Most flat roofs that fail early do not fail because the material ran out of life. They fail because small problems were left to grow. A blocked outlet, a split at an upstand, a patch of standing water that never quite drains away: each is cheap to deal with early and expensive to ignore. Looking after a flat roof is the simplest way to push a costly re-roof further into the future, and it does not take much.
This guide covers what to check, how often, and the point at which maintenance gives way to resealing or replacing the roof.
Why a flat roof needs more attention than a pitched one
A pitched roof sheds water by gravity. A flat roof does not. Even a well built flat roof only has a slight fall, so water lingers, debris settles, and anything that holds moisture sits directly on the waterproofing. In the British climate, with long wet spells and repeated freeze and thaw, that standing moisture is what slowly works faults open. The result is simple: a flat roof rewards regular attention far more than a pitched roof does, and punishes neglect faster.
How often to check a flat roof
Twice a year is a sensible rhythm for most roofs, once in autumn before the worst of the weather and once in spring to see how it came through the winter. Add a quick look after any storm or heavy snow, since that is when damage tends to happen.
A homeowner can do the routine checks from a safe vantage point: clearing debris, looking for obvious damage, keeping outlets clear. What you should not do is walk an old or fragile roof, or climb up in poor conditions. A professional inspection once a year picks up the subtler signs, the early lap failures and tired flashings, that are easy to miss from the ground and costly once they let water in.
What to look for
When you check the roof, work through the same points each time:
- Standing water. A roof should clear within a day or two of rain. Areas that stay wet, known as ponding, add weight and slowly degrade the covering. Mark where it sits so you can see if it is getting worse.
- The surface. Look for splits, cracks and blisters in the covering. Blisters are trapped air or moisture under the surface and tend to spread.
- Laps, seams and upstands. On felt and single-ply roofs, the joints are the weak points. Check where sheets overlap and where the roof turns up against walls, kerbs and parapets. Lifting or open seams are an early warning.
- Penetrations and details. Vents, soil pipes, rooflights and chimneys are all places where the waterproofing is interrupted and has to be detailed carefully. Cracked or pulled-away flashing around them is a common source of leaks.
- Moss, algae and debris. Growth and leaf litter hold moisture against the surface and block drainage. They also hide what is underneath.
- Inside the building. Damp patches, stains on the ceiling or a musty smell often show up before water visibly tracks through. Treat them as a prompt to look at the roof properly.
The maintenance that adds years
The jobs that make the difference are unglamorous and cheap. Keep the water moving: clear outlets, gutters and downpipes, especially after autumn leaf fall, since blocked drainage is one of the most common reasons a flat roof fails before its time. Sweep off debris with a soft brush rather than a pressure washer, which can drive water into seams and damage the surface. Keep growth in check and treat moss before it spreads. Keep foot traffic to a minimum, and use boards to spread the load if the roof has to be walked for access or to service equipment.
Above all, deal with small failures early. Water that gets under a covering does not stay where it entered. It tracks sideways through the build-up and can affect a large area of the roof before it ever shows inside. A split caught and sealed in a morning is a very different job from one found a year later.
When maintenance is not enough: reseal or replace
Maintenance buys time, and good upkeep is one of the biggest factors in how long a flat roof lasts. It does not, though, make an old covering new. At some point a felt or single-ply roof reaches the stage where it is patched repeatedly and still lets water in, and the question becomes whether to reseal it or strip and replace it.
The deciding factor is what lies underneath. If the deck is solid and dry and only the surface is worn, the roof is often a good candidate to be sealed over rather than torn off. A liquid-applied waterproofing system is applied over the prepared existing covering and cures into one continuous, seamless layer, giving the roof a fresh waterproof surface without the cost, waste and disruption of a full strip-off. Because it is bonded and has no welded joints, it removes the laps and seams that tend to fail in the first place.
If instead the deck is wet, rotten or failing structurally, sealing over it only buries the problem, and a full replacement is the right answer. Working out which case you are in is what a survey establishes, and it is worth getting that view before a tired roof turns into an emergency. Our guide on repairing or replacing a flat roof goes into that decision in more detail.
The best time to consider resealing is before the roof starts leaking, while the covering is worn but the structure is still sound. Waiting until water is coming through usually means more has to be put right first.
HYDRONYLON® is a liquid-applied system fitted over a sound, prepared flat roof by Approved Contractors working across London, with a 10-year product guarantee on the materials issued to the contractor who carries out the work. You can see how the system goes on, step by step, on the system page. If your flat roof is worn but not yet leaking, that is the moment to find out whether it can be sealed rather than replaced.