Wet decking and tired joists
The deck is the layer of boards or sheets that the waterproof covering lies on, and the joists are the timbers beneath it that span from wall to wall. When either takes up water, it loses stiffness. Timber that stays damp softens, and in time it rots. A joist or board that has gone soft deflects under its own weight, and the roof dips.
The water has several routes in. A leak through the covering is the obvious one, but others are quieter:
- A failed flashing at the wall, which feeds water into the edge of the deck.
- A blocked outlet or gutter that spills over the edge back onto the boards.
- Condensation from inside the building, trapped under the deck because there is no vapour control layer or ventilation. BS 5250:2021, the current moisture standard for buildings, covers this.
- Old rot that was covered over by a previous repair.
Rot is slow, so a roof can look sound for years while the boards below deteriorate. A sag that appears gradually is often the first visible sign.
Undersized timber and added weight
Not every sag is a sign of damp. Some flat roofs were never stiff enough. Joists that are too slim for their span, or spaced too widely apart, will deflect even when dry. Thin boards between them can dip between supports. Roofs that have been altered, for example by cutting into a joist for a rooflight or a pipe, can also lose strength where the timber was removed.
Weight is the other side of the sum. A flat roof carries its own covering, any insulation, and whatever sits on it. Standing water is surprisingly heavy, and a pond sitting in a dip keeps adding load to the part that has already moved. Paving, planters, furniture, stored materials and a second layer of covering all add to it. Approved Document A calls a roof loading change significant when it goes up by more than 15%, and the Planning Portal suggests getting a structural engineer or surveyor's view first when new work adds weight.
If you suspect the roof was built too light, a structural engineer can look at the span and size of the timbers. That is a job for someone who calculates, not a roofer's guess.
Ponding, and why the fall matters
A sag and a pond feed each other. Once a dip forms, water settles in it. The extra weight deepens the dip, which holds more water. Standing water also ages coverings sooner and finds any weak seam.
BS 6229:2025 keeps a 1:80 minimum finished fall on ordinary roof areas and in gutters inside the building, and IKO advises designing for 1:40. A sagging deck can take the roof below that. The minimum fall answer explains the two figures, and the lifespan comparison sets the coverings' typical lives side by side.
Do not walk on a roof that is visibly dipping or feels springy, and do not pile anything on it. If the ceiling below shows a bulge, stains or cracks that are growing, or a door is sticking, treat it as more urgent and ask for a structural opinion at once. Tell the person looking at the roof whether it has been patched before, and when, because the history of repairs shows where water has been getting in. A note of the dates of any past leaks is worth having to hand.
What to do before re-covering
Putting a new membrane over a dip hides it, and does not cure it. The sequence that makes sense is below.
- Find the cause: wet boards, a leak path, or undersized timber.
- Strip the covering over the sagging area and open up the deck so the boards and joists can be seen.
- Cut out rotten timber. Where a joist has failed, it is replaced or strengthened, a job that may need a structural check.
- Rebuild the deck dry and flat, with a fall designed in.
- Fit the new covering, and the insulation that the renovation rules call for.
The overlay answer covers why a quick cover-up goes wrong, and the same page sets out how Approved Document L treats a new flat-roof membrane, so insulation comes into the plan.
Start at flat roofing if you want it checked, and send photos of the dip, taken after rain when it shows best, along with any marks on the ceiling below. If the dip is deep or the ceiling is moving, say so when you get in touch, so that it is treated as urgent.
