Flat roofs, fewer risks

Does your specifier know what he or she is doing?

We’re not casting aspersions on architects here, but designing a flat roof is not as straight-forward as it seems.

A common reason why flat roofs fail is that someone gets the wind uplift calculations wrong. Although to the uninitiated working out wind loading may sound easy, it is anything but, especially since ‘flat roofs’ are often far from flat these days. Perhaps an architect does not have the right design data for every single component. Or, if the supplier is calculating windlift, they may have received inaccurate data.

Wind-loading calculations have to take into account the site, the building’s orientation, the shape of the roof and the type of fixings you’re going to use. An inexperienced designer might consider attachment without taking into consideration the density of the fixings or the amount of adhesive. That’s why Sarnafil insists on carrying out wind-loading calculations to offer its guarantee.

Unexpected results may also occur from a designer not allowing the correct fall, leading to ponding.

The insulation should be tapered to get a minimum fall of 1:80. And choosing the right attachment method can be a complex decision. For example, a mechanically fastened roof leads to quick and efficient installation, but an adhered roof may be a better solution aesthetically.

There are also environmental considerations. Many specifiers are led to believe that a PVC-based membrane is less environmentally friendly than flexible polyothelene (FPO), or ethylene propylene diene monomer (EPDM)-based membranes. But what they don’t realise is that while all three products have slightly different environmental profiles, they all achieve an A+ rating in BRE EcoHomes guidance for as-built construction. Sarnafil supplies both PVC and FPO-based solutions and specifies the appropriate product based on the projects’ requirements.

Unless you are a flat roof expert, these questions can be taxing. Architects like Sarnafil because it employs flat roof experts in its technical services department and, crucially, it takes on design responsibility for the roof. Its technical advisers visit specifiers early on in the process to give guidance, and this is followed up by a detailed written specification.

What are the implications if you change the specification?

You may think that you have found a comparable system, but the devil is in the details, so check them. Have a good look at the guarantee and BBA certificate.

Then there’s the membrane itself. Is it the same thickness? Is it the same colour? Does it have a laquered finish? (if it does, it will weather better).

Does it contain biocides?

Be aware that some manufacturers will push design responsibility onto the roofing contractor. And if you’re changing the architect’s original specification, they may not be indemnified for the new system.

There’s also the issue of compatibility. If a firm such as Sarnafil has been involved from the design stage, elements such as rooflights may have been designed to accommodate a particular roofing membrane and design. If another supplier comes in who is unaware of these issues, you could end up with problems, leading to delays and possibly a bodged job.

Can you get the materials when you want them?

It’s important to assess the financial stability of the roofing system supplier and to find out where their stock will be coming from. Some contractors have experienced delays because one of the accessories, such as fasteners, hasn’t arrived. You need to be able to guarantee the timely supply of all the components.

Alternatively, some firms may want to deliver all the materials in one go, which can lead to storage problems on tight sites. This also means that if you need more of any material or component you may be faced with a long wait – or have to use another make of product to get the job finished.

Companies such as Sarnafil have a logistics department which co-ordinates deliveries to site from its UK warehouse.

What happens if something goes wrong during installation?

The list of things which could potentially go wrong with any flat roof installation is long. Four of the most common danger areas for single-ply roofs are welding, spacing of fixings, preparation for adhered systems and details.

Getting the welding right is crucial on a single-ply membrane roof. Get something wrong here, such as welding at the incorrect temperature, and you’ve got a leaking roof.

Fasteners need to be correctly spaced. For example, some metal decks require a fixing at every corrugation, some at every other corrugation and some require two per corrugation. What’s happening on site should match the design and specification.

With adhered systems, preparation of the surface to which the membrane will adhered is vital. Some need to be primed, for example. When adhesive is applied and is left for too short or too long a time before putting the surfaces together, the bond quality will be affected. Equally, a wet substrate will affect bond strength.

Finally, details such as pipes or corners can catch out the inexperienced installer, because these tend to be non-standard and must be hand-crafted.

Some flat roof suppliers employ applications experts who visit sites to make sure that the contractor is installing the roof to specification. Sarnafil has 10 field technicians who check things such as the type and number of fixings being used, and whether the vapour control layer is being correctly installed. If there’s a problem, the field technician will visit the site to resolve it.

Sarnafil’s field technicians also visit the job during the project as required and when installation is complete, carrying out a 33-point check. The results of this are sent to the roofing contractor. The guarantee will be issued after a successful inspection.

How welll trained are the roofers?

It’s worth checking how well trained the roofers are. Your roofing contractor may be proud of their 100% CSCS-carded workforce, but what level of CSCS card does its roofers hold?

The reintroduction of a ‘grandfather rights’ type system means that roofers can now get a Basic Skills Card (green) for applied waterproof membranes having completed the health and safety test and submitted a letter from their employer to say that they are competent. Before this rule came in, many of the proactive roofing contractors had invested in putting their workers through NVQ level 2 so that they have Craft Cards (blue).

Manufacturers such as Sarnafil argue that the application of flat roofing requires a skilled worker, especially around detailing such as corners or rooflights. All approved Sarnafil contractors must send their employees on a four-day training course at the firm’s Norwich training and assessment centre, after which they get a Sarnafil link card which is only upgraded to a full card after visiting field technicians are satisfied that the operative has the right skill level.

What does the guarantee say?

When it comes to guarantees, remember that no two are the same. Although they may appear so at first glance. You need to check that you are comparing like with like. Sarnafil, for example, guarantees all the component parts of its system that it sells. The Sarnafil fasteners are manufactured by SFS intec, which counter-idemnifies Sarnafil, and similarly with the insulation, which is manufactured by Kingspan.

Other manufacturers’ guarantees may cover only the membrane, or components are guaranteed separately so that if a fastener, for example, fails, no one has responsibility for replacing the whole system.

Another guarantee sees the roofing contractor taking responsibility. In these cases, check that the contractor is indemnified against insolvency and catastrophic failure caused by a minor component such as a faulty fastener.

It also pays to check the BBA certification. Does its durability statement say ‘in excess of 30 years’ or ‘up to 30 years’?