Gutter Capacity Design

Spouting should be installed with the back lower than the fascia board or cladding to allow for draining of overflow water through the gap between the back of the gutter and the fascia.

Residential spouting installed 5 mm below the top of the fascia at the high point, with a 3 mm gap between the fascia and back spouting, may be considered to have sufficient overflow. Larger spouting will need greater gaps.

This gap is only totally effective if the spouting is correctly maintained and the gap is free of debris.  A designed outlet is preferred.