Nobody wants a new flat roof they did not need, and nobody wants to keep patching one that is past saving. The honest answer sits between the two, and it comes from looking at the roof rather than from a rule of thumb. On a survey we ask the same five things every time: how old the covering is, what condition the deck is in, how far the damage has spread, what the insulation rules say about the work, and what each route costs across the next ten years or so.
This guide is about the decision. It does not list the symptoms of failure; for that, read our guide to the signs of a failing flat roof. The terms you will meet: the deck is the board surface the covering sits on; the membrane is the waterproof layer; an overlay is fresh waterproofing laid straight on top of the existing layer. For the wider service, see flat roofing in Stroud.
Five questions, one decision
A repair is the right call when the fault is local, the deck is sound and the covering has years left. Replacement is the right call when the failure is spread out, the deck has gone soft or the covering has little useful life left. Most roofs sit somewhere between, which is why the decision is made on the roof and not on the phone.
The five questions are not equally weighty. Deck condition and extent of failure usually decide the matter. Age tells us what to expect from the covering. The insulation rules decide how much a replacement has to include. Cost over time is the check at the end, once the other four are known.
What we bring to the survey
We bring a moisture meter, a probe for testing the deck edge, a camera and a tape. The pictures we take on the day go to you with the quote. That lets you see what we saw, and makes a second opinion on another firm's quote easy to get.
Where access is awkward, on a terraced hillside or behind a house with no side passage, the survey also notes how materials will get up to the roof. That affects the cost of both routes, so it goes into the numbers early.
Age and covering: what a flat roof can reasonably give
Ages quoted for flat roofs are ranges, not promises, and the sources disagree. MyBuilder's 2026 flat roof guide gives 15 to 30 years as the general range for a flat roof. Beyond that general figure, each covering has its own range, and the table sets them out with the sources named.
| Covering | Typical life quoted | Source |
|---|---|---|
| Built-up or torch-on felt | 10 to 20 years | Less, 2026 |
| EPDM rubber | 20 to 30 years or more | GreenSpec; Building Centre |
| GRP fibreglass | 25 to 40 years | Less, 2026 |
| Lead sheet | 50 to 100 years | Less, 2026 |
Less puts felt at up to 20 years for a good job and under 10 for a poor one. That gap is workmanship, not material. A ten-year-old felt roof laid badly may need replacing sooner than a twenty-year-old EPDM roof that was laid well.
Age is a prompt, not a verdict
A covering near the bottom of its range is a reason to look harder, not to condemn it. We check the seams, the upstands and the laps first, because those fail before the field of the membrane does. A covering near the top of its range with a clean deck may just need a repair and a note in the diary to check it again. Our page on flat roof lifespan goes into each material.
Manufacturer warranties are not lifespans. An EPDM warranty of 20 to 25 years says what the maker will stand behind, which is not the same as how long the roof will last in practice.
The deck: the question that overrides the rest
A membrane can be perfect and still need replacing, because the board beneath it has gone. Plywood or timber decks that have been wet for months lose their strength and swell. You feel it underfoot as spring or sponginess; you see it at the edges as dark, crumbling board. Our page on a spongy flat roof explains what sponginess means.
If the deck is wet, no patch will hold for long. A new membrane would sit on a surface that moves and draws in moisture. At that point repair becomes false economy, and the right move is to strip the covering, replace the damaged boards and the rotten joist ends if there are any, and lay the new roof on a sound base.
Where the damp has been
A leak does not always come through at the point where water gets in. Water can run along a deck or through insulation before it drops into a room. We use a moisture meter across the deck, then lift a small test area at the worst point, so we can see whether the board is dry, damp or rotten. If wet insulation is found, it comes out as well.
The structure under the deck
Approved Document A, discussing roofs that cannot sustain their loading, mentions deflection leading to ponding. A deck that has sagged between joists will always pond, whatever is laid on it. If the joists are the cause, the fix is structural, and it belongs in a replacement, not a patch.
A dry, flat, firm deck is the best news a survey can give. It means a repair has something solid to hold on to.
How widespread the failure is
Extent separates a repair from a replacement more often than anything else. One split seam, one failed flashing, one punctured area under a tree: these are local faults, and a local repair makes sense if the rest is sound. Failure that is spread across the roof, such as widespread blistering, crazing, many open laps or ponds in several places, tells us the covering as a whole is giving out.
