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.

2 thoughts on “Roof Slate for Low Pitches

  1. hello,
    I need to use a natural slate on a small area of single storey roof in a sheltered position on the lee of a two storey dwelling. The pitch will need to be 18 degrees.
    Could you advise on a suitable specification please?
    The slate needs to be grey, and a reasonable though not exact match to Delabole slate.

    • Hi John

      I’d recommend a completely flat laying slate. The Prestige First is uniform and very flat laying, additionally, the Prestige is a Blue/grey and is commonly used to work with Delabole, particularly in Cornwall/Devon where it’s most common. With regards to the pitch, you will need the Easy slate insert. This is a small plastic “gasket” which closes that gap, guaranteeing a water tight roof. Please call us on 01296 658 396 for more information.

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