Slating a roof is a skilled roofing task that follows defined principles to achieve a weathertight and durable covering. Natural slate in particular requires careful selection, sorting by size, and correct fixing to perform well over decades. This guide covers the key principles applicable to both natural and fibre cement artificial slate.

Gauge Calculation

The gauge is the distance between batten centres and determines how far down the roof slope each slate is exposed. The correct gauge ensures adequate headlap, which is the amount by which each course of slates overlaps the course two rows below. The required headlap varies with the roof pitch and the exposure category of the site; steeper pitches and more sheltered locations allow a smaller headlap, while shallow pitches and exposed coastal locations require a larger headlap to maintain weathertightness.

The gauge formula is: (slate length minus headlap) divided by 2. For a 500mm slate with a headlap of 75mm, the gauge is (500 minus 75) divided by 2, which is 212.5mm. The batten centres are set at this distance across the rafter slope.

Underlay and Battens

As with tile roofing, a breathable membrane underlay is laid horizontally from the eaves upward before the battens are fixed. The underlay provides secondary waterproofing and allows moisture vapour to pass through from the roof space. Treated timber battens of the appropriate size for the rafter spacing are then fixed at the calculated gauge across the underlay.

Laying the Slates

Slates are laid from the eaves upward in staggered courses so that the joints between adjacent slates in one course are covered by the slate above. The side lap, which is the horizontal distance by which adjacent slate edges overlap, must be maintained at a minimum of 75mm to ensure the side joints are always covered by at least one full width of slate above.

Each slate is fixed with two nails driven through pre-drilled nail holes into the batten below. Natural Welsh slate is nailed at the head, two-thirds of the way up from the tail. Centre-nailing, where the nail is placed in the middle of the slate's length, is an alternative method used for some slate types and exposure conditions. The nails must penetrate fully into the batten without being driven so tight that they crack the slate.

Sorting Natural Slate

Natural slate varies in thickness within each delivery. Slates should be sorted by thickness before laying, with the thickest slates at the eaves, progressing to thinner slates toward the ridge. This gradation produces a more uniform finished appearance and helps maintain consistent headlap as the thicker slates near the eaves create a built-up angle that thinner slates cannot.

Summary

Slating requires calculating the correct gauge for the slate size and roof pitch, laying breathable underlay and battens at the gauge spacing, and nailing each slate with the correct headlap and side lap. Slates are sorted by thickness with heavier slates at the eaves. Two head nails per slate through pre-drilled holes fix each course to the battens. Professional contractors experienced with natural slate provide the best results for significant roof work.