Where flat roofs leak
A flat roof covering can be sound across the main area and still leak, because the weak points are the places where it meets walls, chimneys, rooflights, pipes and parapets. At these junctions the waterproof layer has to turn up the wall as an upstand, and be sealed at the top by a flashing, a coping or a chase in the masonry.
What an upstand should do
An upstand carries the waterproofing up the wall so that standing water or splashing from the roof cannot get behind it. The height needs to be enough to protect against ponding and splashing, and the top edge must be protected from rain running down the wall. Poorly built upstands, or ones that have been squeezed in below the ideal height, are a frequent cause of leaks.
Typical faults
- Felt, EPDM or liquid membrane lifting or splitting at the wall
- Failed or missing flashings above the upstand
- Mortar fillets and render that have cracked and let water behind
- Upstands too low, especially at door thresholds
- Rotten timber decking where water has been getting in
- Blocked outlets leading to ponding against the junction
How we repair
We start by checking whether the problem is local or whether the whole covering is failing. Local repairs may involve cutting back the old upstand, renewing the membrane, and refixing with a new lead or purpose-made flashing chased into the wall. If the deck beneath is rotten, it must be replaced or the repair will not hold. Where the roof covering is near the end of its life, it is usually more economical to recover the roof than to keep patching.
Single-storey extensions and rear returns
Flat roofs on rear kitchens, outhouses and extensions are common on terraces in Newcastle, and the junction with the main house wall is often the problem point. Roofs next to a parapet or party wall add further complexity, see parapet wall rebuilding and lead flashing replacement. For wider wet problems, see rainwater penetration remedial works.
Re-covering a flat roof can bring Building Regulations into play, particularly for insulation. See GOV.UK or the Planning Portal.
Frequently asked questions
Why does my flat roof leak at the wall?
The upstand, flashing or sealant at the junction has probably failed, or the upstand was never high enough. Water then runs behind the covering and into the building.
Is it better to repair or replace a flat roof?
If damage is local and the roof is otherwise sound, a repair works. When the covering is old, widely cracked or the deck is wet, replacement is generally better value than repeated repairs.
