarely specified to look pretty, but favoured for their longevity, ease of installation and the peace of mind that they will stay waterproof. However, with many systems offering a range of colours, coatings and decorative profiles, quite striking results can be achieved. Make sure you understand what the warranty is covering, and what maintenance requirements are set out in the warranty documents.
5. Think carefully about sustainability
Identifying the greenest choice is a minefield. The more cost-effective PVC option has traditionally been seen as less green because of its chemistry. It can, however, have advantages if, say, it is manufactured locally compared to the implications of importing other products from abroad. TPO and TPE can claim some enhanced green credentials compared to PVC, but if sustainability is the top priority, specifiers often choose PIB. The product choice will depend on what the architect wants to achieve for the client. There is an element of compromise since, generally, the greener the product, the more expensive it is.
6. Get the sequencing right
Ensure roofing suppliers are involved early on, or expect heartache and cost later. Always ask suppliers how readily available stock is, to avoid long lead times causing delays. Sequencing and the design of interfaces is imperative – for example, early consideration of a balustrade post, which may avoid someone later on sticking a bolt through the membrane and affecting the waterproofing. Most problems are due to a lack of coordination. If you do it wrong, you have to backtrack and pay more or end up with a very weak detail, which will cause problems later on.
7. Choose your installer carefully
Every product is only as good as its installer. If you go for a cheaper option and end up with a contractor who isn’t fully familiar with a particular system’s requirements, you could end up in trouble. Instead, identify suitably trained, registered and monitored contractors who are supported by suppliers that comply with Single Ply Roofing Association (SPRA) guidelines and training regimes. Always ask your supplier how they monitor the quality of their installers, and check they have technical design support available for the installers.
Five main single-ply roof membrane types
PVC (Polyvinyl Chloride) Accounts for around 80% of the UK market. Available in a wide range of colours. Contains plasticisers and chlorine. Requires isolating layer for use with bitumen. Durable, partially recyclable, some UK-made.
PIB (Polyisobutylene) First developed in the 1930s, PIB is long-lasting, 100% recyclable, and the only membrane with a full life cycle assessment to ISO14040 standards. Available in grey and with a copper paint finish. A self-sealing edge makes it simple to install.
TPE (Thermoplastic Polyolefin Elastomer) 100% recyclable and simpler to weld and install than TPO. Small punctures can be resealed using heat. UK manufactured.
TPO (Thermoplastic Polyolefin) Often seen as a more environmentally friendly alternative to PVC because of its lack of plasticisers. However, it can require a solvent to clean it before heat welding. Popular for civil engineering uses. Only available in greys. Partially recyclable.
EPDM (Ethylene Propylene Diene Monomer) Elastic, synthetic rubber roofing membrane, which is popular for domestic projects though it can be hard to detail. Only available in black. Not recyclable, although the membrane can sometimes be reused. Often the cheapest option.
