A box gutter is a rectangular, trough-shaped drainage channel built into the roof structure rather than hung from the eaves. It collects rainwater from one or more roof slopes and directs it towards downpipes. Its enclosed, box-like cross-section distinguishes it from open half-round or ogee gutters.
Design and construction
Box gutters are typically formed from timber framing lined with a waterproof material. Traditional linings include lead sheet, zinc, or copper, while modern installations often use fibreglass, EPDM rubber, or single-ply membrane. The gutter is built with a slight fall — usually a minimum of 1:80 — to ensure water flows towards the outlet.
The cross-section is rectangular or trapezoidal, with vertical sides and a flat base. Dimensions are calculated based on the roof area being drained and the expected rainfall intensity. Building codes generally specify minimum widths, depths, and overflow provisions. A well-designed box gutter includes an overflow outlet or weir set below the surrounding roof level to prevent water from backing up into the building if the primary outlet becomes blocked.
Application
Box gutters are commonly found where two roof slopes meet at a valley, between terraced buildings sharing a party wall, or behind parapets on flat and low-pitched roofs. They are a standard feature in Victorian and Edwardian terraced housing, commercial buildings, and industrial sheds.
Because box gutters are concealed within the roof structure, any leak can go undetected and cause significant damage to timber framing, insulation, and ceilings. Regular inspection and maintenance are essential. Debris such as leaves and moss should be cleared at least twice a year, and the waterproof lining should be checked for cracks, splits, or lifted seams.
Related terms
- Valley gutter
- Parapet
- Downpipe
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