Flat Roofing near you

Small repairs usually stay outside the system. The Planning Portal's re-roofing guidance says some repairs to flat roofs will not require an application for approval under the Building Regulations, and its guidance on work to an existing roof gives the general threshold: a building control application is not normally needed where less than 25 per cent of the total building envelope and less than 50 per cent of the roof are affected.

Replacing the covering is a different matter, and the insulation requirement is the part that surprises people. Approved Document L Volume 1 defines renovation of a thermal element to include replacing the waterproof membrane on a flat roof, and says that where more than 50 per cent of the surface of the individual thermal element is renovated, or the work constitutes a major renovation, meaning more than 25 per cent of the surface area of the external building envelope, the whole of the element should be improved to the U-value in its table of limiting values for existing elements. For a roof that table gives a threshold value of 0.35 and an improved value of 0.16 watts per square metre kelvin. The Planning Portal puts the same point in homeowner language: if a roof with integral insulation is to be replaced you may be required to upgrade that thermal element and reduce the heat originally lost.

There is relief written into the guidance where the upgrade genuinely will not work. Approved Document L says that where achieving the improved value is not technically or functionally feasible, or would not achieve a simple payback of 15 years or less, the element should be upgraded to the lowest U-value that is feasible and pays back within that period, and that generally an upgraded thermal element should not have a U-value greater than 0.7. Its notes add that for a flat roof or a roof with integral insulation, a lesser standard may be appropriate where there are problems with the load-bearing capacity of the frame or the height of the upstand — which is the real-world case of a garage roof that cannot take another 100 millimetres of build-up without the door no longer fitting.

Every flat roof over a heated space is one of two constructions, and the Planning Portal sets out both. In a warm deck the insulation is placed on top of the rafters or joists and the roof covering goes over the insulation, and no ventilation is required for these types of roof. In a cold deck the insulation sits between the joists, and ventilation is required. For a warm deck flat roof the guidance describes rigid insulation laid over the roof joists with a board, normally external ply, laid on top, and the covering laid over that.

The cold deck is the one that goes wrong. The Planning Portal's guidance is that a ventilation gap, usually 50 millimetres, should be provided between the top of the insulation and the underside of the roof covering to allow air to flow across, with ventilation openings at the eaves or the upstand, and a vapour membrane added to the underside of the insulation and tacked to the joists before the plasterboard goes on. Miss the gap, block the openings, or leave the vapour membrane out, and the roof quietly soaks itself from below while the covering above stays intact.

Falls and outlets are the third of the build-up and the most commonly neglected. Water has to be able to leave: where it cannot, it sits, and Approved Document A's guidance on re-covering specifically mentions unacceptable deflection leading to ponding as a sign that a roof structure cannot sustain the loading placed on it. Ask any contractor quoting for a new flat roof how the falls are being formed — firrings, tapered insulation or a re-laid deck — and where the water is going to go once it arrives at the edge.

A patch is the right answer when the failure is local and the rest of the covering is sound: a split at one upstand, a lifted lap, a blocked outlet, a flashing that has come away from the wall. The Planning Portal's guidance supports treating this as ordinary maintenance, saying some repairs to flat roofs will not require an application for approval under the Building Regulations. If the roof is otherwise flat, dry and well drained, spending money on the one detail that failed is the proportionate thing to do.

Re-covering becomes the honest answer once the same roof needs a different repair every year, once the falls are wrong, once the deck has softened, or once the covering has blistered across its whole area. At that point the insulation question arrives with the job, because replacing the waterproof membrane on a flat roof is exactly what Approved Document L calls a renovation of a thermal element, and doing the work twice in five years is far more expensive than doing it once with the build-up corrected. Get the quotation to say what is being done about the falls, the outlets and the insulation, not only which membrane is going on top.

Where the work is notifiable, settle the route in writing before it starts. GOV.UK describes the two options: apply to a building control body, either the local authority's building control department or a private registered building control approver, or use someone registered with a competent person scheme, in which case you do not need to get approval yourself. The published list names NAPIT and CompetentRoofer, run by the National Federation of Roofing Contractors Limited, as the authorised schemes for replacement of roof coverings on pitched and flat roofs, including any necessary connected work but not solar panel installation.

On flat roofs in particular, be clear about what a guarantee covers. A membrane manufacturer's warranty on the material is a different document from an installer's guarantee on the work, and most flat roof failures are workmanship and detailing failures at upstands and outlets rather than material failures in the field. Ask whether the guarantee is insurance-backed, what it excludes, whether it transfers to a buyer, and who honours it if the firm stops trading.