A box gutter is a rain gutter on a roof usually rectangular in shape, that may be concealed behind a parapet, an eave or in a roof valley. Box gutters are designed to suit large water carrying capacity requirements, usually on commercial buildings.
In effect, a box gutter operates like an open drain generally placed between parallel surfaces, as in a valley between parallel roofs or at the junction of a roof and a parapet wall.
Designing for a box gutter requires adequate support for the gutter be provided, both at the sides and below, to provide for the anticipated weight of the water collected. As with all gutters, adequate fall must be provided as well as a rain-head or other large capacity drainage system.