Most flat roofs don’t fail overnight. They send warnings for months — sometimes years — before water ever reaches your ceiling.
The trouble is that those warnings are easy to miss if you don’t know what you’re looking at. A few bubbles in the felt, a puddle that lingers after rain, a hairline crack at a corner. None of it looks urgent. All of it is your roof telling you something.
This guide covers the nine problems we find most often on flat roofs across Deal, Dover and Sandwich, how to spot each one early, and what the repair actually involves. By the end, you’ll know what deserves a closer look and what needs a professional on the roof this week.
If water is already coming through, skip this article and read our guide on how to find and fix a flat roof leak fast instead — then come back to this one once you’re dry.
1. Ponding Water
What it looks like: pools of standing water that remain on the roof more than 48 hours after rain has stopped. You’ll often see dirt rings or moss marking where the water usually sits.
Why it happens: flat roofs aren’t flat — they’re built with a slight fall so water runs to the edge or an outlet. Ponding means the fall has failed somewhere. The deck may have sagged, the insulation compressed, or the roof was built with too shallow a fall in the first place.
Why it matters: standing water is heavy, around a kilogram per litre, and it sits exactly where the structure is already weakest. It degrades felt, finds every marginal seam, and freezes in winter, prising joints apart.
The fix: minor ponding can sometimes be solved by clearing blocked outlets or adding a firring piece to correct the fall locally. Persistent, deep ponding usually means the deck needs attention — a job for a specialist, not a sealant tin.
2. Blisters and Bubbles
What it looks like: raised, spongy patches in a felt roof, anywhere from coin-sized to dinner-plate-sized. Press gently and they give underfoot.
Why it happens: moisture or air trapped between the layers of felt expands in the sun, pushing the layers apart. Each summer the blister grows a little.
Why it matters: an intact blister is a weakness; a split blister is a hole. Once the surface cracks, water enters the roof build-up directly.
The fix: isolated blisters can be cut, dried and patched. Widespread blistering across the roof tells you the membrane has reached the end of its life, and patching becomes throwing good money after bad.
3. Cracked, Lifted or Failed Seams
What it looks like: visible gaps, curling edges or cracking along the overlaps where sheets of felt or rubber join.
Why it happens: thermal movement. A flat roof can swing through a wide temperature range in a single spring day, and every cycle works the joints. Coastal wind and salt air, which we get plenty of along this stretch of Kent, speed the process up.
Why it matters: seams are the natural weak point of any sheet membrane. Most slow, intermittent leaks trace back to a seam.
The fix: individual seams can be re-bonded or taped. If multiple seams are failing at once, that’s age, not bad luck — time to discuss re-covering.
4. Failed Flashing and Upstands
What it looks like: cracked mortar where the roof meets a wall, lead that has slipped or lifted, gaps at the base of chimneys, vents or parapets.
Why it happens: the junction between a flat roof and anything vertical relies on flashing — usually lead — to bridge the joint. Mortar fails, fixings loosen, and lead itself fatigues and splits over decades.
Why it matters: this is the single most common leak source we find on otherwise healthy roofs. Water running down a wall slips behind the membrane and travels, so the damage shows up nowhere near the fault.
The fix: re-pointing, re-dressing or replacing the flashing. Done properly, leadwork outlasts the membrane around it. Done badly with mastic and hope, it fails within a couple of winters.
5. Blocked Outlets and Overflowing Gutters
What it looks like: leaves, moss and silt around the roof outlet, water staining down the fascia, gutters that overflow in heavy rain.
Why it happens: a flat roof drains through a small number of outlets. Block one and the water level on the roof simply rises until it finds another way down — often over an upstand or through a seam.
Why it matters: we’re regularly called to “roof leaks” that turn out to be drainage problems. The membrane is fine; the water just couldn’t leave.
The fix: the cheapest item on this list. Clear the outlets, clean the gutters, fit leaf guards if trees overhang. Twice a year, autumn and spring, keeps most roofs out of trouble.
6. Cracking and Crazing in Fibreglass (GRP)
What it looks like: fine spider-web cracks in the topcoat of a GRP roof, usually starting at corners, edges and trims where the board flexes most. Sometimes a star-shaped crack marks a single impact point.
Why it happens: GRP is rigid. If the deck beneath moves, or the original laminate was laid too thin, the surface cracks rather than stretches.
