Flat roofs (on houses and extensions)

What to look for when building one

A flat roof can be a very good solution if you want to have optimal use of the living space below and above the roof. It's also lends itself very well to the placing of solar panels. You can even plant a garden on your roof, but research must be done first to determine which plants are best to use on a roof and which not. Grass has the ability to effectively contain the moisture in the soil and make it available to the plant.

Flat Roof structure

Flat roofs must have proper insulation that complies - where applicable - with Building Regulations. The normal flat roof composition includes the joists that run from wall to wall, and if the distance is too long, beams to support them. The ceiling would be fitted to the joists from below (optional) and on top of the joists the firring strips are installed, running perpendicular to the joists. The flooring is installed on top of the firring strips. These strips need to be level and if not, wood shims are used to adjust it. If timber boarding is used for flooring it should be tongued and grooved.

The floorboards serve as a deck on which the rest of the roofing will be installed. Upon the floorboard the vapour control layer is added and is necessary because of the condensation that often forms in these spaces. The insulation would be the next layer, usually quite thick, and then the waterproofing.

Types of flat roofing

The waterproofing may be bitumen or some other sealant that is added on top of the insulation and mixed with gravel to provide a walkable surface. The most popular has been coal tar, which could last for 50 to 70 years and has a tendency to ‘heal' itself in high temperatures. However, it is a known carcinogen and therefor not allowed in its traditional form.

A popular medium used now is EPDM (Ethylene Propylene Diene Monomer), a synthetic rubber that is readily available and easy to apply. A newer, much stronger version, called FleeceBack is combined with a polyester fleece backing and is resistant to shrinking, but still extremely stretchable, which allows it to move with the building. Another synthetic rubber is CSPE (Chlorosulfonated Polyethylene), also known as Hypalon. CSPE can be heat welded at the seams or adhered with a solvent-based adhesive.

PVC membrane roofing, also known as vinyl roofing, can, when still in the resin form, be blended with additives such as plasticisers for flexibility, stabilisers for durability and pigments for colours. Thermoplastic PVC roofing, which can also be heat-welded at the seams is very strong and is used as a flexible roofing membrane. It is relatively cheap to produce and has a long life cycle. The light colours of this material make it more reflective and therefor cooler.

Because of the non-flexing nature of flat roofs, leaks develop over the years and cracks develop due to frequent work being done on them. With normal sloped roofs it may be easy to locate a leak, but with a flat roof this may be next to impossible without removing the roof covering and boarding below it.

In warm roofs, where the insulation is above the floorboard, vapour control layers should be fitted between the deck and the insulation and it usually consists of two layers of bitumen felt. Care should be taken that there is sufficient ventilation in the roofing structure so that condensation and the damage it may cause could be dealt with.

Insulation materials include cork, with a relatively low insulation rating, mineral wool, with a slightly higher insulation rating and also fire resistant, polyurethane, with the highest insulation rating and expanded polystyrene. Expanded polystyrene is heat sensitive and needs a protective overlay between it and the bitumen felt. The entire range is subject to specific applications and should conform to the appropriate British Standards The contractor should know the regulations. All polymer-modified materials must have an accompanying certificate from the British Board of Agrément.

Flat Roof Contractors

The contractor should be registered with, to name two, the National Federation of Roofing Contractors or the Flat Roof Alliance. They should also be approved applicators of the materials used, which can be important for warranty purposes. If not, the operatives must posses Construction Skills Certification Scheme cards for the particular job.

When fixing a roof and before the work commence, the quotation from the contractor should contain the location and the area of the job, the cause of the leak and the proposal on how to deal with it. The amount of stripping required must also be stipulated, the materials needed, the quality of the materials, the type of surface protection proposed and the quotation itself. The payment terms must also be clarified as well as the details on insurance and the guarantees on the work and on the materials.

"can you use flashband as a permanent solution instead of torching in bitumen with gravel IE. new pitched roof extension built against flat roof extension with bitumen/gravel and torched. Do you use flashband at the join or torch it. "