The roofs are supposed to be of Baltic fir covered with Countess slates laid on inch boards ; the maximum horizontal wind force is taken at 451bs. per foot super acting only on one side of the roof at a time, equivalent to a normal wind pressure of 30lbs. per square foot for a pitch of 30°, and 401bs. per square foot for a pitch of 45°.

The common rafters to be 1 ft. from centre to centre, but in sheltered positions they may be placed 1 ft. apart in the clear.