Is Standing Water on a Flat Roof a Problem?

A wet roof or a ponding roof?

Flat roofs are never truly flat. They are built with a slight slope, called the fall, that carries water to an outlet or a gutter at the edge. Because that slope is shallow, a roof will look wet for a while after heavy rain, and small damp patches in shallow dips are nothing to worry about. Ponding is different. It is a defined pool with a clear edge, it sits in the same place after every shower, and it leaves a ring of silt, a tide mark or a green stain when it finally dries.

The usual trade rule of thumb is two dry days. If the water is still there after that, the roof has a low spot. Look from an upstairs window rather than climbing up, and take a photo each day so you can see whether the pool is shrinking or holding its size. A pool that is getting deeper or wider from one season to the next tells you something underneath is moving.

Why water collects on a flat roof

The commonest cause is a deck that has sagged. Older extensions and dormers were often decked in chipboard, which swells and softens once water gets into it, and a soft board dips between the joists. Joists that were undersized for the span, or that have rotted at the wall end, produce the same dip on a larger scale. On some older roofs the falls were never built in at all: the joists were laid level and the roof relied on the covering alone, which never works for long.

The other cause is drainage. A blocked outlet, a gutter full of leaves at the edge trim (the strip that finishes the roof edge), or an edge trim fitted too high can all hold water back on a roof that is otherwise sound. Overlaying a roof more than once can raise the edges until water has nowhere to go. Our guide to how flat roofs drain water explains outlets, gutters and scuppers in more detail.

What standing water does to EPDM, GRP and felt

Water is heavy, and a large pond loads a deck that may already be weak, which makes the dip deeper and lets the pond grow. On torch-on felt, the layers of bitumen bonded on with a gas torch, standing water speeds up the breakdown of the surface, washes the mineral chippings away and lets the felt crack and blister as it wets and dries. On GRP, the hard fibreglass laminate, a pond can soften and wear the topcoat and will find any hairline crack at a board joint. EPDM rubber copes with water sitting on it better than the others, but the debris that collects in a pond holds moisture against every seam and lap, and in winter the freeze and thaw works at any weak point. On all three, a pond collects leaves and grit, moss takes hold, and any small defect under it leaks for far longer after each storm than it would on a free-draining roof.

How ponding is put right

A blocked outlet or a high edge trim is a straightforward repair: clear the outlet, add a second one if the roof needs it, or reset the trim so water can leave. Where the deck has sagged, the honest fix is to correct the fall. On a sound deck that can mean tapered insulation boards that build a slope back in. On a soft or rotten deck it means stripping back, replacing the boards, and fitting tapered firrings (timber strips cut to a slope) before the new covering goes on. A new membrane laid over the same dip will pond again, which is why we do not quote for that on its own. Our page on the minimum fall for a flat roof explains how much slope a roof needs.

When the job becomes a full replacement, it normally brings in an insulation upgrade under Building Regulations, and we handle that side where it applies. We start with a free survey, photograph the pond, the outlet and the deck so you can see the cause for yourself, and give you a fixed written quote. If clearing an outlet will do, we tell you that. You can read more about our domestic work on the flat roofing in Basingstoke page.