Both Welsh and Spanish slate are natural stone split from rock in a quarry, and from the pavement they can look much the same. The differences show up later, in how a slate weathers, how evenly it splits, whether it carries minerals that rust, and what it costs per square metre. For a new extension in a plain brick street the choice can safely come down to budget. For a Georgian terrace or a listed villa in Bath it is a different question, because the roof was built in one material and the neighbouring roofs still are. This guide compares the two on the points that matter when you are choosing: durability, colour, grading and thickness, and price. It also covers what to ask a supplier, and where mixing the two causes trouble. The comparison below sets out what we look for on site.
Where Welsh and Spanish slate come from
Welsh slate is quarried in north Wales, in rock that was laid down and compressed long before anyone built a house on it. It has been shipped to English towns for a very long time, and it is the material many older Bath roofs were first covered in. Bath Preservation Trust casework for a Grade I address in St James's Square records Welsh natural slate across the main roof pitch, and a separate case on a terrace at Lambridge Place records the Trust supporting a switch from artificial slate back to natural Welsh slate. Those are individual cases rather than a survey of the city, but they show how the local conservation voice tends to think.
Spanish slate is quarried in Spain, and the word covers a lot of ground. Several quarries supply the UK market, and they do not produce the same product. One may give a fine, even, dark slate with a consistent split; another may give a rougher stone with more variation from slate to slate. That spread is the first thing to understand. When people say Spanish slate is cheaper, or Spanish slate stains, or Spanish slate is perfectly good, they are each describing a different quarry and a different grade.
The same caution applies to Welsh slate, though the range tends to be narrower. Quarries differ in tint, in gauge and in how the slate has been dressed at the edges, and a reclaimed Welsh slate from a Victorian roof is a different object again from a new one. So the useful question is rarely Welsh or Spanish in the abstract. It is which quarry, which grade, and what evidence the supplier can give you for it.
Durability: how each slate ages
A slate roof fails slowly, and usually not because the stone has worn out. The nails fail first, rusting through where they pass the slate, so the slate slides. That is a fixing problem rather than a material one, and it affects Welsh and Spanish alike. The slate itself fails in a handful of ways, and those are where the two differ.
Splitting along the layers
Slate is formed in thin layers, which is why it can be cleaved at all. A good slate is dense and its layers stay tight for the life of the roof. A weaker one can start to delaminate, with the surface flaking in thin sheets that hold water and grow worse each winter. You can see it from the ground as a slate that looks furred or swollen at the tail. We find delamination when we lift a slate on a survey, and we set it against our notes on delaminating slate when we advise on repair.
Iron inclusions and rust staining
Some slates carry iron-bearing minerals, often as small bright specks. Left in the rain, they can oxidise, leaving rust-coloured streaks down the slate and, in poorer stone, weakening it. This is the usual reason people warn against cheap Spanish slate, and it is a fair warning, but it is about quality control more than country. A well-selected Spanish slate from a good quarry should not do it, and a poorly sorted batch from anywhere might. Our page on rust stains on slate goes into the causes.
Water absorption and frost
A slate that drinks water is a slate that suffers in a freeze, because the water expands in the pores. Dense, low-absorption slate is the aim. The way to compare is not by eye but by test data, which suppliers should be able to give you for the product they are quoting. We would ask for it before ordering, and we would be wary of any supplier who cannot say which quarry a slate came from.
Lifespan figures get quoted freely in the trade, so it is worth knowing where they come from. MyBuilder's 2026 slate guide reports good Welsh slate often passing 100 years in service and gives Spanish slate 75 to 100 years, though it is the only source we have for Spanish. The Welsh Slate quarry claims more than 100 years of useful life, which is a producer speaking about its own stone. MyBuilder adds that badly fixed slate of any quality can fail inside 10 years. Read every one of these as a typical figure that weather, exposure and the standard of fixing will move, never as a promise. The safe claim is that a good slate of either origin, fixed with the right nails or hooks onto sound battens, can outlast several generations of the timber below it. We cover that in more detail on how long slate lasts.
Colour and look on the roof
This is where most owners decide, and it is where the two diverge most clearly. Welsh slate has depth and variation. Depending on the quarry, it runs from blue-grey through to purple, heather and a deep green-grey, and within one roof you will see slates that are a shade lighter or darker than their neighbours. That variation is part of why an old Welsh roof looks alive in changing light. When it is wet, the colour deepens and the courses read as bands.
Spanish slate is usually more uniform, a dark grey to blue-black that sits flatter and more evenly across the slope. On a modern house that consistency can look crisp and clean. On an older roof it can look slightly flat beside its neighbours, because there is less of the shade-to-shade play that weathered Welsh slate shows.
Weathering matters as well. New slate of either kind looks sharper and darker than the roof it sits beside, and the gap closes over the first few years. If you are repairing rather than replacing, the new slates will stand out at first whatever they are made of. That is one reason we keep reclaimed slate for patching where the colour will be seen, as our page on matching slate colour explains.
A practical point for terraces: a roof is read as a row. If a single house in a Georgian terrace is re-covered in a flatter, more uniform slate than its neighbours, the join between the properties can be seen from the street, and planning officers in a conservation area tend to notice that kind of change. Whether that matters to you depends on the house, but it is worth looking along the terrace from the pavement before you choose.
Grading, thickness and size
Slate is sorted before it is sold. At the quarry or the supplier, slates are checked for flatness, for the quality of the split, for cracks and for faults, and put into grades. The grade you buy affects the proportion of slates that will be usable on the roof, how much waste there is, and how the finished slope looks. A lower grade that is cheaper per slate can cost more overall if the slater has to reject a fair number on the roof.
