Ponding on a commercial flat roof: is it a problem?

Some water will linger in the low spots of almost any large flat roof for a short while after a shower. That alone is not a fault. What matters is how long it stays, how deep it is and whether it is the same place each time.

We separate three situations at survey. A thin film that clears within a day or two is usually surface tension and a slightly uneven finish. A shallow pool that is still there days later is a sign that the falls or the outlets are not doing their job. A deep, widening pool over one area suggests the deck beneath has dipped, and that deserves attention quickly because water is heavy and the dip tends to deepen it.

Photograph the roof a day or two after rain, from the same position each time, and keep the images. A record over several weeks tells us more than a single visit, and it separates a recurring fault from a one-off blockage.

Despite the name, a flat roof is built with a slight slope, known as a fall, which carries rain towards the outlets. The guidance based on BS 6229 sets this out. IKO, a membrane manufacturer, gives 1:80 (about 0.72 degrees) as the minimum recommended finished fall, and notes that the code of practice wants the roof designed for 1:40 (about 1.44 degrees) so that building tolerances, an uneven deck and some deflection do not eat into it. Rockwool, citing BS 6229:2018, gives the same arithmetic: design at 1:40 to achieve finished falls of 1:80.

In plain terms, you design for a steeper fall than you need, because the finished roof ends up flatter than the drawing. If a roof was built with no allowance, ponding is the likely result. Tapered insulation, or a screed laid to falls, is how that allowance is made on a new or re-laid roof, and it is a major reason a good flat roof specification looks at falls first.

On a big roof there is more to go wrong, and several causes can stack up.

• Falls that were never enough. The roof was built flat, or the fall was lost when a covering was laid over an older layer.
• Deck deflection. Timber boards, metal decking or purlins can sag between supports, particularly if they have been wet or overloaded. Approved Document A itself names overstressed members and unacceptable deflection leading to ponding as grounds for strengthening.
• Blocked or too few outlets. Leaves, moss, debris and old membrane scraps stop water leaving. A large roof may have outlets that are too far apart to clear a heavy downpour.
• Added loads. Plant, solar fixings and stored materials can press a deck down locally.
• Bulges and laps. Layers of felt or overlaid coverings can leave ridges that trap water behind them.

The cause decides the cure, so the first job is finding which of these applies. A roof with a sagging deck needs a different answer from one with a blocked sump.

Water sitting on a membrane keeps its surface wet and loaded. Dirt and algae settle where the water dries off, and seams and laps spend longer submerged. Water that freezes and thaws on the surface adds to that wear. Different systems cope differently. A single-ply membrane with sound welded seams tolerates standing water better than a felt roof with lapped joints, though the manufacturer's own guidance should always be checked. Metal roofs can corrode at laps and fixings, and a deck that keeps dipping is a structural concern as much as a waterproofing one.

There is also weight. Water is heavy, and a large pond on a roof that has already dipped adds to the load that caused the dip. This is why structural advice can be part of the fix. The Planning Portal points owners to a structural engineer or surveyor when loading changes, and a moving deck deserves the same caution. Our guide to commercial roof systems compares the coverings and how each handles water.

We survey a ponding roof from above and below. On the roof we check outlets, upstands, laps and the profile of the surface. Underneath, we look at the deck, the purlins and any signs of movement or damp. We note where the water sits, how deep it is and how fast it clears.

The remedies range from simple to substantial. Clearing outlets and adding a sump or an extra outlet costs far less than lifting a membrane. Where falls are the problem, tapered insulation laid under a new covering can create the slope the roof lacked. Where the deck has failed, the repair is structural, with the covering renewed on top of it. A roof that is otherwise sound may only need a local correction, and a roof that is worn out may be better replaced. Our guides to church hall roofs and barn roof sheets show how large and unusual roofs add their own drainage points.

Send photographs and the roof's approximate size. If the survey finds ponding that needs work, the written quote covers the whole job and our workmanship is guaranteed for ten years. Our commercial roofing page covers everything else we do on larger buildings.