Why Is My Flat Roof Sagging?

Under the waterproof covering of a flat roof is a deck, usually plywood or boards, nailed to joists. The joists run from wall to wall and carry everything: the covering, insulation, the deck, rain, snow and anybody who walks on it. If they cannot carry that load, they bend, and the roof visibly dips between the walls.

Three things decide whether a joist copes. Its size, in particular its depth, controls how stiff it is. The spacing between joists sets how much deck and load each one carries. And its span, the distance it crosses without support, matters most of all, because the bending grows quickly as the span gets longer. Many garage and extension roofs were built with timbers that were adequate for a light covering but are marginal for a heavier modern build-up with insulation and a new deck.

A sag that has been there from the start often points to undersized timber. A sag that has appeared over time points to something changing: damp, rot, a new load or the loss of a support. Telling these apart is the first job of any survey, because the repair differs.

Timber that stays wet loses strength. If a leak has run for months or years, the deck and the tops of the joists soak up water, soften and eventually rot. A rotten joist end where it sits in a wall, or a deck that crumbles when probed, no longer carries its share, and the load shifts to the sound timbers around it.

This sets off a cycle. As the roof dips, water collects in the low spot. Standing water is heavy: a puddle a few centimetres deep weighs a surprising amount across a whole roof. That extra load bends the roof further, which deepens the puddle. The water also finds the weakest point in the covering, so the leak worsens, the timber stays wet, and the rot spreads. What began as a small dip becomes a cycle that only structural repair can break.

You may notice a spongy feel underfoot, a ceiling that bulges or has gone brown, doors that start to stick, or cracks in the plaster at the wall edge. Our pages on flat roof ponding water and finding a flat roof leak help with the early signs.

The right repair depends on what the survey finds. Where the deck alone is soft, it can be cut out and replaced with new boards, fixed down to sound joists. Where joists have rotted at the ends, a roofer can cut away the bad timber and bolt a new section alongside, called sistering, or fit a new joist and bearing. If the joists are simply too small, the options are to add new ones between the existing ones, replace them with deeper timbers, or add support such as a steel beam or a new wall plate.

Joists that are sound but dipping can sometimes be levelled by fixing tapered firrings on top, which also builds the fall back into the roof. That corrects the shape, but it does not add strength, so it only suits a roof whose timbers are fit for the load.

Replacing or adding structural timbers is building work, and an engineer may need to size them. Whether the job needs approval is a question for your building control body. The Planning Portal also tells owners to consult an engineer or surveyor before the weight of a roof covering is altered. See our guide to Building Regulations for flat roofs for more.

A sagging roof is rarely a job for a patch. A new covering laid over a bowed, wet deck will fail early and hide the rot. A decision on how far to go usually follows the same logic: where the joists are healthy and just the waterproofing has gone, renew the waterproofing; where the structure has failed, it has to be put right first, and the covering follows. The page on repair or replace a flat roof sets out how this decision is made.

Ask the roofer to show you the timbers, with photos, before and after opening up. Ask what size and spacing they propose for the new joists, whether an engineer has been asked to check, and how the new fall will be set. If a roofer offers to lay a new covering straight over without looking underneath, be wary.

Price depends on the extent of the damage, which cannot be seen until the covering is lifted, so a written total follows a survey and any opening up we can do safely. Until then, keep people off the roof, and clear standing water only if it can be done safely from a ladder. Our flat roofing page lists the wider options.