How are flat roofs over shops repaired?

Finding where the water gets in

Shop roofs are often flat or very shallow, sitting behind a parapet, a low wall that runs round the edge of the roof. They carry rainwater to outlets or to a gutter at the back, and they are crossed by flues, extract ducts, air conditioning pipework and signage brackets. All of these are places where water can enter.

The stain on a ceiling tile rarely sits under the hole. Water runs sideways through the deck and insulation before it drips, which makes tracing it the first job. We check the covering for blisters, cuts and lifted joints, then the upstands against the wall, the outlets and each item of equipment fixed through the surface. Standing water that stays after rain tells you the fall is poor. A flat roof is designed to a minimum fall of 1:80 and built to 1:40 to allow for tolerances, according to BS 6229:2018 as set out by manufacturers, though many shop roofs fall short of that.

Leaks over a shop are frequently caused by the things the roof carries rather than the covering. A flue that has moved, a cable pushed through without a seal and a rotted timber upstand are all common. A roof that is sound in the middle can still leak at the edges.

Repair methods by roof type

Which repair is right comes down to the covering.

• Built-up felt. Blisters and splits are cut out, dried and patched with new felt, with the edges sealed. Where the felt is crazed all over, a new layer on top may be better value than constant patching. Our page on built-up felt roofing goes into this.
• EPDM. This synthetic rubber membrane is repaired with glued patches and sealing tape, as long as the surface is clean and dry.
• GRP. The fibreglass system, resin worked into a woven mat on site, can be repaired locally by grinding back, re-laminating and finishing with a new topcoat.
• Single-ply (PVC or TPO). Seams and patches are heat-welded, and the membrane is detailed carefully round outlets.
• Lead and metal. Splits in lead flashings or gutters are repaired by welding or replacing sections, with care that new material matches the old.

The materials do not mix freely. Some adhesives and solvents attack some membranes, so a repair has to be compatible with what is already there. Our page on commercial roofing materials explains the differences.

Repair, overlay or recover

A patch repair is right when the roof is mostly sound and the fault is local. It is the cheapest and least disruptive option, and it should be offered first. Once a roof has been patched again and again and still weeps somewhere new each winter, patching is no longer the answer.

An overlay lays a new membrane over the existing one. It avoids stripping, so there is less noise, dust and waste, which suits a trading shop. It needs the existing roof to be dry and reasonably flat, with insulation that is not saturated. Trapped wet insulation is a good reason to strip instead, as the water does not go away. See our page on the commercial roof overlay.

A full recover strips the covering and often the insulation, and replaces them. That is a bigger job with a cost to match. For flat roofs, MyBuilder (2026) reports national typical rates per square metre of about £70 to £100 for bitumen felt, £90 to £130 for EPDM and £95 to £150 for GRP, and a commercial job is priced on survey, with your fixed quote following the survey. Scale matters for Building Regulations as well: re-covering that touches under 25 per cent of the building envelope and under 50 per cent of the roof normally needs no approval, whereas a larger job may.

Keeping the shop trading

Disruption is a matter of planning rather than luck. Before work starts we agree the days, the access route, where materials will be stored and how the area below will be protected. The roof is usually reached from the rear or from a side yard, with a ladder or scaffold that keeps clear of the shop entrance. Where scaffold or a hoist must stand on a pavement, the council may need to licence it, so allow time to arrange that.

Noise and smell are the main nuisances. Torch-on felt uses a flame and has a distinctive smell, and some adhesives are strong. Cold-applied systems and single-ply membranes reduce both, which is a real advantage above a food shop, a salon or a café. Dust from cutting out old material can be controlled with careful sweeping and sheeting.

Inside, ceilings below are checked for damp and for anything that could fall, and stock is moved from the wet area. Work can be sequenced so that the roof is watertight each evening, with a section completed at a time rather than the whole roof opened at once. Ask about working hours with neighbouring businesses in mind, because a shared service yard is a shared nuisance.

Roof lights, flats above and what else to check

Many shops have rooflights over storerooms or stairwells, and these are leak sources in their own right. Our answer on industrial rooflight replacement describes how they age, and the same signs apply on a shop. The occupants of flats above a shop add another layer, since the roof may belong to the landlord and a lease decides who pays for what.

Shops in Bath are often in older buildings. A parapet gutter, a gutter set behind the front wall of the building and lined with lead or another sheet, is a feature of many older terraces. It needs a different approach from a modern membrane roof. Where the building is listed, B&NES Council decides consent, and an unconsented alteration is an offence. Another type of occupied premises with its own timing demands is the school, so see school roof repairs for how we plan around it.

The survey costs nothing, and it comes back with pictures taken on your roof. A fixed sum follows on paper, and if the shop must keep trading, tell us your quietest hours so the noisy stripping can be planned around them. Other commercial jobs are listed on the commercial roofing hub.