Ponding

The designed slope built into a nominally flat roof so that rainwater drains towards gutters or outlets. Falls may be formed by the structure, screeds, firring pieces or tapered insulation, and design gradients are normally set steeper than the minimum finished fall to allow for construction tolerances and deflection.

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.

Rigid insulation boards manufactured or cut to a wedge profile and laid to a designed scheme to create drainage falls on a flat roof, avoiding the need for screeds or firrings. Schemes combine tapered and flat boards to achieve both the required fall and the target U-value.

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.