Sagging Flat Roof — what's actually going on

Water pooling on a flat extension or garage roof rather than clearing.

Common causes

Soft or delaminated deck under the covering
Firrings rotted where rainwater has tracked in
Outlet set above the lowest point of the fall
Original build laid without proper falls at all

How we fix it

Lift a section to inspect the deck and insulation honestly
Re-board and re-fir to restore the fall to the outlet
Rebuild as a warm-roof EPDM or GRP system to current regs
Integrate gutter aprons so run-off clears the wall

Sagging Flat Roof by area

Locally specific notes for sagging flat roof across the towns and neighbourhoods we cover.

Sagging Flat Roof in HamptonTW12 Sagging Flat Roof in RichmondTW9

Our guides on this

Sagging or dipping roof: what it means

A gentle, even sag in an old roof is usually historic timber settlement and is not dangerous. A new dip, a sharp local hollow, or a sag with cracked plaster or sticking doors below it is structural and needs looking at quickly.

Flat roof drainage and falls: why ponding wrecks flat roofs

A flat roof should be built to a designed fall of at least 1:80, and in practice 1:60 to allow for deflection. Water left standing more than 48 hours after rain counts as ponding and will shorten the roof's life through UV concentration, freeze-thaw stress and debris build-up.

Flat roof systems compared and how long each lasts

Felt is cheapest and lasts 10 to 20 years. EPDM rubber is seamless on most domestic roofs and lasts 20 to 25. GRP fibreglass is the toughest underfoot at 20 to 30 years but costs more and dislikes a flexing deck. The deck and the detailing decide the outcome more than the brand.