New and replacement flat roofs

From the top down, a flat roof is a waterproof covering, usually a layer of insulation, a vapour control layer that stops warm, moist air from inside the house reaching that insulation, and a structural deck, most often timber boards on joists. The order those layers go in decides which of three kinds of flat roof it is.

A flat roof is laid to a slope so water runs to an outlet or gutter. BS 6229 asks for a finished fall of at least 1 in 80, and because timber deflects and boards are never perfectly true, it tells designers to aim for 1 in 40 so that 1 in 80 is still there once the roof is built. Water left standing in a hollow, and any part of a roof that falls the wrong way, are treated as faults, not as something a flat roof just does.

Where the roof meets a wall, the covering turns up the wall as an upstand, normally at least 150mm above the finished roof, with a flashing over the top. Since 2018 the standard allows 75mm at a level door threshold, which makes the drainage near that door all the more important.

Upstands catch people out on a re-cover. Insulating a roof from above raises its surface, so the upstand under a door or a window cill can end up too short. Approved Document L allows for this: where upstand height makes the full insulation standard impractical, a lesser one may be acceptable.

Replacing the covering on a flat roof is building work. Government guidance lists it among the jobs that must be notified to building control, unless the roofer is registered with a Competent Person Scheme for roof coverings, such as CompetentRoofer, and certifies the work themselves. With a scheme member, you should receive a certificate of compliance within 30 days of the job finishing. Work to less than half of the roof, and less than a quarter of the building's whole outer envelope, does not normally need an application.

Approved Document L treats replacing the waterproof membrane on a flat roof as renovating it. Renovate more than half of the roof and the whole roof should be insulated to a U-value of 0.16 W/m²K, unless that is not technically feasible or would not pay for itself within 15 years, in which case it should be taken as close to that as is practical.

According to the Planning Portal, a re-cover also needs building control approval if the new covering performs significantly differently in a fire from the old one, or is 15 per cent or more heavier, which a green roof or a paved finish can be. So does any structural alteration.

Approved Document B limits which coverings can be used near a site boundary. Within six metres of one, only a covering classed BROOF(t4) is acceptable; the lower classes are only allowed further away, and the two lowest not at all on a terrace of three or more houses. The rule does not apply along the line between the roofs of a pair of semi-detached houses.

On most rear extensions the boundary is well inside six metres, so the fire classification of a system is worth checking before anything else about it.

The fire risk during the job matters as well. Some coverings are laid with a gas torch, and the roofing trade's Safe2Torch guidance says that wherever a fire risk is found next to the work, or cannot be ruled out, the specification must default to a torch-free method.

Every common covering can make a good flat roof when it is laid properly. The more useful question is what this particular roof has to cope with.

A flat roof you can step out onto is a different project from one you only look at. The government's planning guidance is clear that balconies and raised platforms, which it says include roof terraces, are not permitted development, so they need planning permission. A Juliet balcony, with no platform to step onto, normally does not.

It is a Building Regulations matter too: the roof has to be strong enough to carry people, it needs guarding at the edge, and the covering has to be one designed to be walked on. If the house is listed or in a conservation area, ask the local council before changing how a roof looks.

A flat roof feels safe to stand on, which is the danger. The Health and Safety Executive says almost one in five deaths in construction involve roof work, many of them smaller builders working on houses, and that falls from flat roof edges are prevented by simple edge protection: a secure double guardrail and toeboard around the edge.

Rooflights are the other hazard. The HSE says every roof should be treated as fragile until a competent person has confirmed it is not, and warns that some rooflights are hard to see in certain light. Expect edge protection and covered rooflights on any flat roof job, and ask about them if a quote does not mention access.

A quote that answers most of these before you ask is a good sign.