The short answer
Water pools on a flat roof because of too little fall, a sagging deck, a blocked or badly placed outlet, or a build-up of debris. A thin film that clears soon after the rain is a nuisance. Water that remains for several days is a warning, because it speeds ageing, grows algae and tests every joint. The fix depends on the cause.
Why water stops where it does
Rain runs downhill, so a puddle means there is a low spot, or something in the way. The most common reasons are an original fall that was too shallow, a deck that has sagged over the years, an outlet that sits higher than the surrounding surface, and debris that has built up and dammed the flow.
The covering itself can cause it. A thick overlap, a lump where an old repair was patched on, or a wrinkle in a single-ply sheet can hold a small pool. The fall on a flat roof is meant to carry water to an outlet, and our page on the minimum fall for a flat roof sets out the figures.
Trees add to it, and so do gutters that overflow onto the roof edge in autumn. Leaves and seeds settle in the low corners, rot into silt and slow the drainage, so a roof beneath a tree can start ponding even when its fall was fine.
How long is too long?
There is no single limit, and guides use different language for it. As a practical rule, a thin film that dries out soon after the rain stops is part of life on many flat roofs. Pools that are deep, that persist for several days, or that return in the same place every time deserve attention.
The colour of the surface tells you more than the water does. Dark staining, a green film of algae and a tide mark of silt show where water has been sitting for a long time. Soft ground underfoot near a pool suggests the deck below may be damp. A small roof such as a dormer cheek has little room to shed water, so even a shallow dip matters there. Our page on a dormer flat roof covers the details, and welded sheet roofs are described in PVC single-ply for a house.
Does standing water affect a warranty?
Some manufacturers set limits on how much standing water their single-ply and liquid systems tolerate, and some warranties exclude it. If your roof carries a manufacturer warranty, read its terms. A warranty is separate from a roofer's workmanship guarantee, and it covers the product rather than the pools on it.
Ponding adds weight too. Water is heavy, and a pool across a sagging deck loads it further, which deepens the sag. Approved Document A refers to overstressed members or deflection leading to ponding as a reason to strengthen or replace roof members. Heavy pooling on a deck that has visibly dipped is a matter for a survey, not a wait-and-see.
The fixes, from simple to major
Start with the simple causes. Clear leaves, silt and moss from the surface, and free any outlet or gutter that is blocked. If the pool is shallow and the roof is otherwise sound, that may be all it needs.
If the roof is shaped wrongly, the repair is bigger. Options include adding tapered insulation boards over the existing deck to create a fall, fitting a new or lower outlet, or cutting a scupper, a small opening through a parapet wall that lets water leave. If the deck has sagged or the boards are rotten, the deck needs repair or renewal before any new covering goes on. A new waterproof layer over a dip keeps the dip.
Do not be tempted by a quick coat of paint or sealant over a pool. It may hold for a season, but the water returns to the same spot.
Sometimes the cure is outside the roof. A downpipe that is partly blocked, a gutter that is full of silt or an outlet that has been covered by a later repair will all hold water back. Checking the whole path from the roof surface to the drain is the least costly first step, and it settles many cases before any building work is considered. A pool that drains again once the outlet is clear was never a fault in the deck.
Why some coverings tolerate it better
Jointless coverings such as GRP and fully glued single-ply sheets have fewer laps for water to find, so they cope with shallow standing water better than a multi-layer felt roof with many seams. Rubber sheet is described in our EPDM rubber roofing guide.
None of them is a licence to ignore a pool. Even a material that tolerates water will age faster under it, and the details around the edge of the pool are where problems start. A roof with a planted layer has its own drainage needs, and our page on a sedum green roof explains them.
What to do now
Photograph the pool a day after rain and again two days later, from a safe spot such as a window. Note where it sits and whether the ceiling below is stained. Please keep off the roof if the deck might be weak.
Send us the pictures, or ask for a survey. We check the fall, the outlets and the deck, and the written quote follows with photographs of what we found. We suggest the lightest fix that clears the cause. Drainage design for a whole roof is a bigger subject, covered in our guide to flat roof drainage, and the work we carry out is listed on the flat roofing page.