The short answer
A leaking flat roof can be made safe quickly, usually by cleaning the area, locating the split or failed joint and applying a temporary patch suited to the membrane: a cold-applied felt patch, an EPDM repair tape or patch, or a GRP repair. Standing water is cleared first. A patch buys time. If the membrane is old, widely cracked or soaked beneath, the lasting answer is a new covering over a checked deck.
What a roofer can do on a failing flat roof at short notice, how it differs by covering, and when patching stops making sense.
The first steps on a leaking flat roof
Flat roofs fail differently from pitched ones. Water sits on the surface until it finds a split, a lifted seam or a gap at an upstand, and then drops through the deck into the room below. The first job on site is to get a safe look at the whole roof, clear leaves and debris from the outlet, sweep off standing water and find where the water is going in. The stain on the ceiling rarely lies directly below the fault, because water travels across the deck or along the insulation before it drips.
The roofer then dries the area as far as conditions allow and applies a temporary patch. Where the roof is out of reach or the weather is too wet to bond anything, a tarpaulin held down with battens on the edges can stop the worst of the water for a short time. Our page on emergency roof tarpaulins describes how to fix one that stays put.
Inside, lift wet insulation and keep the room ventilated so the structure can dry. A wet deck left covered is one of the quickest ways to turn a small repair into a re-cover.
Temporary patches by membrane type
The right patch depends on what the roof is made of. Traditional felt, bitumen based, can be patched with a cold-applied repair layer or a torch-on patch once the surface is dry, though a naked flame has no place near anything combustible. EPDM, the single-sheet rubber membrane, takes a purpose-made primer and repair tape or patch, and the surface has to be clean and dry for it to bond. GRP, the fibreglass and resin system, can be repaired by abrading the area and laying new resin and glass matting, but the resin stays tacky if the air is damp.
Each system has a lifespan. Published guides give felt as roughly 10 to 20 years, EPDM 20 to 30 years and GRP 25 to 40 years. Where a membrane is close to the upper end of its span, a patch is only a holding action, since the membrane around the repair is just as tired as the area that failed.
Whatever the covering, a good patch overlaps the damaged part by a generous margin, runs under or over the laps the right way for water to shed, and is rolled or pressed down firmly. Ask the roofer what has been used and how long they expect it to serve.
Ponding water and what causes it
Puddles that stay on a flat roof for days are a warning. Standing water adds weight, speeds up wear on the membrane and finds every weak spot. Ponding can come from a clogged outlet, a sagging deck, squashed insulation, or a roof built without enough fall in the first place. The current British Standard for flat roofs, BS 6229:2025, keeps a minimum finished fall of 1:80 for general areas and internal gutters. IKO advises designing for a steeper fall of 1:40, so that the finished roof still sheds water after allowing for tolerances.
Clearing an outlet and cleaning a gutter can cure a minor case. If the puddle remains after that, the roof is holding water because of its shape, and no patch will change that. Options then include adding tapered insulation to build a fall during a re-cover, or fitting an extra outlet. The cause should be settled before money goes on a covering that will only collect water again.
Take photographs of the ponding after rain, from a window or a safe vantage point, and show them to the roofer.
When a patch ends and a re-cover begins
A patch is the right answer for a single, clear defect in a membrane that is otherwise sound. It stops being sensible when the splits are many, when the surface is blistered or crazed across its whole width, when the deck feels soft underfoot, or when you have patched the same roof more than once. At that stage each repair costs money and the roof still leaks.
A re-cover strips the failed membrane, checks and repairs the deck, adds insulation and lays a new covering. Typical UK costs per square metre, from Checkatrade's September 2026 guides, are £80 to £100 for EPDM and £50 to £80 for felt. GRP is quoted at a covering-only figure of £36 per square metre, rising to around £210 per square metre fully insulated. What your own roof costs turns on its area, its access and the state of the deck, and the written total follows our survey. Ask whether VAT and scaffolding are included.
Our notes on emergency roof repair costs and the emergency and general roof repairs page help you weigh patching against replacing.
More on this in Emergency & General Roof Repairs in Crawley
Two more flat roof questions
Can I patch a flat roof myself?
Small jobs on a low, safely reachable roof are within some people's skill, but a flat roof is still a fall risk, and a patch laid over a wet or dirty surface fails quickly. A roofer can also read where water is really entering. If you try it, work from a secure position and use a product made for your type of covering.
Is the leak always where the stain is?
Often it is not. Water can run along the deck, through insulation or across a ceiling void before it appears, sometimes a metre or more from the split. That is why the roofer checks seams, upstands, outlets and the edges as well as the area above the stain, and why guessing at one spot often fails.