Slate-Hung Weatherboarding & Clun Forest Framing: Upland Rain Shielding & Timber Care

The borderlands of Shropshire and Wales feature some of the highest driving-rain indices in Britain. Uplands like the Clun Forest, Long Mynd, and Kerry Ridgeway experience over 1,400 mm of annual rainfall, driven horizontally by gale-force Atlantic westerlies. In these conditions, traditional exposed black-and-white timber framing—celebrated in lowland England—faces rapid decay.

When rain is driven directly against the junction between shrinking oak studs and infill panels, water penetrates deep into the frame joints. To combat this, vernacular builders developed the rainscreen principle centuries before modern building science: wrapping the exposed weather-facing facades (primarily south and west gables) in an impervious, ventilated outer cladding layer of slate or timber boards.

In areas adjacent to Welsh slate quarries, builders utilized Slate Hanging (also termed slate cladding or vertical slating). High-grade metamorphic slate from quarries like Ffestiniog or Corris was split into thin, dense shingles that absorb less than 0.2% water by weight, making them impervious to frost spalling.

Horizontal timber battens (laths) were nailed directly across the exterior face of the timber frame. Slates were punched with nail holes near the head (head-nailing) and hung in overlapping courses with a minimum double-lap (each slate overlapping the course two below it by at least 75 mm). Because the slates hang vertically, gravity holds them tightly against the wall, shedding torrential rain instantly without allowing capillary draw.

In heavily forested dingle valleys where slate was expensive to transport, builders cladded timber frames in horizontal Weatherboarding (feather-edged clapboard). These boards were traditionally riven (split) from straight-grained oak rather than sawn, ensuring continuous wood cells that resisted warping and water ingress.

Each board was sawn or riven with a tapered cross-section—typically 150 to 200 mm wide, tapering from 18 mm at the thick bottom butt to 6 mm at the top feather edge. Installed with a 30 to 40 mm overlap, each board was secured with a single nail placed just above the lap of the board below. This critical detail allowed the boards to shrink and expand across their width during seasonal humidity cycles without splitting.

The key to the longevity of slate-hung and weatherboarded timber frames is the continuous ventilated air gap behind the cladding. Air entering through bottom vents sweeps upward through the cavity, evaporating any microscopic moisture that penetrates the cladding joints before it can contact the structural oak frame.

A catastrophic modern mistake during barn conversions is stuffing fiberglass or rigid foam insulation tightly between the cladding and the timber frame. Trapped condensation causes accelerated wet rot in structural studs. Modern conservators install breathable counter-battens and micro-porous wind-barrier membranes, securing slates with silicon bronze or copper nails that will not corrode in acidic upland air.