Thickness is the second variable. A thinner slate gives a finer line at the eaves and ridge, which is why heritage roofs often look delicate. A thicker slate is tougher underfoot and heavier. Thickness differs between quarries and between grades, so it should be read from the supplier's data rather than assumed from the country of origin. It also bears on weight, which matters for the timbers and for Building Regulations: the Planning Portal asks for approval whenever a replacement covering is at least 15 per cent heavier, and the structure gets checked. Changing from a thin slate to a noticeably thicker one is one of the few ways slate-for-slate can tip over that line, so we ask about it at survey.
Then there is size. Slates come in a range of lengths and widths, and the size sets the gauge, the distance between battens, and therefore how many slates the roof takes. A traditional roof may have diminishing courses, with large slates at the eaves getting smaller towards the ridge. That is a skilled laying pattern and it needs a supplier who can provide the right range of sizes, which is easier from some quarries than others.
Finally, there is the dressing of the edges and the quality of the holes. Slates can be hand-holed or machine-holed, and the placement of the hole sets the headlap, the overlap that stops water being driven up under the slate above. For fixings, minimum pitches and headlaps, we point owners to the manufacturer's data and to BS 5534, the standard that governs fixing slate and tile. Cupa Pizarras, a slate producer, quotes NHBC's 20-degree minimum for slate (with the right headlap) and puts hook fixings at 25 degrees or steeper, so a shallow roof narrows your choices whichever slate you pick.
What each costs, and what the price does not tell you
On price the two are clearly separated. The published UK figures below come from one third-party source, MyBuilder (2026), and they are typical rather than fixed.
| Origin | Per square metre laid | Whole roof, where quoted |
|---|---|---|
| Welsh | £140 to £180 | Not given |
| Spanish | £100 to £130 | £7,000 to £10,000 for a semi-detached house |
Treat them as national guide figures, not as an Empire price list. After we have surveyed your roof, the price we give you is a fixed one on paper that names the slate, the grade and the quarry.
The gap between the two ranges is real, and on a large roof it is a considerable sum. It is worth being clear about what it does and does not buy. The difference is partly the cost of the stone and its transport, partly the cost of the sorting that goes into a higher grade, and partly the market for the finish. It is not a measure of how long the roof will last, because a roof fails at the nails, the battens, the lead and the underlay as readily as at the slate.
So a sensible way to read cost is to compare the whole specification. A Spanish slate of known quarry over new battens, with breathable underlay and fresh lead, is a better roof than a Welsh slate relaid on tired timber. If you want to see how the rest of a job is priced, our guide to slate roof cost breaks down scaffolding, lead, ridge and Building Regulations alongside the slate rate.
Reclaimed slate sits outside this table. It can be the best match for an old roof, and its price depends on supply from the batch, so we price it when we have seen the stock.
Choosing between them for a Bath house
The right answer starts with the building rather than the price list. A short set of questions covers most cases.
- Is the house listed? Swapping the covering material on a listed building takes listed building consent, and B&NES says clearly that unauthorised work is a criminal offence. Planning officers and the conservation team will generally expect like for like, which for many older Bath houses means natural Welsh slate.
- Is it in the conservation area? About half of Bath sits inside the conservation area. An unlisted house there that is re-slated in matching slate should be uncontroversial, but a new material is worth raising with the planning team first.
- Is it a terrace or a semi? Shared roofs and visible rows push you towards matching what is already there.
- Is it a new build or an extension? Then there is no historic roof to match, and a good Spanish slate is a reasonable choice.
On an older roof where only some slates are failing, you may not need to choose at all. Reclaimed slate or a few matched new ones can repair a roof without recovering it, and we recommend repair where repair works. Where the real choice is patching against re-covering, repair or re-slate shows how we weigh up one against the other.
One firm rule: do not mix the two on a single slope. Welsh and Spanish slates are cut to slightly different gauges and tints, so a patch of one in the other shows as a patch, and the different dimensions can complicate the lay. Where a full slope is being renewed, use one material from eaves to ridge.
Finally, ask for the slate to be named in the written quote, with the quarry, the grade and the size. A line that just says natural slate leaves too much open, and it is the commonest reason two quotes for the same house cannot be compared. The main slate roofing service page at slate roofing in Bath explains how we survey and specify.
What to ask a supplier or roofer before you order
A few direct questions will tell you more than any brochure. They apply equally to Welsh and Spanish slate, and a roofer who is comfortable with the stone will answer them readily.
- Which quarry, and which grade? The answer should be a name. Vague origin is a warning sign for any natural slate, whichever country it is from.
- What test data covers this slate? Ask for the figures on water absorption and for how the slate performs in frost, and ask whether the sample you were shown is the product being delivered.
- What is the thickness and the weight per square metre? This feeds into the structure, and into the weight rule if the covering changes.
- How are the slates holed and fixed? Nails or hooks, copper or stainless, and the headlap that has been designed for your pitch and exposure.
- What happens with a faulty batch? Slate is a natural material, and occasionally a delivery has more rejects than it should. Know in advance who bears that cost and what the replacement route is.
It also helps to ask to see the slate in daylight, wet as well as dry, because colour can shift considerably. A sample held in a showroom is not a roof, and a slate that looks even in the box can show banding across a whole slope.
If you are still weighing slate against another covering, our page on slate versus tiles compares them, and on site we can lay the options side by side, and our survey photographs are of your own slope rather than a stock picture.