Repair or Replace a Flat Roof?

Most flat roofs are covered in one of three things: bitumen felt, a liquid-applied or glass-fibre layer such as GRP (glass reinforced plastic), or a rubber sheet called EPDM. Each ages differently, but the warning signs overlap, and reading them in the right order saves money.

Which signs point to a cheap repair?

A single split, a lifted edge, a small blister or a cracked patch of sealant tells you the covering has failed in one place. If the rest of the surface is flat, firm and dry, mending the one spot makes sense first. The cause is usually something you can name: a seam that opened, a fixing that rusted, a spot where foot traffic wore the surface thin.

Which signs point towards a new roof?

Look for trouble that is spread out rather than local. Cracks across the whole surface, blisters in several places, water that stands for days after rain, stains on ceilings under more than one area and a deck that feels spongy all push the decision the other way. One problem can be mended. A pattern suggests the whole system has reached the end of its working life.

Age matters too, though only as a guide. Less.co.uk, in a page updated March 2026, gives GRP as lasting 25 to 40 years and torch-on felt 10 to 20 years. GreenSpec puts EPDM at 20 to 30 years. These are typical lifespans, never promises, and a roof laid badly can fail early while a well-laid one outlasts the range. If yours is near or past its typical age and showing a pattern of faults, treat each repair quote as one step towards a decision about the whole roof.

Three faults account for most of the phone calls about flat roofs. They look similar to a homeowner standing at a bedroom window, but they point to different causes and different bills.

Is a blister a problem?

A blister is a bubble where trapped moisture or air has lifted the covering from what lies beneath. A small, unbroken one on a sound roof can be watched. If it has split, or the roof has several, water is getting under the covering and travelling sideways, which is how a leak ends up well away from the hole. Treat a blister that has split as a leak waiting to show itself.

What causes splits and cracks?

Roofs move. They expand in sun and shrink in frost, and a covering that has gone brittle will crack at the points where it is stretched, such as upstands, corners and joints. Splits along a line often mean movement in the deck below rather than a defect in the membrane itself. Patching a split without finding the reason for the movement means the same split returns.

Why does water sit on the roof?

Standing water, called ponding, means the surface is not draining. The cause may be a blocked outlet, a deck that has sagged between joists, or a fall that was too shallow from the start. BS 6229:2025 keeps a minimum finished fall of 1:80 across the main roof area, while IKO, citing the 2018 edition, says a roof should be designed for 1:40 so the finished roof still reaches the minimum. A roof that ponds after the covering is renewed has not been corrected, so the fall deserves as much attention as the membrane. See flat roof ponding water for the detail.

The deck is the structure under the covering: boards, plywood or concrete. It decides the whole question. A covering can be replaced as often as you like, but a rotten deck has to be cut out and renewed before any covering will last.

What can you check from inside the house?

Start with the ceiling below. A stain that grows after rain, paint that bubbles, a damp smell or a patch of mould in the corner of a room all point to water getting in. In a loft or void above, a roofer looks for dark timber, damp insulation and daylight where none should show. Photograph anything you find with the date, because the pictures help whoever surveys the roof.

What does a soft deck feel like on top?

Walking on a roof you do not know is risky, so leave that to the person doing the survey, who will have safe access and will spread their weight. They press along the boards with a gloved hand and a stout probe. Boards that spring back, a surface that dips between joists or a screwdriver that sinks into the timber mean the wood is wet and likely rotten. A hollow sound or a crunch under the covering can mean the same.

Rot rarely stays inside one board. Water runs along joints and settles at the lowest point, so a roofer opens the covering near the leak, and on a suspect roof will lift further to see how far the damage runs. If wet timber is found across a large area, patching the covering above it is wasted money. The deck work becomes the main cost and, at that stage, it makes sense to lay a new covering over the new boards in the same visit.

A repair is the right answer more often than people expect, and a roofer who recommends one when it is the better buy is doing the job properly. The honest test is whether the repair removes the cause or just covers the symptom.

Where does a repair work well?

Repairs work where the fault is local and the surrounding roof is sound. Typical examples are a split at an upstand, a failed joint around a pipe or rooflight, a worn edge trim, a blocked outlet that needs clearing or a lifted seam on a single-ply sheet. Checkatrade's roof repair guide, with a table dated June 2026, puts repair work at £100 to £140 per m² for bitumen, £130 to £180 for GRP and £120 to £170 for EPDM, all excluding VAT and for patches of up to 3 m². Those are national typical ranges and not a price for your roof.

Where does a repair fall short?

A repair cannot make a failing covering sound. Sticking new material to old felt that has gone brittle will often give a second leak a few feet from the first. It cannot cure a deck that has rotted, a fall that is too flat, or insulation that has been soaked. Neither does it add insulation, and a repaired roof keeps whatever thermal performance it had. It buys time, and that is useful if time is what you need, perhaps because an extension or a sale is coming. Ask your roofer to say in the written quote what the repair is expected to achieve and what it leaves untouched.

A full replacement strips the old covering back to the deck, repairs or renews the boards, corrects the falls where needed, and lays insulation, a vapour layer if the design calls for one, and the new waterproof covering with its edge trims, outlets and upstands. The old material is taken away, and scaffold or edge protection is set up first so the crew can work safely at the edge. Insulation is laid to suit the design, whether that sits under the covering or between the joists below, and the outlets and trims are fitted before the surface is sealed. Falls are checked while the work is open, because that is the only cheap moment to correct them. At the end the roof is tested for drainage and the site is swept clear.

Does a new flat roof bring energy rules with it?

It can. Part L of the Building Regulations counts a new membrane as renovation, so insulation is part of the conversation whenever the covering is renewed. Where a roof is renovated, Approved Document L (2021 edition, 2023 amendments) sets 0.35 W/(m²·K) in Table 4.3 as the threshold and 0.16 as the improved figure to aim for. Paragraph 4.13 sets a fallback: where the full figure cannot be reached sensibly, the target drops to the lowest U-value that is feasible and repays its cost within 15 years. That fallback is a point to raise with your building control body, not a loophole to assume. The 2026 edition takes effect on 24 March 2027 and renumbers these as Table 3.3 and paragraph 3.14.

The practical advice is short. Ask your roofer whether insulation is part of the new roof, and check with your building control body before work begins. For homes in Crawley, that means Crawley Building Control. Our guide to warm roof and cold roof construction shows where the insulation goes, and building regulations for flat roofs explains the wider picture.

Three numbers frame the choice: what the repair costs, what the replacement costs and how long each is likely to last. The written quote after the survey fixes the first two for your roof. The national ranges below give you something to measure it against.

What do new flat roofs cost nationally?

Checkatrade's roof installation guide, in its October 2026 table, puts flat roofing at £50 to £110 per m². Its rubber roofing guide, September 2026, gives EPDM at £80 to £100 per m² and felt at £50 to £80. Its fibreglass guide, same month, quotes GRP at £36 per m² for covering only and around £210 per m² fully insulated. Its flat roof replacement guide, with a table last updated December 2024, shows whole roofs from £2,800 to £7,500, so treat that one as dated. Scaffold, where needed, adds to the total: Checkatrade's fascia and soffit table names £300 to £600 a week per elevation, which is a national figure and no guide to your site. VAT at the standard 20% may need adding, so check whether your quote includes it.

A simple way to decide

Repair if the fault is local, the deck is firm, the covering is within its typical life and the roof drains. Replace if the deck is soft, faults are spread over the surface, water ponds, or the roof has already had repeated patches. Whichever way it goes, the written quote after the survey should give a single total for the job. The flat roofing hub lists every covering we fit and repair.