Slate Roofing Terminology

General guidance on design is given here, detailed technical advice is freely available from the company’s highly trained and experienced technical department.

With an unrivalled knowledge of natural roofing slates, our technical department can provide  assistance with roof specification, cost estimates and standard CAD details.

The focus of production is on the core sizes (Penrhyn; 500 x 300 mm, 400 x 250 mm & 300 x 200 mm.  Cwt-y-Bugail; 500 x 250 mm, 400 x 250 mm & 300 x 200 mm) but it has the flexibility to produce bespoke sizes to order.


Bond or side lap - The horizontal distance between the side of a slate and the side of the slate immediately above it. The bond should never be less than the lap.

Gauge - The length of tile/slate exposed after it has been installed. It equals the distance between the top of one batten and the top of the next.

Headlap - The amount by which a tile/slate overlaps the course below it.

Margin – This is the visible or exposed area of a slate from the bottom or ' tail ' of one slate up to the point the slate above overlaps it. Another name for the margin is the ' exposure ' . If you measure the margin it will be the same measurement as the Gauge.

Tail -The lower edge of the width of the slate when laid.


Minimum Recommended Headlaps

The recommendations for minimum pitches and laps for slate apply to normal situations. In general, the recommendations apply to rafter lengths of not more than 9m in moderate driving rain exposures and 6m in severe driving rain exposures. Specifiers should take account of any abnormal condition that might apply and may need to specify greater values than the recommended minima. If it is necessary to use pitches lower
than the recommended minima please contact Welsh Slate technical department.


Slates fixed in accordance with the details given in the Roofing Download Brochure guide will have adequate resistance to wind loads, wind uplift and rain penetration under most conditions. Detailed guidance on wind load calculations is given in BS 5534 and BS EN 1991.


Sales Office
Penrhyn Quarry
LL57 4YG

+44 (0) 1248 600656

Download our Roofing Brochure for a Comprehensive Guide to Technical Requirements and Standards

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