Capillary Action

Capillary action occurs when the surface tension of water causes it to climb into narrow gaps or crevices, defying gravity. In roofing, this most commonly happens at the side laps between metal roof sheets, at the overlaps between flashing pieces, and in the gap between tiles and valley irons. If two metal surfaces are close enough together (generally less than 3mm apart), the surface tension of the water creates a capillary bridge that draws moisture up and through the joint. Roofing professionals combat capillary action through several techniques. Anti-capillary gaps (sometimes called capillary breaks) are small channels or ridges formed into the overlap area of roof sheets that interrupt the capillary path. Sealant tape or butyl strips placed within the overlap also break the capillary connection. On low-pitched roofs where capillary risk is highest, additional measures such as wider overlaps, sealant beads, and specific sheet profiles with built-in anti-capillary features are specified. In Gippsland, capillary action is a common cause of mysterious leaks on metal roofs, particularly at side laps on low-pitched roofs exposed to wind-driven rain. The leak may only occur during certain wind conditions that force water into the lap, making it difficult to reproduce during a dry-weather inspection. Town & Country Roofing is experienced in identifying and resolving capillary-driven leaks, using anti-capillary sealants and correct overlap techniques.