The short answer
Water ponds when the roof cannot move it to an outlet. The causes are too little fall in the first place, a deck that has sagged since, an outlet that is blocked or set too high, or a covering that has stretched. A shallow film that dries within a day is common, but deep or long-lasting ponds wear the membrane and add weight.
Fall, deflection and settlement
A flat roof is meant to drain. The target in BS 6229:2025 is a finished fall of at least 1:80 for the main roof areas and internal gutters, with the roof designed for 1:40 so that building tolerances still leave the finished slope above that line. Every pond marks a place where the finished fall is lost.
Fall can go missing in the first place. A roof laid dead level, or with joists running the wrong way for the outlets, never had the slope. Cutting corners at the design stage is the commonest origin of a roof that has ponded since it was new.
Fall can also disappear later. Timber joists creep under constant load and deflect between their supports, particularly if they are undersized or have been damaged by damp. A deck made of thin boards may dip between joists. Extra weight, such as plant, stored items or an extra layer of covering, deepens the dip.
Settlement of the walls that carry the roof, or the movement of a new extension relative to the old house, can tilt a roof away from its outlets.
The pattern of ponds helps. A single hollow in the middle of a span suggests deflection, a line of ponds along one edge suggests a wall has dropped, and ponds around an outlet suggest the outlet is set too high.
Blocked or badly placed outlets
Not every pond is a structural problem. An outlet clogged with leaves, silt or moss turns the roof into a tray, and the whole surface can hold water until it is cleared. In autumn and spring, check outlets and gutters, and clear them by hand.
The outlet's position and height matter. If the outlet body stands proud of the surface, water has to rise before it can drain, and a permanent pond forms around it. Replacement of a roof covering can raise the surface by a few millimetres, so outlets that worked on the old roof may need resetting.
There may be too few outlets, or they may be too small, for the area. A large roof with a single small outlet at one corner needs a long fall to reach it, and in heavy rain it can overwhelm the outlet. Adding an outlet or overflow solves some ponding without touching the fall.
Gutters on a flat roof need a fall of their own. Brett Martin suggests 1 mm for every 600 mm of gutter length, and a flat gutter holds water along its run.
Our page on a blocked flat roof outlet covers clearing one safely, and the sequence for diagnosing the cause starts there.
When ponding matters
Small amounts of water on a roof are normal. A film left in a shallow hollow that evaporates within a day or so will do little harm on most coverings. What matters is depth, duration and what the water sits on.
Deeper ponds that remain for several days load the structure. Water weighs a kilogram a litre, so a pond covering a few square metres adds a substantial load to a deck that may already be deflecting, which makes the pond deeper still.
Standing water also ages the covering. Felt, in particular, deteriorates faster beneath permanent water, and algae and moss grow in wet spots, holding more moisture. Seams, laps and edges under water are tested constantly, and any hairline defect there will let water through. Roof coverings have limited lives, with HomeBuilding giving felt 10-15 years and Less giving 10 to 20 years, so ponds shorten a short life further.
In frost, a pond turns to ice and the freeze-thaw cycle can open up laps. Met Office averages for South Farnborough show around 53 days of air frost a year, so winter conditions are a real factor here.
Many manufacturers also make their guarantee conditional on adequate drainage, so persistent ponding can affect a warranty. Ask the supplier of your covering what its terms say.
Putting it right
The cure follows the cause. Blocked outlets are cleared. Outlets set too high are lowered or replaced. Roofs with too few outlets get extra ones or an overflow through the parapet.
Where the fall itself is wrong, the options are structural or additive. Firring pieces can be fixed to the joists to restore a slope, a tapered insulation scheme can be laid over the deck, or the deck can be rebuilt. Our page on tapered insulation on a flat roof explains the second option, and the one that also meets the energy rules.
A sagging deck needs its cause found first. If joists are rotten or undersized they need repair or sistering before any covering goes on, and sometimes the deck is better replaced.
On a survey we measure the falls with a level, note where the ponds sit and photograph the roof after rain where we can. We share the readings, explain the likely cause and say which fix we recommend. If the roof is sound and the pond is shallow, we may advise monitoring only.
The work is priced as one total in writing. The minimum fall for a flat roof page sets out the figures, and the flat roofing hub covers the coverings.
