Roof Slate for Low Pitches

In this article, we will breakdown the use of natural slate on what is deemed to be a “low pitched roof” – 22.5 degrees or lower. With this information, you should be able to make an informed decision on which slate to use, or whether you use natural slate at all (fear not, natural slate can be used on pitches as low as 12.5 degrees. More information on that later).

There are many variables that need to be taken into consideration when choosing the slate, or even deciding whether slate is a viable choice.

Consider your rain exposure. 

Moderate Exposure: (Driving Rain index less than 7m2/s). Severe Exposure (Driving Rain index greater than 7m2/s)

Weather tends to be more severe towards the West. Central UK and, in particular, the East, have been somewhat blessed with lighter wind driven rains. The below graph should give you an idea of your exposure. 

Consider the size of the slate

The wider and longer a slate, the lower the pitch on which it can be successfully applied. The table below show the recommended headlap for your slates (in mm) for a MODERATE EXPOSURE area.

  20 Degrees 22.5 Degrees 25 degrees 27.5 Degress 30 Degrees 35 Degrees 40 Degrees 45 Degrees
Slate Size (mm)                
600×300 130mm 115mm 90mm 80mm 75mm 75mm 65mm 65mm
500×300 115mm 105mm 90mm 80mm 75mm 75mm 65mm 65mm
500×250 125mm 110mm 90mm 80mm 75mm 75mm 65mm 65mm
450×250 125mm 110mm 90mm 80mm 75mm 75mm 65mm 65mm
400×250 85mm 75mm 75mm 65mm 65mm

The table below show the recommended headlap for your slates (in mm) for a SEVERE EXPOSURE area.

  20 Degrees 22.5 Degrees 25 degrees 27.5 Degress 30 Degrees 35 Degrees 40 Degrees 45 Degrees
Slate Size (mm)                
600×300 120mm 120mm 115mm 110mm 90mm 80mm 70mm
500×300 125mm 115mm 110mm 110mm 90mm 75mm 65mm
500×250 135mm 115mm 110mm 110mm 90mm 75mm 65mm
450×250 115mm 110mm 110mm 90mm 75mm 65mm
400×250 100mm 95mm 75mm 65mm

As overwhelming and complex as this seems, we operate on a much more basic rule. Don’t use natural slate below 22.5 degrees, unless you have the ability to hole the slates. All of our 500×250 slates are holed at 190mm/195mm. suitable for pitches as low as 22.5 degree and upwards. For shallower pitches, we have unholed 500×300 slates which are suitable for 20 degree pitches (based on a headlap of 115mm). Our 600×300 slates are typically holed at 230mm, suitable for pitches down to 22.5 and upwards. Please call us as these holes can vary ever so slightly. 

Can I use natural slate on a pitch lower than 20 degrees? 

YES! There is now a product called “Easy Slate”. Easy slate is, basically put, a plastic gasket that sits between the slates, sealing the gap and creating a seamless course of slates. This product will allow for natural slate to be used on pitches as low 12.5 degrees. Call us on 01296 658 396 now for further information.

What is Pyrite in Roofing Slate?

What is pyrite in roofing slate?

Pyrite is iron sulphide (also known as “fool’s gold”) which is commonly found in roofing slates. Pyrite has an unfortunate reputation of rusting on the roof, but what many people don’t know is that you can have 2 types of pyrite inclusions; stable and unstable.

Stable Pyrite Inclusions

A stable inclusion is found in a T1 slate. These inclusions tend to be visible flecks of “gold” with refined edges, typically a cube shape which, ironically, when found in its raw form, is really quite a beautiful thing. When on the roof, and exposed to the elements for a number of years, these inclusions, if anything, will rust as they are, without bleeding down the roof. These are completely harmless, they don’t discolour the slate and, most importantly, don’t affect the performance of the slate.

stable pyrite in roofing slate

(a small, clearly defined fleck of pyrite)

Unstable Pyrite Inclusions

Unstable pyrite is sometimes very difficult to spot as the inclusions tend to be a lot smaller but greater in numbers, covering large areas of the roofing slates (see image below). These inclusions are commonly found in T2 slates. While these inclusions very rarely compromise the performance of the slate, they do rust and bleed down the roof, creating a bit of an eye sore. We recommend checking the A1-S1-T1 DOP (declaration of performance) prior to making an order, to ensure you’re receiving a T1 slate.

unstable pyrite inclusion in roofing slate

(clusters of unstable pyrite inclusions which can rust and bleed down the slate)


As stated above, not all pyrite is going to discolour the slate. A certified T1 slate is almost guaranteed not to rust, whereas a T2 slate can be prone to it. Here at Buy Roof Slate, we openly divulge all details of our slates, including their origin, to our customers. Your peace of mind and satisfaction is our number 1 priority. If you’d like to discuss this further with one of our knowledgeable staff, please don’t hesitate to get in touch.

How many roof slates 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

If you know the surface area of your roof, you can use our simple Roof Slate Calculator to give you the exact amount of slates you require to complete the job.

Please Note: These figures do not allow for any wastage, therefore we recommend adding a further 5%-10% on top of your order to account for this.