The correct head-lap for a roof will differ depending on the roof and the recommendation of the roof slate installer. A 75mm head-lap is the minimum recommended, however this is subject to change due to the pitch of the roof and whether it will be exposed to severe weather (within approx 20 km of the coast).
Slate quarries supply information about head-laps and recommended pitches. The data these quarries recommend comes from centuries of knowledge. Some Australian slate roofers will fix roof slate to a 50mm head-lap, enabling them to save on materials.
Roof slate can be fixed to a head-lap of 10mm, which will keep out 90% of the weather, allowing only 10% to enter the property. The sarking may hide any signs of leaking, but the battens the slates are fixed to will begin to rot, and mould will begin to occur.
Premier Slate recommends fixing to a 100mm head-lap for roof pitches under 45 degrees. For roof pitches over 45 degrees, 75mm is acceptable. A quality slate is the ideal material for any roof 22 degrees and over, and when installed correctly and protected from foot traffic, will give any building a long life span.
A licensed slate roofer should carry out repairs or renewal of slate roofs.