Slate Roof Headlaps - Hidden Hurdles
Headlap is a technical term that relates to how slates are lapped and fixed on a roof. In Australia, the usual headlap for a roof with a pitch of 40 degrees is 75mm. This means that 15.4 slates per covered metre will be required if 500x300mm slates are used and 18.5 slates per covered metre if 500x250mm slates are used.
Flatter roofs e.g. 30 degrees, require bigger slates and headlaps. The reverse is true for roof pitches greater than 45 degrees.
If lower headlaps are used, fewer slates are required to do the job which either increases roofing margins and/or lowers cost but this may not be good practice. Headlaps are specified by architects for all new buildings. If you are re-roofing an old building, make sure that the quotes for work clearly state the headlap to be used.
Welsh Slate http://www.welshslate.com has a headlap calculator based on a spreadsheet format; contact us if you would like a free copy.