Reliable Solutions for Damaged and Leaking Flat Roofs
Flat roofs can develop problems through weather exposure, standing water, movement, impact damage and the gradual deterioration of joints, edges and roof coverings.
Visible signs can include cracks, splits, bubbles, lifting materials, damaged trims and water entering the property. The source of a leak may not always be directly above the point where dampness appears internally, as water can travel beneath the covering before becoming visible.
Where the damage is localised, and the surrounding roof remains serviceable, a repair may provide an effective solution. Where deterioration is widespread, or the roof deck has been affected, partial or complete replacement may offer better long-term value.
Which Flat Roof System Suits Your Roof?
In short: felt, rubber (EPDM) and fibreglass (GRP) can all give a sound, watertight flat roof. The useful question is which one suits the shape of your roof, the details on it, how it is used and your budget. Whatever goes on top, the deck, the falls and the finish at edges and upstands usually decide how well the roof performs.
This table starts from the roof rather than the material.
| If your roof… | Often worth considering | Points to weigh up |
|---|---|---|
| is a plain rectangle, such as a single garage or simple rear extension | EPDM, as one sheet can often cover the whole area | Felt and GRP suit it too, so budget and appearance may decide |
| has several rooflights, soil pipes, flues or tight internal corners | GRP, laminated around each detail as one continuous layer | Felt and EPDM can be detailed well, but every penetration needs its own carefully formed piece |
| sits close to timber fascias, cladding or overhanging eaves | EPDM, GRP, or a cold-applied or self-adhesive felt | Torch-on felt is laid with a naked flame, so it is not always appropriate beside combustible materials |
| covers an outbuilding or store where cost matters most | A well-specified modern felt system | Ask how many layers are included and what the top sheet is, as felt specifications vary widely |
| can be seen from upstairs windows or the garden | GRP for a smooth coloured finish, or felt with a mineral surface | EPDM is usually black, which some owners prefer on roofs that are out of sight |
| is large, or on a commercial or rental property | Multi-layer felt or seamed EPDM | Drainage, safe maintenance access and working around occupants matter more on bigger roofs |
Many roofs fit more than one row, so we talk the options through after inspecting. Each covering is explained on our GRP fibreglass roofing, EPDM rubber roofing and felt roof repair and replacement pages.
Warm Roof or Cold Roof Construction?
If there is living space below the roof, for example a kitchen extension, a bedroom over a garage or a dormer, the way it is insulated matters as much as the waterproof layer. There are three common build-ups.
Cold roof
Insulation sits between the joists, below the deck, with a ventilated air gap above it. Many older extensions were built this way. If the edge vents are blocked or were never fitted, warm, moist air from the room can condense on the cold underside of the boards, leading to damp patches, mould and eventually rot.
Warm roof
The insulation goes on top of the deck: a vapour control layer first, then rigid insulation boards, then the waterproof covering (for GRP, a new board and then the laminate). The timbers stay on the warm side, so condensation risk is much lower and no ventilation is needed. When the roof above a lived-in room is renewed, this is usually the more dependable choice.
Inverted roof
Insulation sits above the waterproofing and is held down by ballast or paving. It is mainly used on commercial roofs and roof terraces.
A warm roof finishes noticeably higher than the one it replaces. At walls, the flashing may need raising so the upstand still clears the finished surface by the 150 mm or so that industry guidance generally calls for; at open edges, deeper fascias or timber kerbs may be needed. If old quilt stays between the joists, insulation manufacturers can provide a condensation risk calculation for the combined build-up. Building Regulations treat replacing over 25% of the covering on a roof above living space as thermal element renovation, so Building Control normally has to be told; our guide to Building Regulations for roof replacement sets out the rules.
Falls, Drainage and Standing Water
A flat roof is never truly flat. It is built with a slight slope, known as the fall, that carries rain towards a gutter or outlet. BS 6229 recommends designing for a fall of around 1 in 40, so the finished roof still achieves at least 1 in 80 once deflection and building tolerances are allowed for.
How falls are created
- Firrings: tapered timber strips fixed along the tops of level joists, so the deck slopes.
- Sloping joists: joists set at an angle when the roof is first built.
- Tapered insulation: on a warm roof, boards cut to a fall can form the slope, or correct a poor one, without altering the timbers.
Why water ponds
Water still lying on a roof a day or two after rain usually comes down to falls that were too shallow from the start, a deck that has sagged between joists, an outlet or gutter choked with leaves and moss, or an edge trim holding water back like a dam. Standing water adds weight, leaves silt as it evaporates and keeps any defect beneath it permanently under water, so a small split under a puddle lets in far more than one on a well-drained area.
Getting water off the roof
Most domestic flat roofs drain over a drip trim into an eaves gutter. Roofs enclosed by parapet walls drain through internal outlets, or through chutes in the wall into a hopper; on those it is good practice to add a second outlet or an overflow, so one blockage cannot back water up over the upstands and into the building. Re-covering a roof that ponds without correcting the falls usually just moves the problem on, so we look at the falls as part of any replacement. Our guide on how to stop water pooling on flat roofs covers what you can check yourself.
