The amount of slates you need per square metre (m2) depends on which slate size you’re using. For example, if you’re using a 20×10 slate (500mmx250mm), you will require 20 slates per m2.

A 24×12 slate (600mmx300mm) demands only 13 slate per m2. These figures tend to be slightly different throughout the industry, but we, along with most of our roofing slate suppliers, believe the following figures to a good guide.

Most of our slates are pre-holed to suit 80 to 100 mm head lap.
As a general guide in trade the numbers of slate per square meter of roof covering inclusive overlaps are:

• 12x8 (300mmx200mm) = 49 slate per m2
• 16x8 (400mmx200mm) = 33 slate per m2
• 16×10 (400mmx250mm) = 26 slate per m2
• 20×10 (500mmx250mm) = 20 slate per m2
• 24×12 (600mmx300mm) = 13 slate per m2

When multiplying the above figure by the total roof area to be covered will give the amount of slates required.

It is the general practice in the trade to allow extra 5% - 15% quantity for wastage.

Working Out the Quantity of Slate and a Half

Slate and half is one and half times width of the slate. It is used to verges, hip and valleys.

Firstly, work out the gauge=(slate length – head lap)/2
e.g. 500x250mm slate at 100 mm head lap the gauge will (500-100)/2 = 200 mm

If each edge of verge, hip or valley on a roof is 4m (4000mm)
There will be 4000/200 = 20 rows of slate. You only require the slate and half on alternative row.
The 500x375mm slate and half will be 20/2=10 + 1 to top = 11 slate and half for one edge.