Why it matters: hairline crazing in the topcoat alone is cosmetic at first. Cracks that penetrate the laminate let water into the boards beneath, and GRP hides saturation well until the deck is soft.
The fix: topcoat crazing can be re-coated. Structural cracks need the laminate cut back and re-built. A specialist can tell the difference in minutes; from the ground, you usually can’t.
7. Shrinking and Cracking from UV Exposure
What it looks like: felt that looks dried out, crazed like old leather, with exposed bitumen where the protective mineral surface or solar chippings have worn away. Edges may pull away from upstands as the material shrinks.
Why it happens: ultraviolet light slowly breaks down bitumen. The felt loses its oils, becomes brittle, and shrinks. South-facing roofs and roofs that have lost their chippings age fastest.
Why it matters: brittle felt cracks rather than flexes with the first hard frost. Shrinkage also pulls the membrane off upstands, opening a path for water at the roof’s most sensitive points.
The fix: restoring chippings or applying a solar-reflective coating slows the decline on a roof with life left in it. Once felt is brittle and shrinking, replacement is the honest answer — and a modern EPDM rubber system handles UV far better, with Firestone membranes carrying a life expectancy of around 40 years.
8. Vegetation, Moss and Organic Growth
What it looks like: moss cushions, lichen, even grass or small shrubs taking root in silt that has gathered on the roof.
Why it happens: debris collects, holds moisture, and becomes a growing medium. Ponding areas and shaded north-facing sections grow the best gardens.
Why it matters: roots work into seams and laps, and moss holds water against the membrane permanently. Anything green on your roof marks a spot that never dries out.
The fix: remove growth gently — wire brushes and pressure washers damage membranes — then deal with whatever let it establish, usually ponding or blocked drainage.
9. Sagging and a Spongy Deck
What it looks like: a visible dip in the roof line, a ceiling below that bows, or a roof that feels soft and spongy underfoot rather than solid.
Why it happens: this is the end stage of the other eight problems. Long-term moisture has rotted the timber deck or saturated the insulation beneath the membrane.
Why it matters: of everything on this list, this is the one not to sit on. A sagging deck means the structure itself is compromised, and every week of weather makes the eventual repair bigger. If the ceiling below is staining or bowing, treat it as urgent and call an emergency roof repair service rather than waiting for a routine appointment.
The fix: there’s no patching a rotten deck. The affected section has to be stripped, the timber replaced, and a new waterproof system installed.
Patch Repair or Full Replacement? How the Decision Actually Gets Made
Here’s the question behind every problem above: is this roof worth repairing, or are you paying to postpone the inevitable?
A fair assessment weighs a few things:
- A built-up felt roof typically gives 10–20 years. If yours is at year 18, a £250 patch buys time, not a solution.
- One blister is a repair. Blisters, cracked seams and ponding together mean the system is failing as a whole.
- The deck. If the timber beneath is dry and sound, almost any repair is viable. If it’s wet, the membrane question is secondary.
- Your plans. Selling in two years and re-roofing in ten lead to different sensible answers for the same roof.
This is where who you ask matters as much as what you ask. A national firm sending a salesperson on commission has one answer ready before they’ve climbed the ladder. A family-run local contractor — whose name is on the van and whose reputation lives or dies on word of mouth in towns the size of Deal and Sandwich — has every reason to tell you when a £200 repair is genuinely all you need. We’ve been doing exactly that since 1997, and a fair share of our roof repair call-outs end with us recommending less work than the customer expected.
Independent guidance backs this up: the National Federation of Roofing Contractors advises homeowners to look for an established local trading history and a contractor willing to explain their reasoning, not just quote a price. For a sense of how long each system should last and what replacement involves, the Federation of Master Builders’ flat roof cost guide is a useful independent reference point.
Catch It Early, Pay Less — Every Time
The pattern across all nine problems is the same:
- Spotted early, almost everything on this list is a modest, contained repair
- Ignored for a year or two, the same fault works down into the deck and multiplies the cost
- Twice-yearly checks — a look from an upstairs window after autumn leaves fall and again after winter — catch most problems while they’re still cheap
Your flat roof will tell you when something’s wrong. The only question is whether anyone’s listening.
If you’ve recognised your roof anywhere in this article and you’re in Deal, Dover, Sandwich or the surrounding areas, we’ll take a look and give you a straight answer on repair versus replacement — no pressure either way.
Call Weathered Roofing on 07379 187815 or request a free quote.