The short answer
Repair a flat roof when the fault is local, the covering is otherwise sound and the deck underneath is dry. Replace it when the covering is cracked or worn over most of its area, when leaks recur at different points, or when the deck is wet or rotten. Age matters, but condition and extent of damage decide the question.
The three questions that decide it
Three questions settle most cases, and they are better than a rule about age alone.
How much of the covering is damaged? A split at one corner, a lifted flashing or a single blister is local. Cracking, thinning or open laps across the whole surface is general. Local faults suit a repair, and general ones rarely do.
Is the deck dry and sound? The deck is the timber board or plywood that the covering sits on. If a leak has been running for months, the deck may be damp, stained or soft. A new patch over a wet deck seals the moisture in and the problem returns.
How often has it been repaired? One repair that has held for years suggests a sound roof. Repairs at three or four points, or the same spot repeatedly, suggest a covering that is failing faster than it can be patched.
A fourth consideration is whether other work is coming, such as an extension, a rooflight or new insulation. That can make replacement the more sensible moment.
When repair is the right answer
Repair is the first option we weigh, and on flat roofs it fits more cases than people expect. A young roof with a local fault is the clearest case.
Typical repairs include re-sealing a lifted flashing, replacing a failed section of lead at a wall, re-forming a corner on a rubber roof, patching a puncture, renewing a split trim or clearing and re-lining a blocked outlet. A GRP roof with a hairline crack along a deck joint can often be cut out and laid again. A felt roof with a single failed lap can be re-stuck.
Repair works best when the surrounding material is still supple and firmly adhered, so a new patch can join to it. It works when the cause is known and removed. Patching over a split without finding out why it split, for example a springy deck or standing water, only delays the next one.
MyBuilder (2026) gives typical UK repair costs of £100 to £300 for minor jobs and £500 to £1,000 for moderate ones. Those are national figures, not our prices.
When replacement is the right answer
Replacement becomes the sensible choice when repairs can no longer keep ahead of the failure.
- The covering is worn across its area. Crazing, cracks, bare patches or open laps in many places mean the whole membrane has aged.
- The deck is wet or rotten. Timber that is soft, stained or discoloured needs renewing, and that means lifting the covering anyway.
- Leaks keep returning. New leaks in new places are a sign of general failure.
- The roof has more than one layer of old covering. Extra layers add weight and trap moisture.
- The falls are wrong. A roof that holds water across large areas may need its falls corrected, which means lifting the covering.
If one of these applies, a replacement is the honest recommendation, and replacing in good time protects the structure under the roof from damage that costs far more.
Age is a clue, not a verdict
Age helps you read the signs, but it should not decide alone. Felt is typically 10 to 20 years, EPDM 20 to 30 years or more, and GRP 25 to 40 years, according to published guides, with the usual caution that exposure and workmanship change the figures. See how long a flat roof lasts for the detail.
A fifteen year old felt roof with a dry deck and a single failed lap may need nothing more than a repair. A twelve year old one, laid in a single layer over a damp deck, may be beyond saving. A well-made EPDM roof may carry on long past its quoted range if the edges remain tight.
That is why a survey is better than a guess. We look at the surface, the deck where we can see it, the edges, the outlets and the ceiling below, with the findings laid out in photographs.
Falls, ponding and the deck
Many replacement decisions turn on drainage. If water lies on the roof in large, deep pools, the cause may be insufficient fall, a deflected deck or a blocked outlet. A blocked outlet is a simple fix. A sagging deck or a roof built without enough fall is not, and correcting it usually means lifting the covering and laying tapered insulation or firring strips, which are tapered timber battens that create a slope.
Guidance based on BS 6229 sets a recommended minimum finished fall of 1:80, with a design target of 1:40 to allow for building tolerances. Standing water has its own page, which explains the position.
If a deck needs renewing, it is far more economical to do it as part of a replacement than to patch a covering on a deck that is going. The wider design questions, such as insulation and ventilation, are covered in flat roof design.
Cost and what the survey tells you
Typical UK flat roof replacement runs £2,800 to £7,500, averaging about £75 per square metre, according to MyBuilder (2026). Those are third-party figures, and your own price turns on roof size, the system chosen, deck condition and the number of edges.
A repair is nearly always cheaper on the day. The question is cost over the next ten years. A series of repairs on a failing roof can add up to more than a replacement, and each leak risks damage inside.
The survey is free, and it comes with photographs from your own roof. Afterwards you receive a fixed written price, and where a repair will do, repair is what we put forward. When a replacement is the answer, booking it for late spring or summer gives the deck time to dry out and the new covering a better start. If a re-cover goes past the Building Regulations thresholds, building control may apply, and we explain that at the survey. See the flat roofing service page for the other work we do.
