The short answer
A single blister, a small split or a loose trim can often be repaired. Replacement comes into view when faults are widespread: many blisters, splits across the open surface, a deck that feels soft underfoot, water that stays for days, or stains that keep returning after repairs. One sign on its own is a prompt to look closer, and several together usually settle it.
Blisters and bubbles in the surface
A blister is a raised bubble where the covering has lifted from the layer below, usually because moisture or air was trapped when the roof was laid. On felt you can see a rounded hump, sometimes soft when pressed. Blisters are not always an emergency. A single small one that is intact can sit for a long time, and a roofer can cut it open, dry it and patch it.
The worry is a cluster. Where blisters spread across a slope, water is probably moving between layers, and each one that splits becomes a new way in. A roof covered in them rarely repairs well, because the layers beneath have already been wetted through. Our page on flat roof blisters covers what causes them in more detail.
Splits, cracks and open laps
Look along the joints first. A lap is the overlap where one strip meets the next, and a split is a tear across the surface. Both open up as a covering ages, stiffens in cold weather and moves with the building. Walk the line of every upstand with your eye, because the corners take the strain.
Hairline cracking, sometimes called crazing, shows the top surface is weathering and losing its flexibility. That is a warning rather than a fault. Splits that run through the full thickness, or laps that lift when you press them with a finger, are different: they let water in now. A split at one corner can be patched. Splits in several places, on a roof that is also stiff, curling or shedding grit, say the material is spent.
Rubber membranes fail differently. EPDM, a synthetic rubber sheet, seldom cracks across its open area. Trouble tends to start at seams, corners and the glued edges.
Water that stays after the rain stops
After the weather clears, a sound flat roof drains and dries. A shallow film that disappears the same day is a nuisance. Water still lying there several days later suggests the deck has sagged or the original slope was too gentle, and it hastens the ageing of whatever lies underneath. Algae, silt lines and a ring of dirt mark where it usually sits.
Ponding on its own does not always mean replacement. A blocked outlet is quickly cleared. A low spot over a failing deck is another matter, and recovering over it only hides the dip. We look at the cause in our guide to water pooling on a flat roof.
A soft or bouncy deck
The deck is the board or sheet the waterproofing sits on, normally plywood or older timber boards. Look from a window or a ladder rather than walking the roof, and note any spongy patches, dips or a bounce that was not there before. Soft patches usually point to a board that has stayed damp for months.
This is the sign that matters most. A rotten deck cannot hold fixings, so a new covering laid over it will fail early whatever its quality. If boards have gone, the work is renewal of the deck as well as the covering, not a repair. Our guide to a spongy flat roof goes through what we check.
Stains, damp patches and mould indoors
Look at the ceiling beneath the roof, and at the walls where it meets the house. Brown rings, bubbling paint, a musty smell, black spotting or a patch that dries and returns after rain are all signs of water getting in. The stain is often not directly under the fault, because water travels along joists before it drops.
One stain after one storm does not mean a new roof. A pattern across several rooms, stains that come back after earlier repairs, or timber that feels damp when pressed make replacement more likely. Wet insulation is another sign. Once it has absorbed water it stops working and holds moisture against the deck. Winter brings many of these to light, and our page on replacing a flat roof in winter covers the timing.
Mark the edge of any stain in pencil and date it. If the ring grows after the next spell of rain, the leak is live. If it stays put, the damage may be old. That simple record helps a roofer trace the entry point, and it stops you repainting over evidence.
Roofs in a conservation area carry extra rules about materials, covered in flat roofs in conservation areas, which is worth reading before you settle on a replacement.
Deciding between repair and replacement
Take the signs together. A few isolated faults on a roof of reasonable age usually favour repair. A roof with blisters, splits and soft boards at once, or one that has been patched repeatedly, favours replacement. Age matters too, and typical lifespans for each covering are on our page about how long a flat roof lasts. Whether a leak is down to age or to a detail is the subject of do modern flat roofs leak.
At survey we photograph the roof, and the price you receive in writing arrives with pictures taken on your roof, giving you the evidence to judge it. If a repair cures the cause, we recommend that and nothing larger. Where replacement is the right answer, we set out why. Details of the work are on our flat roofing page.
Safe checking matters. Do not climb onto a flat roof you are unsure about, as a weak deck can give way underfoot.