Why Flat Roofs Fail

Most flat roofsFlat roof (roof terrace)The flat concrete roof on top of a building. It is the most common kind of roof in Greece and the part that takes the most sun and rain. in Attica were built in the '70s, '80s and '90s, with plain cement screedScreedA layer of sand-and-cement mix or lightweight concrete poured over the slab to create falls and a solid surface for the next material. or bituminous feltBitumen membraneWaterproofing that comes in rolls of reinforced bitumen. It is torched onto the roof in sheets that overlap at the seams. as the only waterproofingWaterproofingThe work and materials that stop water getting through on flat roofs, balconies, bathrooms, basements, planters and tanks.. Both materials have a realistic lifespan of 10 to 15 years — not because they're poor quality, but because the roof is the most stressed surface on any building.

That stress comes from three directions at once. First, sunlight (especially UV) gradually breaks down the bonds in bituminous or cementitious materials, making them brittle. Second, sharp temperature swings — from 60°C at midday in July to 5°C on a winter night — cause expansion and contraction that fatigue the material as if you were bending it back and forth continuously. Third, rainwater pooling in areas with insufficient slopeFalls (drainage slope)The gentle slope built into flat roofs, balconies and bathroom floors so water runs to the drain or gutter instead of pooling. has all the time it needs to work its way into every hairline crackCrackA split or gap in render, concrete, a wall, a coating or wood. Fine cracks are usually only on the surface; ones that grow, keep reopening or run diagonally need attention. the first two factors have already created.

The result is a vicious cycle: hairline cracks widen with every rain-dry cycle until they're large enough to let water through the slab. From there, water travels through the concrete's reinforcement, eventually reaching the ceiling of the floor below — often quite far from where the leak actually started, which is what confuses many owners when they go looking for the cause.

A roof's useful life isn't the same for every building. Orientation, shading, the quality of the original construction, and whether the roof sees foot traffic (used as a terrace, say) all significantly affect how quickly the first problems show up.