Key points
- Repair a single, isolated defect on a sound, dry roof; replace when failure is widespread or keeps returning.
- A wet or rotten deck, or saturated insulation, means a new covering on top will not last.
- Overlaying can save strip-out costs, but only on a dry, well-bonded roof with a sound deck and good falls.
- Renovating more than 50% of a flat roof over a heated room usually means upgrading the whole roof with insulation.
When is a flat roof repair enough?
A repair is usually enough when there is one identifiable defect, such as a split lap, a lifted flashing or a damaged outlet, and the rest of the roof is sound, dry and draining.
Typical repairable faults include a puncture from a dropped tool, a cracked joint where the covering meets a wall, or an outlet blocked and cracked by leaves. Our repairs page lists the common ones. Fixing them early protects the deck below; leaving them lets water spread through the build-up, which can turn a small job into a re-roof.
What are the signs a flat roof needs replacing?
The clearest sign is widespread failure rather than a single fault: the problem is in the covering as a whole, not one spot on it.
- Cracking and crazing across large areas of the surface.
- Blisters in many places, which point to trapped moisture or air under the membrane.
- Exposed, bare bitumen where the mineral chippings have worn away across the roof.
- Split laps and lifted seams in several places rather than one.
- Upstands and flashings failing around the whole perimeter.
- A soft or spongy feel underfoot, or a deck that visibly dips between joists.
- Damp patches on ceilings in more than one place, or that return after repairs.
Water standing on the roof for days after rain is a design problem as well as a wear one. A patch cannot correct a fall, so ponding that has damaged the covering usually calls for re-firring or tapered insulation during a replacement. See ponding water on flat roofs.
Does the age of the roof decide whether to replace it?
Age is a strong guide but not the deciding factor: a roof near the end of its typical life with recurring problems is a candidate for replacement, while an old roof in good condition may still be worth repairing.
As a rough benchmark, modern torch-on felt typically lasts 15–25 years, GRP 20–40 and EPDM 25–50, all with good specification and maintenance. Older felt laid before modern reinforced membranes generally has a shorter life than that. If you do not know when the roof was laid, look for signs of earlier patching, which suggest it has already needed attention. Our lifespan guide goes into more detail.
Why does the condition of the deck matter so much?
The deck is the structural board the waterproofing sits on, and if it is rotten or saturated no new covering on top will last.
Older roofs often have chipboard decks, which swell and lose strength once wet. Signs include sagging between joists, a bouncy feel and edges that crumble when the old covering is lifted. A roof with sound timber and a single leak is a repair job; a roof with soft, stained boards across the area needs the deck replaced as part of a new roof. Insulation matters too. Wet insulation loses much of its thermal value and keeps feeding moisture into the deck, so a roof with saturated insulation is rarely worth patching.
How do you find out if there is moisture inside the roof?
The reliable way is a survey that looks beneath the surface, combining a visual inspection with checks from inside the building and, where needed, test openings in the roof.
Staining on the ceiling, a musty smell in the room below, or water running out when a covering is cut are all clues. Water also travels: a leak can enter at an upstand and appear on a ceiling a metre or more away, which is why patching the spot above a stain often fails. A flat roof survey and leak investigation costs £150–£300 and gives a written report of what we found and what we recommend. Our leak guide covers the common entry points.
When do repeated repairs cost more than a new roof?
Once you are paying for repairs every year or two on an ageing roof, the total often approaches a large share of a replacement while the roof keeps deteriorating underneath.
Repairs typically cost £150–£500 for a localised patch, £250–£650 for an upstand or flashing and £120–£350 for an outlet. A single garage re-roof in EPDM costs £1,600–£2,800 on a sound deck. Add up what the last few years of call-outs have cost, then add the damage to ceilings and decorations inside. If that running total and the likelihood of further leaks are both rising, replacement is usually the better use of money. The full figures are in our flat roof cost guide.
Can you lay a new flat roof over the old one?
Sometimes. Overlaying a new covering onto an existing roof can work where the old roof is dry, firmly bonded, well drained and on a sound deck, and where the chosen system is approved for that substrate.
It avoids the cost of stripping and disposing of the old layers, and some liquid-applied and felt systems are designed for it. It is the wrong choice if there is trapped moisture, a rotten deck or poor falls, because those problems are sealed in rather than fixed. Extra weight and height at upstands and door thresholds also need checking. Our guide on whether to overlay or strip a flat roof sets out the tests in detail.
Does replacing a flat roof bring building regulations into play?
It can. In England, Wales and Northern Ireland, renewing more than half of a flat roof that sits over a heated room normally means the whole roof should be brought up to the current insulation standard, so far as it is reasonable to do so.
That is one of the less obvious costs of replacement: the insulation, the taller edges and the approval fee all need to be in the quote. A patch repair does not bring in the insulation requirement, and neither does re-roofing an unheated detached garage. Wales asks for a check on most re-roofing jobs, Scotland runs its own building standards system, and in England a much heavier covering or any structural change can need approval on its own. Our building regulations guide sets out each nation's rules, and where a re-roof needs approval we arrange it as part of the job.
What happens once you decide to replace?
The next step is a free, no-obligation survey, after which you receive a fixed written quote naming the system, the build-up, a rate for any rotten decking found and how approval is handled.
Vetted roofing teams working under S&K then carry out the work, and you receive a written workmanship guarantee for the re-roof, its length stated in the quote. If you are not ready to replace yet, a targeted repair can buy time while you plan. Get a range for your roof with the guide-price tool, or see flat roof replacement.
More questions
How many times can a flat roof be patched?
There is no fixed limit, but if patches keep failing or new leaks appear elsewhere, the covering or the deck beneath has usually reached the end of its useful life.
Is it worth repairing a 20-year-old felt roof?
It can be if the fault is isolated and the deck is dry, but a felt roof of that age is near or past the end of its typical life, so get a survey before spending much on repairs.
Can I just replace the part of the roof that leaks?
Small areas can be cut out and renewed, but the new section must bond properly to the old and both must be sound. If more than half the roof over a heated room is renewed, the building regulations rules for the whole roof usually apply.
Do I need a survey before deciding?
For anything beyond an obvious single defect, yes. A survey shows whether the deck and insulation are dry, which is the main factor in whether a repair will last.