Random Roofing – How to Install Diminishing Courses

One of the many advantages of using natural roofing slate is that you can achieve unique aesthetics like random effects.

So, what are the random roofing effects you can design, and how do you get the desired result?

Download the complete random roofing and diminishing courses brochure.

Random Effects

A traditional and increasingly popular slate specification is “random roofing”. This finish gives your roof a cost-effective way of either reproducing traditional random slating during a restoration project or creating a stunning aesthetic or visual effect on a brand new building.

Here is what you need to know when specifying a random roofing effect:

There are two random slating patterns:

Random Width – using roofing slates of the same length but in random widths

Random Width in Diminishing Courses – using roofing slates of random width in courses of progressively shorter length (towards the roof’s ridge)

Size and coverage

To achieve random slating effects, pallets are supplied in “Random Width” – all roofing slates are the same length but are of random width.

Length – ranges from 300mm to 500mm in 50mm increments

Width – ranges from 150mm to 450mm in 25mm increments (but most are between 200mm and 300mm)

For example, SSQ Riverstone Random roofing slate can cover between 22m² and 24m² per tonne using an overlap of 75mm to 100mm.

A Diminishing Course effect – roofing slates of the same width but in courses of progressively shorter length – is achieved by ordering slates of random width and in the various lengths required.

For more product information, including details about headlaps and E factors, download the brochure now.


Successfully achieving a random slate effect relies on careful planning and accurately marking up the roof.

The roofing slates should be carefully sorted and graded, first by length (if appropriate) and then by thickness. Once sorted, the number of slates of each length will determine how many courses using that length can be laid across the roof.

When the number of courses is known, the gauge of the battens must be carefully worked out to maintain the minimum headlap required.

If slates are being laid to create either a Random or Random Width in Diminishing Courses effect, an even and gradual reduction of the batten gauging from eaves to ridge must be maintained.

In all random width slating, care must be taken to maintain the minimum sidelap required between courses. This is determined by multiplying the batten gauge by the E Factor (see brochure) which, according to BS 5534: 2003 (the Code of Practice for Slating and Tiling), should be no less than 70mm.

Need Help?

If you’d like advice about random roofing slates, or any other ‘special effect’, our technical experts are here to help you.

Contact our specialist experts today and they’ll get back to you immediately – click here to contact the experts.