Roof Slate Calculator – How to Calculate Roof Size

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 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 and 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:

  1. Quantities will differ depending on which slate size you choose (more info on this below)
  2. 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)

We’ve built a simple tool to calculate the amount of slate you need for your roof. Simply select the size of slate you need and enter the size of your roof in m2 into the box. Our Slate Calculator will do the rest (remember, when checking out, to add 10% more onto the total in order to cover wastage).

Slate & Half Slates

At the verge – outer edge of the roof – it is general practice to alternate courses of standard size slates and the slate and half’s which are 1.5 the width of normal slates. They are used to maintain the half bond needed. As a guide, a slate of 500 x 250 sizes uses a slate and half of 500 x 375. You would need 2.5 of these slate per linear metre. Therefore, if your rafter length was 4 linear meters, your calculation would be 4 x 2.5 = 10 Quantity slate & Half’s. Likewise, you can use this guide to calculate the same around chimneys and skylights or anywhere where cutting is involved. Again, our team is here to advise you.

Please Note

Due to the level of daily enquiries we can accept plans via email, we simply do not have the time to do take-offs from drawings, we will assist those who call with roof dimensions and have any questions relating to our products and use. For most the slate calculator is a simple useful tool that is used daily.