What a repair can cover
Typical repairs include re-bonding seams, renewing a flashing or an upstand, patching a puncture with a matching piece, sealing around a pipe or rooflight, and clearing and refitting an outlet. In MyBuilder's 2026 repair guide, minor roof repairs run £100 to £300 and moderate ones at £500 to £1,000. Checkatrade's repair guide gives per m² rates for flat coverings: bitumen £100 to £140, EPDM £120 to £170, GRP £130 to £180, excluding VAT and access.
A repair has to match
A new patch must suit the old covering, and a liquid coating on an old surface relies on that surface being sound. Where no patch will hold reliably, you will hear that from us.
It is also worth asking whether the fault is a one-off or a symptom. A single puncture from a fallen branch is a one-off. A seam that has opened in three places is a symptom of the covering aging. The first suits a repair. The second often means the other seams are close behind. Our guide to flat roof blisters covers one common sign that the covering is breaking down.
Insulation rules: what a replacement has to include
Replacing a flat roof covering is not only a roofing job. In Approved Document L, swapping the membrane over a flat deck ranks as renovating a thermal element, which brings the insulation into play. When over 50% of the surface is renovated, the entire element should reach the document's improved U-value, 0.16 W/(m²·K) for a roof. Where a new element is built, the limit is 0.15.
The Planning Portal thresholds apply as well: approval is not normally needed when work affects less than 25 per cent of the building envelope and less than 50 per cent of the roof. Go past either and approval applies, with the roof possibly needing an upgrade. A repair that stays below the thresholds does not usually have this effect, which is one reason a repair costs less. The detail is in our guide to flat roof Building Regulations.
Insulation adds depth and cost
Bringing a flat roof up to current standard usually means insulation above the deck, which raises the finished roof level. That in turn affects door thresholds, upstands, parapet heights and rooflight kerbs. Allow for it in the design, and we include those knock-on items in the quote.
On VAT, the insulation element at a home is zero-rated until 31 March 2027, while the remaining roofing carries the standard rate, so expect two treatments on one quote.
Overlay as a middle route
Some roofs can take a new membrane over the old, which saves stripping and disposal. It needs a sound deck and dry insulation, and it does not suit every old covering. See our guide to overlaying a flat roof for when it is sensible.
Cost over the next ten years
The lowest price today is not always the lowest cost over ten years. A repair that holds for three years and is followed by a second repair, then a third, can add up to the cost of a new roof, with the worry of each leak in between. A replacement spends more now and buys the years in a single go.
National ranges for a full replacement, from MyBuilder's 2026 flat roof guide, are £70 to £100 per m² for bitumen felt, £90 to £130 for EPDM, £95 to £150 for GRP and £95 to £140 for PVC single-ply. The guide puts a typical replacement at £2,800 to £7,500. Checkatrade gives scaffold at roughly £700 to £1,300 for a re-roof. Our guide to what a new flat roof costs sets these out by material.
A way to compare
Ask each quote to state what happens to the deck, the insulation, the edges and the outlets. A low figure that leaves old edge trims and a tired outlet in place is not the same job as a higher one that renews them. Compare the lines, not only the total.
Small jobs carry setup
Repair rates per square metre look higher than replacement rates because a small job carries the same setting-up as a large one across less area. That is no reason to avoid a good repair, but it is a reason to ask how many years of use the repair should reasonably give.
If a repair quote creeps close to the price of a replacement, ask the roofer to explain why they still recommend repairing.
How we put the decision to you
After the survey we send you a plain statement of which of the five questions pointed which way, and a written quote for the route we recommend. If a repair is sensible, that is what we recommend; if replacement is the safer course, we show you the deck, insulation and edges behind that view. Where a repair is possible but a replacement is the better long-term spend, you can be given both as separate quotes.
Both routes are done by us with the site cleared and waste taken away, and our work is covered by a 10-year workmanship guarantee in writing. Where the roof is a terrace or a balcony over a room, the stakes are higher because the roof is walked on; see balconies and roof terraces over living space.
What to send first
A few photographs from the inside and the outside, a note of when the leak first showed and whether it follows rain from one direction, and the age of the roof if you know it. You can reach us on 01453 933613, by WhatsApp, or send the enquiry form.