Jan 12 Self-builder Q&A
We all like the creature comforts, but when it comes to building a house, few can argue that it’s the big things like structure, services and roof that determine whether the project runs smoothly. Thirty years ago, flat roofs (‘membrane roofs’ is a better description) were purely functional, except in the few cases where a garden was placed over them. Now, they are frequently an integral part of the architectural concept, the ‘fifth façade’ as Le Corbusier once put it. Has waterproofing technology kept up with change and what can it offer the self-builder?
Polymeric single ply membranes have rocketed to number one in popularity for new build and are an increasingly strong contender in refurbishment. The reasons are not only technical as we shall see later, but key has been their design flexibility, which means a wide range of geometry can be waterproofed without compromising low carbon objectives
This short article will help the self-builder meet or even exceed their expectations for the roof and most importantly, to ‘get it right, first time and on budget’.
What are the key design criteria?
Waterproofing used to be the big challenge, now its thermal insulation. This has dictated big changes to roof system design. Mandatory building fabric energy efficiency levels are on a steep curve to meet zero carbon by 2016. Whilst the Building Regulations average U-value[1]requirement of 0.20W/m2.0K has not changed since 2006 (and probably will not after 2013) the designer is unlikely to achieve overall carbon compliance without working to a U-value of 0.16 or so. That is a lot of insulation (e.g. approximately 150mm of high-efficiency cellular material or 225mm of acoustically superior dense mineral wool.
This has effectively signed the death warrant for cold deck roofs, the traditional arrangement where the insulation is placed above the ceiling and the space between it and the underside of the deck is ventilated to the outside. Cold deck roofs have been replaced by the more reliable and straightforward warm deck roof (see sketch 1). Those requiring a very heavy roof use (e.g. big planters or other unknown point loads) on a suitably massive structure may opt for the inverted roof (see sketch 2) but it is essentially only a variant of the same principle.
Highly effective ‘thermal break’ tube fasteners have now replaced the old steel-only version. Check that you have these in your design or you may have to pay an expensive penalty in increased insulation thickness (and cost).
Key design criteria:
· Ensure sufficient height at details: this should be a minimum 150mm in the vertical or equivalent up a slope (e.g. where a membrane roof meets a discontinuous element roof with slates or tiles).
· Provide generous falls: aim to achieve at least 1:80 slope in the finished roof, which means starting out at 1:40 or perhaps 1:60 if you have tight tolerances.
· Aim for a simple drainage arrangement: try to arrange the falls with mitres so that they run to a single point for internal outlets or over an eaves edge if you have external gutters. Avoid internal box gutters at all costs; they introduce their own problems and are simply not required these days. Remember that drainage capacity depends on rainfall intensity, not frequency, so design in Cambridge is more demanding than in Carlisle. It sounds obvious, but rainwater outlets should always be at the low points!
· Design to resist wind load: wind causes an uplift force on low-pitch roofs which is greatest at the perimeter, especially the prevailing wind corner. A suitably qualified person – check who this is – should have designed the attachment of all the components to resist this force with a suitable margin of safety depending on the attachment method.
· Design all details carefully so that they provide clearance for the installer and do not rely on sealant alone. All upward-facing edges of waterproofing should be protected by a separate flashing (e.g. an ‘apron’ on a pipe or a lead flashing against a wall) or alternatively welded to a profile set into the abutting masonry.
· Make sure your as-built roof system has current certification for external fire performance: fire performance is tested from the outside and is a Building Regulations issue. The result depends not only on the single ply membrane but also the insulation beneath.
How can I get the appearance I want?
· If you want a smooth finish, discuss thoroughly the options for securing the insulation and single ply membrane and request photographic evidence from the membrane supplier. Good setting out - i.e. where the seams will be - is important, as is correct use of membrane-metal trims at edges. It may be worthwhile considering a plywood overlay to the insulation and/or a fleece-backed membrane to further improve appearance, but much will depend on careful, low-tolerance installation.
· Replication of metal finishes is straightforward, using pre-formed profiles for copper and zinc or larger batten-roll details for lead (see photo). Here again, careful planning and design to match the dimensional discipline of the metal is critical for success.
· A wide range of colours is available in most product types. Decide carefully on the colour to suit any replication of metal.
How should product type be selected?
· Selection of insulation is critical:
· Fit as much thermal resistance as you can in the space available. The pay-back period is getting shorter with every increase in delivered energy cost. Manufacturers can provide calculations of U-value and design to eliminate condensation risk.
· Make sure you have enough acoustic insulation. Manufacturers can advise on how to avoid rain noise intrusion in lightweight roofs.
· Selection of single ply membrane type is also important: PVC is a proven performer which gives excellent appearance. Flexible Polyolefin (FPO, sometimes called TPO) is an excellent alternative but requires great care at installation to achieve good appearance. Both have A+ ratings for environmental impact on timber panel or steel decks (visit www.thegreenguide.org.uk for more details.
[1] U-value is the measure of how much heat passes through unit area of a building element (wall, floor, roof) for unit temperature difference between outside and inside. The lower the U-value, the better the insulation.
