Why does my flat roof hold water, and does it matter?

Flat roofs are not flat: they are built with a designed fall so that water runs off to an outlet. When you see a puddle that is still there two days after the rain, the fall has gone. Something has moved — the firrings have compressed, the deck has sagged between joists, or the joists themselves have deflected, usually because they have been getting wet.

Safety

What you'll need

• Binoculars or a view from an upstairs window
• Camera on a pole (optional)
• Torch for the ceiling void below (optional)

Step by step

1. Look at the roof from above rather than from on it.An upstairs window, a neighbour's window, or a phone camera taped to a pole. Photograph it a couple of days after heavy rain, which is when ponding shows.
2. Note where the water stands.A single puddle in the middle usually means the deck has sagged between joists. A puddle along one edge means the whole slope has gone or the outlet is on the wrong side. Water standing against the abutment wall is the worst position of all.
3. Check the outlet.Flat roofs drain to an outlet or a gutter, and those block with leaves, moss and detritus more readily than a pitched roof's gutters because nothing washes them. A blocked outlet creates ponding all on its own and is the cheapest thing to fix.
4. Identify the covering.Three-layer built-up felt, single-ply membrane, EPDM rubber, mastic asphalt, or lead. Each has different failure modes and different repair options, and each has a very different expected life.
5. Look at the covering's condition.Blisters, splits at the folds, seams lifting, crazing and loss of the mineral surface. In a ponded area those failures happen faster because the water is permanently loading the joints.
6. Look at the upstand and the flashing.Where the roof meets a wall there should be an upstand of adequate height with a flashing or a cover over the top of it. A low upstand with water ponding against it will overtop in heavy rain regardless of the covering.
7. Look underneath.From the room below or the void: staining on the ceiling, sagging, a musty smell, or soft plasterboard. That tells you whether water has already been getting into the structure.
8. Decide between patching and recovering.An isolated split in a covering with life left in it is a patch. Ponding caused by structural deflection is not repairable by recovering alone — the falls have to be reinstated, which usually means stripping back and adding firrings or replacing the deck.
9. Get the ventilation and insulation strategy right if it is being rebuilt.A rebuilt flat roof is either a warm deck, with the insulation above the deck and a vapour control layer below it, or a cold deck with a ventilated void. Warm deck is the modern default because cold deck flat roofs are notoriously prone to hidden condensation.

Tips

• Some ponding on an older flat roof is common and not always urgent. What matters is whether the covering is intact, whether the structure is sound, and whether it is getting worse — photograph it annually.
• Clearing the outlet and keeping the roof free of leaves and moss is the single cheapest thing you can do for a flat roof's life, and it needs doing at least twice a year.
• If the flat roof is over a habitable room, insist that whoever rebuilds it explains the vapour control strategy. Hidden condensation in a cold deck flat roof rots the joists silently.

Common mistakes

• Applying a liquid coating over a ponding roof — It bridges the current splits and does nothing about the fall or the deflected structure. Water still stands, the coating fails at the same points within a few years, and the timber underneath keeps getting wet.
• Walking on it to check — Flat roofs over garages, extensions and bays are often lightly built, and one that has held water may have decayed timber under the covering. This is how people fall through roofs.

Questions people ask