The short answer
An overflowing outlet means water is arriving faster than it can leave. The usual causes are leaves, moss and grit sitting over the grating, an outlet too small for the roof it serves, or a gutter or downpipe further down that is blocked or has lost its slope. Clear what you can reach safely, then check what lies downstream, because a clean outlet feeding a choked pipe still backs up.
Water pouring over the lip of a flat roof is a message. It is telling you where the route out is narrowing.
Leaves, moss and grit on the grating
The commonest cause is also the simplest. Leaf fall, seed heads, moss that has drifted from nearby tiles and the loose grit that works off a gravel-topped roof all end up at the lowest point, which is where the outlet sits. Over weeks they knit into a mat that lets only a trickle through.
Can I clear it myself?
If the outlet is within reach of a safe ladder, you can. Wear gloves, lift the debris off the grating by hand, and put it in a bag rather than pushing it down the pipe. Then pour water over the outlet and watch it drain. A good flow means the problem was on the surface. A slow gurgle means something is stuck further in.
Don't walk about on the roof to do this unless the roof was made to be walked on, since many membranes puncture easily. If the only way to the outlet is across the surface, hand the job to a roofer with proper access gear.
Roofs under overhanging trees need clearing more often. Our guide to moss on a flat roof explains where it comes from and what to do about it.
Outlets that were never big enough
Sometimes the outlet is clear and the roof still floods in heavy rain. That points to a drainage design that was marginal from the start, and no amount of sweeping will cure it. A single small outlet serving a large roof, or a gutter with a narrow downpipe, can cope with ordinary showers and then spill when a storm drops a lot of water in a short time.
How do I tell a design fault from a blockage?
Watch what happens. If the outlet runs clear in light rain and only overflows in a downpour, with no debris in sight, the capacity is probably at fault. If it overflows in light rain, something is blocking it.
BS EN 12056-3:2000 is the BSI standard for sizing roof drainage, and BSI still lists it as current. A roofer or designer can check the outlet size against the area it serves. Fixes include adding a second outlet, enlarging the existing one and the pipe below it, or fitting an emergency overflow through the parapet or upstand so that the water has a way out before it reaches the doors.
A re-cover is the best moment to put the drainage right.
Gutters and downpipes downstream
Many an outlet is blamed for a fault that sits lower down. Water leaving the roof has to run along a gutter or pipe to a drain, and any blockage, crushed section or loss of slope along that route sends it back up.
Where does the water stop?
Check the downpipe at its foot, where leaves and silt collect, and look at joints and bends, which snag debris. If the pipe feeds a gutter, look along its length for sagging sections that hold a pool. Manufacturer guidance from Brett Martin is 1mm of fall for every 600mm of gutter, which leaves very little slack. A sagging section will hold a pool even when it is spotless.
A downpipe that runs into a drain can also be at fault if the drain is blocked. Water standing at the base of the pipe after rain points that way, and a drain clearance is a job for a drainage contractor, not a roofer.
If the gutters are failing as well, our gutters, soffits and fascias page covers repairs and replacements. Overflow running down a wall can soak the brickwork, so deal with it promptly.
When to stop clearing and call a roofer
Clearing an outlet is a routine job. Repeated overflow after a proper clear is not, and neither is any sign that water has got inside the building.
What signs mean the problem is bigger?
Look for a ceiling stain below the outlet, damp on the wall where overflow runs, standing water that stays for days after rain, an outlet that has sunk below the surrounding surface, or a cracked or lifted flashing around it. Any of these suggests that water has been getting into the roof, not just spilling over it.
A roofer will check the outlet and its seal to the membrane, the falls around it and the pipework beneath, then explain what is wrong. Some jobs are minor: a new grating, a resealed collar, a cleared pipe. Others involve the deck or the drainage layout. Price turns on access, on how many outlets are involved and on what the inspection turns up beneath the covering.
We look first, show you photographs of the outlet and what is behind it, and give one written price. See the answer on flat roof maintenance for the upkeep routine, or the flat roofing page for the full service.
More on flat roof drainage
Should I put a bigger outlet in my flat roof?
It may be the right fix if the outlet is clear but the roof still overflows in heavy rain. The pipe below has to carry the extra flow, so the whole run needs checking, not just the top. A roofer can set the roof area against the drainage and say whether a larger or a second outlet is needed.
Is overflowing water on a flat roof an emergency?
Not by itself, but it should not be left. Standing or overflowing water finds weak spots and can enter the roof or the wall below. Where a ceiling is already dripping, the roof gets made safe first. Otherwise, clear what you safely can and book a survey ahead of the next stormy week.
The full picture: Flat roofing in Taunton ยท More short answers