The short answer
Water has nowhere to go. It collects round the outlet, deepens, and starts to work into the weak points: laps, upstands, flashings and any small split. If it rises far enough it can overflow an edge or come in at a wall. The weight and the long wet spell also age the covering. Clearing the outlet is quick; the damage left by ignoring it is not.
Ponding and overflow
A flat roof is built to a slight fall so water runs to an outlet or gutter. When that exit is blocked, water has no route. It backs up across the roof, and the pool gets deeper with every shower.
The first effect is ponding. Water that lingers adds load to the deck, keeps the covering wet and speeds up its ageing, especially on felt. The second effect comes when the pool is deep enough to reach the upstands, the vertical turn-ups at the walls. Flashings and laps there are built to shed running water, not to hold it back, and a pool deeper than the upstand can reach the top edge.
If the roof has a parapet or a raised edge, an overflow may be provided to let water out before it reaches that point. Without one, the water takes the lowest route it can find, which is often into the wall or over the edge onto the ground below.
Wind and cold add to the risk. Frost can lift standing water into a crust that blocks the outlet further, and a heavy downpour on a full roof is a bigger load than the deck was designed to carry for long.
None of this gives a fixed time for failure, but it explains why a blocked outlet is worth clearing the same week you spot it.
What blocks an outlet
Leaves block outlets, especially in autumn and under overhanging trees. Moss and silt wash into the outlet and build up, and moss sheds spores that settle in the corners. Over time, grit and bits of broken covering add to it.
Birds' nests, toys, balls and even builders' debris can end up in the outlet. In some roofs, a scrap of material left from the last repair is the cause.
Not all blockages sit on top. A pipe below the outlet can be furred or silted, a bend can be crushed, or the downpipe may be blocked at the bottom with leaves from the gutter. If water drains slowly even with the grating clear, the fault is lower down.
The outlet itself can be at fault. Outlets are set into the roof and fixed to the covering, and one that has dropped below the surrounding level will pond around it, while one set too high holds water back. Cracks and loose fixings are other possibilities.
BS EN 12056-3:2000 is the standard BSI shows as current for roof drainage layout and calculation, which covers how many outlets a roof needs.
Leaf guards and checking outlets
A leaf guard is a cage or dome over the outlet that stops leaves entering the pipe while letting water through. Where trees overhang a roof, it can save a lot of clearing. It does not remove the need to look, because leaves collect on top of the guard and can seal it as effectively as a blocked pipe.
Choose a guard that fits the outlet and is fixed securely. A loose one can blow off in a gale, and a badly shaped one can trap debris.
For the check itself, look from a safe place at each outlet and the surface around it. After heavy rain, see whether water is draining or sitting. Check gutters and downpipes too, and make sure there is water coming out at the bottom when it rains.
A garden hose gently run into the outlet shows whether the pipe is clear. Slow draining means a blockage below.
Do not climb on the roof to do this unless it is safe. Some coverings are fragile, and a wet flat roof can be slippery. Our answer on walking on a flat roof explains what each covering tolerates, and our maintenance answer sets out a yearly routine.
Clearing it, and checking for damage
Clearing is simple in principle. Scoop out the leaves and silt by hand, wearing gloves, then flush the outlet and downpipe with water to check the flow. A flexible rod can free a stubborn blockage in the pipe.
Once the water is draining, look at what the pooling has done. Check the covering for blisters, soft spots and splits. Check the upstands for flashings that have lifted. Look at the ceiling below for stains. If the water was deep for some time, the deck may be damp.
If you find a problem, a repair is usually small if caught early. A split can be patched, a flashing re-fixed, and an outlet reset. If the roof has ponded for months, the deck may need checking, and our answer on rotten flat roof decking explains what that involves.
When the same outlet keeps blocking, or the roof always ponds, the cause is deeper: a fall that is too shallow, or an outlet in the wrong position. A survey identifies it, and our flat roofing page lays out the fixes. The written price for any work is a single total after the survey.
