Roof Slate Calculator – How to Calculate Roof Size

How many roof slates do I need per square metre (m2)?

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 21 slate 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 all of our roofing merchants, believe the following figures to be the most accurate.

• • 16×8 (400mmx200mm) = 29 slate per m2
  • 16×10 (400mmx250mm) = 27 slate per m2
  • 18×10 (450mmx250mm) = 23 slate per m2
  • 20×10 (500mmx250mm) = 21 slate per m2
  • 24×12 (600mmx300mm) = 13 slate per m2

How to Calculate Roof Size

This is a question we get asked all the time. In order to calculate the amount of roof slate you will require, an accurate measurement of the roof is required. We’ll go into calculating slate quantities further on. We now need to calculate the square meterage of your roof, often shown as m2.

Firstly, you measure the length of the rafter which normally runs from the gutter to the ridge line of the roof. For the sake of this article, let’s say the rafter length is 4 linear metres (represented by X in the image below). We now need the ridge line length (represented by the Y). Again, for the sake of this article, let’s say this is 10 linear metres. We now know that Face 1 of the roof is (X) x (Y) which is 40 metres squared 40m2. As there are 2 sides to a standard up and over roof, you can simply multiply each side by two, therefore 40m2 x 2 = 80m2. It is worth noting at this stage any deductions for Sky Lights etc, in addition Sky Lights and Chimneys may raise the need for further Slate & Half Slates, but you can always call our team for assistance.

Calculate Roof Slate

Now we have the m2 size of the roof, we can start calculating how many slates you need to cover the entirety of your roof. There are a couple of things to note here:
Quantities will differ depending on which slate size you choose (more info on this below)
A 10% wastage allowance for handling, cutting and fitting is normally added to your order to ensure you aren’t left short.

Slate Sizes and Quantities

You will need the following amount of slate per m2

Metric / Imperial

• 600×300 / 24×12 (13 slate per m2)
• 500×250 / 20×10 (21 slate per m2)
• 450×250 / 18×10 (23 slate per m2)
• 400×250 / 16×10 (27 slate per m2)

To calculate the amount of slate you need for your roof, simply multiply the m2 by the number of slate per m2 as mentioned above.