Blue roof

A roof deliberately covered with growing medium and vegetation over a waterproofing system, usually incorporating root barrier, drainage and filter layers. Extensive green roofs use shallow substrates with low-maintenance planting such as sedum, while intensive green roofs support deeper, landscaped schemes. They contribute to rainwater attenuation, biodiversity and reduced urban heat island effect.

The drainage fitting through which water leaves a flat roof or gutter into the downpipe system, incorporating a clamped or welded connection to the waterproofing and usually a leaf guard or grating. Outlets are positioned at the low points of the falls, with secondary overflow provision to guard against blockage.

Rainwater that remains standing on a flat roof after the rest of the surface has drained, typically caused by inadequate falls, structural deflection or blocked outlets. Persistent ponding adds load, accelerates deterioration of some coverings and can invalidate system guarantees.

A roof with a very low pitch, in UK practice generally taken as no more than 10 degrees to the horizontal. Flat roofs rely on a continuous waterproof covering rather than overlapping units, and must be laid to falls so that rainwater drains to outlets.