Design Guide for Rural Northern Ireland
Traditional Qualities: Roofs
Although it is not a large area, Northern Ireland has considerable variations in climate. Certain areas are much more exposed than others and rainfall in some parts is more than twice that in sheltered areas. For generations Northern Ireland builders have used the pitched roof. Principally, it depends on shedding rainwater quickly. Overlapping units of thin stone (slate) replaced the layers of straw or reeds used in thatching, but the pitch remained the same.
In sheltered areas, traditional materials could be used as low as 30 degrees to the horizontal; exposed conditions could raise this to over 40 degrees; but the range 32-38 degrees covers most circumstances. Galvanised corrugated steel sheet ('corrugated iron') was sometimes used for cheapness, but generally on lean-to sheds (stores) and outbuildings rather than dwelling houses. This thin roofing material could be laid at a lower pitch - and often was.
Wind was another prevalent factor in roof design. The most exposed parts were the edges at the gables (the verges). Thatched roofs are seen with cement or lime render or heavy stones set on the verges to hold them down; sometimes ropes tied down to walls were used to hold the roofing in place.
Latterly, cement render or paving stones were used along the edges of slated or corrugated steel roofs, giving the familiar sight of walls which continue above the roof.