Water pools on my roof after rain. Is that a problem?

A flat roofFlat 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. isn't completely flat: it has falls that lead water to the drainsFloor drainThe outlet in a floor where water drains away on roofs, balconies and in bathrooms, with a grate on top and usually a trap below. and guttersGutter and downpipeThe channel and pipe that collect rainwater from a roof and carry it down to the ground or the drains.. When water sits in hollows for days after rain, the waterproofingWaterproofingThe work and materials that stop water getting through on flat roofs, balconies, bathrooms, basements, planters and tanks. there stays wet all the time, dirt collects and the coating wears faster. On old white roofs, the spots where water used to sit often show up as grey, worn patches.

On a roof crowded with solar water heater frames, the surface had hollows where water collected. Once it was cleaned and the damage was patched, new material was laid to fill the hollows and level the surface, and only then did the primerPrimerThe thin first coat that goes on before the main material. It binds dust, seals the pores and helps paint or waterproofing stick firmly. and waterproofing go on. On another roof with a new 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., the surface was levelled with a straightedge while still wet, precisely so no hollows would be left.

The drain itself is one of the most sensitive spots, because all the water ends up there. In this job the material around the old drains had rottedWood rotDecay of wood caused by fungi when it stays damp for a long time. Rotten wood goes soft, gives when pressed or crumbles, and can't be saved with paint. and rusted, and water was getting into the slab. At the outlet through the foot of the parapetParapetThe low wall around the edge of a flat roof or balcony. The joint between it and the floor is one of the most common places for leaks., the area was cleaned and the white waterproofingLiquid waterproofing membraneA waterproofing product applied as a liquid by roller, brush or spray. Once dry it forms one continuous, flexible membrane with no seams. was carried inside the opening, reinforced with fabricReinforcement meshA mesh or fabric, usually glass fibre or polyester, set into render, a coating or liquid waterproofing to hold it together and stop cracks from opening. so it wouldn't 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. at the corners.

One thing you can do yourself is keep drains and gutters clear of leaves and soil, especially before winter. But if water sits because the surface has sunk or doesn't have proper falls, cleaning won't fix it, and someone needs to look at what exactly is wrong.