Seven Common Mistakes When Replacing Slipped or Broken Slates
A slipped slate looks like a ten-minute job, but wrong nails, mismatched sizes and torn underlay turn a...
If you have ever priced a slate roof, you have met the terms best, second and third. They sound like a simple ranking of quality, and in some ways they are. But they are also quarry-yard shorthand for thickness, consistency and how many slates will survive the journey from pallet to ridge. Knowing what the grades mean in practice helps you avoid two common mistakes: paying over the odds for slate you do not need, or saving money now on a roof that will cost more later.
Traditional slate grading is not a single national standard. A quarry in Wales, a merchant in Scotland and an importer of Spanish slate may all use the words differently. Broadly, though, the hierarchy works like this.
Best grade — sometimes called first grade or A-grade — is the thickest and most uniform slate a quarry produces. It has been sorted to remove visible flaws, cracks, delamination and rust-forming inclusions. The slates are closer in size and thickness, so they are easier to lay and more predictable in service. Best grade is the specification for listed buildings, heritage roofs, exposed coastal sites and anywhere access is difficult.
Second grade is the workhorse. It is slightly thinner and less uniform than best, but still sound. You may find minor variation in thickness, more colour difference and occasional slates to set aside. For a typical domestic re-roof, second grade is often the sensible middle ground: a long service life without the premium of best.
Third grade is the thinnest and least consistent. It may include small defects, uneven edges and a wider spread of thickness. It can be serviceable on a shed or garden office, but it is not the right choice for a main house in an exposed location. It is also more likely to break during handling and fixing.
Some producers sell "heavy", "medium" and "light" slates instead. The names change; the questions do not. Ask for the actual thickness range, the source quarry and the test data.
Thickness is the clearest physical difference between grades. A best-grade slate feels solid in the hand. A third-grade slate feels thin and brittle. Thicker slates hold their shape, resist wind uplift and tolerate the small movements that happen as a roof expands and contracts. They are also heavier, so check that the roof structure, battens and fixings are up to the load. On a re-roof, the existing timbers may have been designed for a thinner slate.
Thinner grades generally need more generous headlap to shed water reliably, especially on lower pitches or in driving rain. If you are tempted by third-grade slate for a main roof, check the minimum pitch and exposure conditions first.
Wastage is where grade names become real money. Best-grade slate arrives sorted, so there is less time spent discarding broken or odd-sized pieces. Second-grade needs more sorting and a bigger allowance for breakages. Third-grade can be genuinely wasteful: a pallet may contain a noticeable proportion of slates that are too thin, chipped or uneven for the visible face of the roof. Whatever grade you choose, order a margin above the calculated roof area. The lower the grade, the larger that margin should be.
Well-fixed best-grade slate can outlast the battens, the underlay and sometimes the timber beneath it. Second-grade slate is a long-term material too, but it may need more attention over its life. Third-grade slate is a shorter-term proposition — suitable for outbuildings, but not for a roof you expect to leave alone for decades.
Use this rough guide when specifying:
Local exposure matters as much as the grade. A second-grade slate on a sheltered urban roof may outperform a best-grade slate that has been poorly fixed on a wet, windy hillside. Fixing quality, headlap, underlay and ventilation all play a part.
Best-grade slate is the most expensive per slate and per square metre. Second-grade sits below it. Third-grade is the cheapest. The gap between best and third can be wide enough to change the whole budget for a large roof, but it narrows when you look at the full cost of the job.
Remember that the slate is only one line in the quote. Labour, scaffolding, battens, fixings, underlay, lead work and waste removal often make up the larger share. A cheap slate that takes longer to sort and lay, or that needs replacing sooner, may not be the bargain it first appears. On a small roof, the difference between second and best grade might be a modest part of the total; on a large or complex roof, it will be much more significant.
Prices also depend on quarry, region, size, surface finish, colour and availability. Reclaimed slate is a separate market with its own grading. Always ask for a full roof costing, not just a per-slate price, and compare like for like on thickness and source.
Thin slate breaks more easily. It can suffer from hairline cracks that are invisible until the first hard frost. Inclusions in the stone may rust and stain the roof. If you are buying third-grade slate, inspect a sample carefully. Look for delamination, tapered edges, chipped corners and rust-coloured marks.
Grade names alone are not enough. A good specification names the source, the thickness range and the relevant standard. Slate for roofing in the UK should comply with BS EN 12326, which covers product characteristics and testing. Ask for a certificate or test report.
If the building is listed, or in a conservation area, check with the local conservation officer before you order.
Think of the grade as a starting point, not a guarantee. Match it to the building, the exposure, the pitch and the budget. For most homes, a sound second-grade slate is a practical, long-lasting choice. For heritage work or a roof you never want to think about again, best grade is worth the premium. For a shed or temporary building, third grade will do the job.
If you are unsure, ask a roofer who works with slate regularly, or a surveyor who understands traditional roofing. Get the specification in writing, including thickness and standard. A little time spent on the grade now will save money and worry later.
Photo: analogicus / Pixabay
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