Architects & designers have been inspired by the natural beauty of slate for centuries, both for its aesthetic potential as well as its practical qualities. In prehistoric times slate has been used for hearth stones & then buildings ever since.
Slate quarrying dates to the 12th Century & early stages roofing slate was restricted to important buildings. Documentary evidence suggests that it has been exported from Cornwall to Winchester in early 14th Century. By the 15th Century it was more commonly used as a replacement to alternative roofing materials on many different buildings. Early slates came in lots of different shapes & sizes which meant that they required more skill to use or fit, by the 17th Century some more standardisation had begun to come in.
It’s non-combustible & resistance to chemicals are just two of its qualities. The low water absorbency reflects its resistance to rain along with resistance to frost are great properties of natural slate. Energy efficiency is also created as there is a cushion formed between temperatures hot or cold, just remember to insulate.
Durability means that it is an ecofriendly choice with some roofs could be as much as 150 – 200 years old, but the lifespan is usually determined by the fixings holding them in place. In the lifespan of many historic roofs, they would have been replaced or significantly repaired 2, 3, or 4 times. The durability of slate means that it is unaffected by extremes of temperature, yet the same can’t be said for wind.
Slate itself is some 350 million years old formed in the Devonian period. Metamorphic slates are formed by the action of heat & pressure acting over hundreds of millions of years on consolidated clays (muds) deposited under water. The heat & pressure causes clay minerals to recrystallise & to align themselves with their long axis perpendicular to the direction of pressure. It’s this alignment which gives the slate it’s cleavage properties & allows the rock to split into its strong sheets.
The strength of individual slates is a product of the rock’s strength & the slates thickness. A thin slate (4 mm) made from a strong rock is equal to a thick slate (8-12 mm) from a weak rock.
This is why they have stood the test of time over hundreds of years. Unlike cheaper imports they can be used under moderate to severe atmospheric conditions & are available in a range of sizes.
Roofing slate tiles have been used on modern buildings through to heritage property repairs. These can be prestigious contracts for private clients to listed buildings & churches. They may be looking at maintaining the character of a property or adding character & appearance. When carrying out repairs to a property you are frequently respecting the historical construction methods or the appearance of the building. Vertical slating or cladding is often used to alleviate damp walls or on those surfaces that will expect excess wind or rain exposure (frequently it will use smaller slates).
The basic stages of stone extraction & splitting of slates have not changed for hundreds of years, the first step in creating a roofing slate is the hammering out of the slab of rock ready for it to be taken to the saw for its first cut. Slabs are inspected for impurities or quartz filled fissures which could affect the quality of the slate. The saw man’s job is to use a diamond blade to cut the slab into handleable pieces. These pieces then get passed along to the splitting shed where are struck with a hammer & chisel along the cleave planes to the required thicknesses (5-8 mm) with any waste being discarded. Finally, before they are ready for the customer they are dressed into their distinctive shape & sorted into crates as one size or a mix of sizes dependent upon the order.
When you start to talk to those in the know about roofing slates you need to learn a whole new language of Duchesses (24x12), Peggies, Randoms, Doubles (12x6), Laps, Gauge, Head lap etc. Also, there is a published a code of practice for slating & tiling.
The number of slates you need for your roof maybe dictated by the pitch or the laps (laying style), the size of the slates you are using, size may be restricted by weight, aesthetic appeal may also play a part alongside cost for coverage. Are you intending to fix with clips or nails or have you considered mortaring to help with wind resistance? So many elements may come into play.
Traditionally you may have chosen rag slates – large irregular slabs of slate, laying the largest on the eves & verges. These will get smaller as you move up the roof. Unfortunately rag slates have somewhat fallen out of favour these days being replaced with sized slates.
You may still choose to use slates of varying sizes laying larger ones on the eaves & verges, filling the roof with smaller slates decreasing as you reach the ridge (diminishing courses). Scantle slates or Peggies can be 12” - 6” long with random widths, these can be higher in labour, waste & cost to lay but look beautiful. An apprentice splitter will typically start on peggies before being allowed to move on the splitting the larger slates.
Slate length is measured from the tail to the head (bottom to top edge), and some will choose to use the same size of slate all the way up a roof to the ridge. It is generally recommended that you use the thickest slates on the lower courses & thinnest near the ridge. There may be more wastage and the overall look will be different. You may need additional slates or to cut slates for the verges & abutments in order to maintain a good lap & stop water penetrating in.
The lap of a slate as you have probably already worked out is how much the tail of the slate overhangs one on the course below. You can single lap where course 2 overlaps course 1. Double lap where course 3 overlaps course 1 (as well as 2). Triple lap where course 4 overlaps course 1.
Most recently we have supplied roofing slates to the Tregannell Hill project, Nansledon where they wanted to use local materials on a local project (Dutchy development – Newquay). Slate roofing is not only a practical choice but it also looks good giving great kerb appeal.