undercloak

An undercloak is a rigid substrate laid directly onto roof timbers or battens, serving as a bed for mortar and the tiles or slates that sit above it. It does the physical work of supporting weight and distributing loads while providing a stable, level surface for the mortar joint that bonds the visible roofing material. Without it, tiles would rest directly on timber, which would rot, and mortar would have nothing reliable to grip.

The material is usually fibre cement board, typically 6 to 10 mm thick, or occasionally lime mortar render applied in situ. Fibre cement has become standard because it resists moisture, does not decay, accepts mortar adhesion reliably, and is easy to cut and fit. Older work sometimes used lime and hair, or clay tiles bedded directly in mortar on timber: these are far less durable and now regarded as repair problems rather than best practice.

Undercloak is installed before any visible tile goes down. The boards are cut to width, fitted tightly edge to edge across the rafters or battens, and sometimes nailed or clipped to prevent movement during the mortar bedding process. The mortar (typically hydraulic lime or cement mortar, depending on the roof age and material) is then trowelled onto the top surface of the undercloak, and tiles or slates are bedded into it. The joint thickness is typically 10 to 15 mm.

Where it matters

Undercloak is essential on pitched roofs where tiles or slates are set in mortar rather than hung on battens alone. It is particularly important on clay tile roofs, where the load is heavy and even bedding is critical to prevent cracking and water ingress. It plays little role in modern batten-only installations with dry-fixed tiles, but remains the standard for traditional work and heritage roofing. Failure of undercloak, through delamination, rot, or mortar loss, usually means water finds its way into the rafter space, making undercloak failure a common trigger for full roof remediation.

The term reflects its position in the assembly: it cloaks (covers) the structural members underneath, supporting the visible roof above. Specifications often call it a bedding layer or setting coat substrate, but undercloak remains the trade norm in the UK and Commonwealth building sectors.