When choosing roof slates for your renovation or self build, there are several factors to consider, including:
- whether to choose British or imported slates
- deciding whether to use natural or manufactured slates
- ensuring quality
- installing and maintaining your slate.
Locally sourced slate is an enduring sight on our roofs. The introduction of BS EN 123261 in 2004 also guarantees that newly quarried British slate is frost- and fire-proof.
Welsh slate, grey-blue in colour, is universally recognised as the highest quality slate available. Other prestigious slates from Britain include:
- Burlington blue-grey
- Green-coloured Westmorland slates from the Lake District
- Dark-grey Cornish Delabole slate
- Scottish Ballachulish and Easdale slates.
Natural slate is worth the investment if you are renovating a period property, or matching a new build to suit the local vernacular. Planners often favour slate as it’s a key part of the local vernacular architecture in many areas. That’s worth remembering if you’re considering a project in a conservation area or area of outstanding natural beauty.
What About Imported Slate?
Imported slate offers competitive prices and an array of different hues, making it a popular choice. Spain is by far the largest exporter, providing nearly 75% of all slate worldwide. Canadian slate is also a good alternative to Welsh slate.
Imported slate has been subject to bad press, but it can be a good purchase if researched thoroughly.
Chinese and Brazilian slate tends towards the lower end of the price scale. Unlike other slates that are formed naturally under 500 million years of tectonic pressures, slates from these countries are formed from sedimentary processes that can result in less robust roofing material.
Clockwise, from top left: Harris Slate & Stone’s Brazilian Graphite slate (01267 233824); Forna slates, by Harris Slate & Stone, are imported from Galicia, Spain, POA (as before); Imerys’ integrated solar photovoltaic slates (imerys-rooftiles.com); Black Mountain Quarries’ Carmen slate from Spain; Cembrit’s Glendyne blue-grey with Trinity heather blue slate (020 8301 8900); East Moorland slate is imported from China, but similar in hue to Cumbrian slate, from China Slate (01246 865222)
How much do roof slates cost?
For an average roof size of 150m², 3,000-3,500 slates would be needed, with costs for natural slates (excluding installation) ranging from £18/m² to £30/m².
Benefits of Natural Slate
Natural slate has many benefits:
- a lifespan of up to 150 years
- can endure extreme temperatures
- is snow- and fire-resistant
- is fully waterproof
- as a mineral product, natural slate is not subject to any chemical processes during production
How to Ensure the Quality of Natural Slate
- Ask to see samples.
- Ensure products are tested to BS EN 123261 (the recognised standard for natural slate quality).
- Look for slate graded T1. The higher the grade, the less the likelihood of oxidising pyrites in the slate, which lead to rust-coloured staining. Generally, as slate becomes more expensive it is less prone to this staining process.
- Slate is also graded by its tendency to absorb water (W1) and here the lower the number, the better. Low water absorption is linked to long life. The maximum water absorption in the UK is 0.6.
- Look out too for S (carbonate content). Slate graded S1 is the best (and so not affected by acid rain).
Once delivered, test a random sample — good-quality slates produce a clear ring when tapped, whereas poor-quality slates produce a dull thud and should not be installed.
Man-made products lack the durability and charm of their natural counterparts. However, they are an affordable alternative and so are widely used. Manufactured to consistent size, shape, texture and colour, they can create a uniform, smart finish.
Consistency also means they can be laid single bond and still provide watertight protection (unlike natural slates, which are typically laid in a double-lapping bond) and, coupled with the convenience of pre-drilled holes, makes installation far easier — saving time and money.
Concrete slates and slates manufactured from reconstituted slate dust are both viable options. Clay-based slates are another option and offer good resilience to the elements. Fibre-cement slates are made from cement, organic fibres and mineral additives and they are ideal for:
- complex designs
- steep roof pitches
- for roofs where a lightweight solution is required (copper rivets are usually required to keep this lighter material in place).
Unlike natural slate, however, weathering may expose the base colour of both concrete and fibre-cement slates over time.
Clockwise, from top left: Sandtoft’s Rivius slate is made from natural alluvial clay and has a ceramic finish (sandtoft.co.uk); The Beauvoise slate from Imerys looks like slate, but is actually made from clay (imerys-rooftiles.com); Harris Slate & Stone’s Wanit Repro are fibre-cement slates (01267 233824); Stoneleaf’s Celtic Grey slate is imported from Spain, but similar in hue to Welsh slate (01277 841555); Bellvue Natural Slate’s Imperial range, imported from China and available in two sizes: 500 x 250 and 600 x 300, call 01767 650201 for prices; Slates from the Crofter range, by Bradstone Structural, have the appearance of stone slate, but are made from Portland cement (01285 646884)
Installing Roof Slates
Installing a natural slate roof is a time consuming, highly skilled job, and therefore a substantial cost to factor into your budget.
It’s worth finding a roofer with knowledge and experience of British Standard 5534 (the national code of practice for slate tiles) and BS 8000 Part 6 (which concerns the workmanship of roofing).
Expect a simple gable-to-gable roof to take one week. For large complex roofs, where you have valleys, hips and intersections and the slates need cutting to fit, you can expect the work to take weeks or even months. Bear in mind that you need a pitched roof to install slates. Installing slates on a flat or nearly flat roof is not normally possible.
Ensure slates are safely stacked when delivered on site, to help prevent discolouring through efflorescence. This is a chemical reaction that occurs when slates, stacked too closely together, become damp or wet.
Re-slating Period Properties
If you’re re-slating a period roof, ask your roofer to preserve as many of the existing slates as possible. Match these with natural slates of a similar size, hue and texture, and re-lay in a random assortment to create a characterful roof.
Alternatively, visit a reclamation yard — good-quality reclaimed slate is in high demand, and this is both a cost-effective and sustainable solution. Generally you will pay a little less than you would for new slates, although if the size is in demand you may pay a bit more.
Re-slating period properties provides an ideal opportunity to review a roof’s existing insulation. Adequate insulation can reduce fuel bills by 40% and so renovators and self builders alike should give considerable thought to the insulation systems available, before adding slates.
However, once roofing commences, ensure adequate ventilation space is left between the sheathering (boarding to which slates are nailed) and the insulation. This will allow air to circulate and the roof to ‘breathe’, which prevents unnecessary complications caused by condensation.
How to Maintain a Slate Roof
Once installed, regular inspections are key to prolonging the life of your slate roof. Inspect your roof from ground level at least twice a year. Nail sickness or nail ‘rot’ is where nails corrode, causing slates to slip, and it is one of the most common problems with slate roofs. If possible, access your roof space and check for leaks or damp, as this could mean a slate has slipped or been damaged.
- Find out more about repairing a roof