Busting the myths about slate pitches

Neil Rowland, commercial manager – aggregates for Welsh Slate (now part of the Breedon Group) discusses alternative roofing uses for natural slate. 

Thought natural slate could only be used down to 20° roof pitches? Think again.

While that may be the minimum pitch for natural slate to comply with BS 5534:2014 and hence Building Regulations, the material can actually be used on “flat” roofs*, defined by BS 6229 as having a pitch of less than 10°. 

It manages this by “mineralisation”, that is, the natural slate that can be used on flat roofs does so in mineral, not a finished slate or tile, form.

Slate minerals have been used to add multiple qualities to the green roofing felt typically sold through consumer retail outlets such as B&Q for use on garden sheds for almost half a century (when crushing machines were first introduced), and for the heavier-duty black torch-on bitumen felts and roofing systems produced by the major manufacturers for just over half of that.

In addition, for the past 30 years a blend of coarse and medium granules has comprised 60% of the Cambrian man-made roofing slates manufactured in Ebbw Vale, South Wales, by Redland, like Icopal now part of the BMI Group. This part-resin slate can be installed on pitches as low as 15%.

A by-product of the production of roofing slates, the mineral material is washed, crushed and dried to produce granules of three sizes – coarse granules measuring 1.5mm to 3mm, medium measuring 0.6mm to 1.5mm and fine powder granules up to 0.6mm – which are rolled onto the bitumen coating. Coarse and fine each comprise 40% of produced material while medium granules comprise 20%.

And such is the demand from construction and infrastructure projects, from as far afield as EMEA and Pakistan, that £250,000 worth of investment in people, plant and machinery has been made at the minerals quarry in Snowdonia, North Wales, where the old slate tips are being revisited for minerals purposes. The new plant includes a drier with the capacity to treble primary output.

Originally produced at what is still the world’s biggest roofing slate quarry at Penrhyn, minerals manufacture was centralised at Ffestiniog several years ago. Some 20% more was produced in 2018 than in previous years and an additional 33% is forecast next year.

Some of this demand is reflected in the increasing trend for volume housebuilders to include a garden shed with their properties. Even the fine granules/dust are used to make briquettes for gas BBQs.

And even at that fine a composition, Welsh slate retains its 500 million-year-old legacy of density, with minerals from this part of the world preferred against the alternative materials produced from French, Spanish and Chinese slate.

Slate as a mineral brings a variety of benefits to the membrane industry, which is much larger in Europe (18 roofing felt manufacturers in Italy alone) compared to the UK (five), with water and fire resistance complemented by its inertness. 

It is also available in a variety of colours – grey being pigmented black for bitumen purposes but also white for solar reflection, red and even pink! Although green is completely natural, bespoke colour greens are produced to complement retailers’ corporate colours. Whichever colour, it goes a long way – one 1.5-tonne bag making up to 300 rolls of felt.

Trials are currently underway at Penrhyn, supported by distributor giant SIG, on a large-format slate “sand” (up to 4mm) to help bring the roof slate aesthetic to concrete tiles which typically can be installed on pitches down to 17.5° depending on rafter lengths, exposure and so on.

And an architect has recently asked for a sample of a larger format (20mm) crushed Penrhyn Heather Blue slate to use loose on the flat roof areas of an unusual new roof at Barts Hospital in London.

But in addition, in an industry where the yield is typically 4% of the original product, the focus on best practice and best use of the planet’s resources maximises eco points.

The roofing industry may also get to benefit from a current trial by the Welsh Office and World Health Organisation on the healing properties of slate powder. Cream for roofers’ sunburn perhaps?