Why is my flat roof leaking?

Ponding water

A flat roof is never truly flat; it needs a gentle fall so that rain drains away. When the fall is too shallow, the deck sags or an outlet is partly blocked, water collects in pools. Standing water ages the covering faster, adds weight to the deck and searches out any weak joint. Pools that remain for a couple of days after rain are a warning sign.

Splits, cracks and blisters

Coverings move as they warm in the sun and cool at night. Over time, older felt in particular can crack, craze or blister as trapped moisture expands. A split can be very small and still let a surprising amount of water through. Blisters are not always leaking, but they show the covering is no longer fully bonded.

Upstands and wall junctions

Where a flat roof meets the wall of the house, the covering should turn up the wall and be protected by flashing above. If the upstand is too low, the flashing has slipped or the mortar holding it has crumbled, water runs behind the covering. This is one of the most common causes of leaks on rear extensions.

Edge trims and drip details

Edge trims hold the covering down at the perimeter and throw water into the gutter. When trims come loose or the covering pulls away from them, wind can get underneath and rain can track back onto the deck. Leaks near the edge of the ceiling below often start here.

Outlets, gutters and downpipes

Leaves, moss and debris block outlets and gutters, so water backs up over the edge of the covering. Internal outlets on larger roofs are particularly vulnerable. Clearing them is simple, but if water has been backing up for a long time the deck around the outlet may need attention too.

Rooflights, vents and pipes

Every opening in a flat roof is a joint that must be sealed. Rooflight upstands, soil pipes and vents all need properly formed collars or flashings. Sealant applied as a quick fix tends to crack and let water back in.

Condensation can look like a leak

On a cold flat roof without enough ventilation or insulation, warm air from the room can condense under the deck and drip back down. It typically appears in cold weather rather than after rain, and is often spread across the ceiling instead of in one spot. The fix is about insulation and ventilation, not the covering.

What to do now

Until the roof is looked at:

• catch drips and move belongings away from the area
• keep water away from light fittings and switch off affected circuits
• photograph the ceiling and, safely from a window, the roof surface
• note whether it leaks in all rain or only in heavy or windy weather
• clear a gutter only if you can reach it safely from the ground or a stable ladder

Repair or replace?

One defect on an otherwise sound roof can usually be repaired. If there are several leaks, widespread cracking or a soft, sagging deck, replacement normally gives a better long-term result. For pitched roof leaks, our general roof leak guide covers tiles, flashing, ridges and valleys.

Rayner’s Roofing repairs and replaces flat roofs on homes, garages, extensions and dormers across Staines, Surrey and West London.

Frequently asked questions