With LABC approval, it is suitable for all pitches but, unlike square-edged boards, it will not provide an air leakage path. Permo therm is installed over rafters and not only provides added height within the roof space but removes the need for separate underlay installation. Boards can be laid either from left to right or right to left with no ventilation of the batten/counter-batten cavity needed other than for impervious or tight-fitting roof coverings.