Commercial Roof Drainage Problems

A flat roof is designed to drain gently towards its outlets. For flat and low-pitched roofs, BS 6229:2025 sets a minimum finished fall of 1:80 over general areas and inside gutters, and IKO's guidance, quoting the 2018 edition, advises designing for around 1:40 so that the finished roof still meets the minimum. Over the years the deck can settle, insulation can compress, and new plant can be added, and the fall quietly disappears.

Blocked outlets cause the other half of the problem. Leaves, windblown litter, moss and the grit that comes off ageing felt or mineral surfaces all collect around the grating. On a big roof there may be only a few outlets, so one blockage affects a wide area.

Standing water is not always an emergency, but it is always worth recording. Water that disappears within a day or two of rain points to minor undulation. A pond that stays and leaves a ring of dirt or algae suggests a real low spot. Ponds add load, speed up the ageing of seams and add weight to a deck that was designed for less. We photograph them during the survey and mark them on a sketch of the roof.

BS EN 12056-3 sets out how roof drainage inside buildings is laid out and calculated. The principle is simple: the outlets and pipes must carry the water that falls on the area they serve in heavy rain, with allowance for the rainfall the designer chooses. A roof that was drained correctly when it was built can become under-drained later, if the roof area grows, if a plant deck sheds water onto it, or if outlets are covered by new insulation or ballast.

Large spans have their own difficulty. A long roof deflects under load, and the lowest point may move away from the outlet. Sheet laps then act as dams, and water sits behind them. Parapets complicate things further, because a parapet with no scupper or overflow turns the whole roof into a tank.

Fixes range from adding a rainwater outlet at the low point and re-laying tapered insulation to correct falls, through to replacing the covering with a drainage design that suits the structure. Which one applies depends on the survey, and we describe the options with photos before giving a single total.

On very large roofs, designers sometimes use a siphonic system. Instead of a conventional outlet, a baffled inlet lets the pipe run full, which draws water away at speed through small-bore pipes that run horizontally above the ceiling. The advantages are fewer downpipes and no need for drains falling to the ground. The drawbacks are that the system works only if it is designed and installed exactly as calculated, and a partly blocked inlet cuts its capacity sharply.

If you have a siphonic roof, ask for the original design drawings and keep the inlets clear. Any alteration to the roof, such as new plant or extra insulation, changes the calculation, so the designer should be asked first.

Overflows are the safety net for every kind of roof. An overflow outlet, set a little above the main outlet level, lets water escape in a visible place if the main drains block. Without it, water rises until it finds a lap, an upstand or a rooflight, and then it comes through the ceiling. Where a parapet surrounds the roof, an overflow through the wall is not a luxury, and it is often the quickest fault to correct.

You can do the first part yourself from the ground or from a safe access point: look at the downpipes after heavy rain, listen for gurgling, and look for staining where water has overflowed. Do not walk on a roof unless it has proper access and edge protection, since many commercial roofs include fragile rooflights, as covered in our guide to fragile roof safety.

For the roof itself, a tradesperson should clear and photograph each outlet, measure and record the depth of any ponds, trace any internal pipes that overflow in storms and check the box gutters. Our notes on box gutter lining show what happens when a gutter fails.

If the cause is a simple blockage, the repair is cheap. If it is the fall, the price depends on the roof area, the build-up and the access, and our written price is one total once we have surveyed. Ask every contractor whether VAT and scaffold are in their figure. For the wider picture of the work we take on, see our commercial roofing page.