Green roofing a subterranean house

House on Dartmoor

A number of alternatives were explored for the roof. Single ply membrane was seen as the most appropriate for working on this slightly inaccessible site where maintenance must be kept to a minimum. The roofing subcontractor was convinced that with Rhepanol we would receive good quality technical advice and it would be the most cost effective answer. Square edge softwood boards were laid on large section softwood joists. Rhepanol HG was laid over the boards and topped with the compatible water-retaining layer. Grass turfs were laid on topsoil from the site. At the perimeter, the turf was rolled into a “sausage” that was pressed over the stainless steel angle edging; lengths of the same stainless steel angle were glued to the Rhepanol on longer or steeper slopes to inhibit migration of the turf. Very steep areas were restrained with geojute and small chestnut pegs. The owners decided to prevent access for sheep, but there were unanticipated problems concerning local wildlife such as badgers and field voles that do not respect fences and gates – a badger dug out a bumble bee nest in the earth and voles have built a network of tunnels through the turf. The badger did no real damage because of the protective layer over the Rhepanol, but the voles ate through the fiberglass insect mesh in roof ventilation slots and gained access to parts of the house (Stainless steel mesh has now replaced the fiberglass). Weight was reduced in several areas (where the earth would have been deep) by laying large slabs of polystyrene under the topsoil. Access to the roof is limited to seasonal grass strimming; there are fixing points for safety harness at key positions in the grass.

Peter Hall is an Australian architect who came to England and was happy to stay for family reasons. After some years working in London, he moved to Devon to join Allen Van der Steen to form Van der Steen Hall Architects. Based in Chagford on Dartmoor, the practice works throughout the West Country. With a wide range of project types, the practice has an abiding interest in finding and developing contemporary answers that respect local traditions on sensitive sites and within sensitive and historic environments.