How the two differ in kind
Start with the structure, because it is the constraint that cannot be designed around later. Both coverings weigh several times what asphalt shingles weigh, and a roof framed for a light covering — common on twentieth-century construction and on anything re-roofed cheaply — will not carry either without strengthening. That is a calculation before it is a preference, and discovering it after the old covering is off is the expensive order to do it in.
Then pitch. Neither covering is waterproof in the way a membrane is: both shed water by overlapping, and both have a minimum pitch below which wind-driven rain travels up the lap faster than gravity carries it down. As pitch falls, the required HEADLAP grows — which means more courses per unit of roof, more units per square metre, and a heavier, more expensive roof at the shallow end. The quantity is not a fixed rate per square metre; it is a function of the pitch.
The materials then differ in how they fail. Slate is metamorphic stone and essentially inert: a good slate does not change chemically, and its published grades describe expected life in decades against water absorption and the presence of minerals that oxidise. Clay is fired earth, and its vulnerability is FROST — a tile that absorbs water and then freezes spalls its face away, which is why frost-resistance classification matters far more in a hard climate than any other property on the datasheet.
The factors that actually differ
Show
| Clay tile | Natural slate | |
|---|---|---|
| Weight on the structure | Heavy, and heavier again in the deeper interlocking profiles. A rafter and purlin check comes first. | Heavy, varying with thickness grade — and thickness is also what buys life, so the long-lived slate is the heavy one. |
| Minimum pitch | Interlocking profiles reach lower pitches than plain tiles, because a side channel does work the lap would otherwise do. | Generally wants a steeper pitch than an interlocking tile, and the lap grows quickly as pitch falls. |
| How quantity behaves with pitch | Rises as pitch falls, through increased lap and closer battens. | Same mechanism and more sensitive to it, because headlap on a slate roof is the entire weathering strategy. |
| What ages it | Freeze-thaw. An underfired or highly absorbent tile spalls; a correctly classified one does not. Moss and lichen are cosmetic on clay. | Delamination and mineral decay in poor slate. Good slate is effectively inert; the grading system exists because the range between sources is enormous. |
| Replacing a broken unit | Lift the units above and slide the new one in. A morning's work with no special parts. | The nails are inaccessible under the courses above, so a replacement is held by a TINGLE — a metal strap hooked over the slate below. A visible, accepted, slightly ugly repair. |
| Variation between products | Wide but documented: profile, frost class, and manufacturer's fixing specification. | Enormous and not always documented. Two slates of the same size and colour from different quarries can differ by decades of life, which is why the grade and the source matter more than the appearance. |
| Fixing | Nailed, clipped or both, with the schedule tightening at edges and in exposed positions — the same zoning that governs any covering under uplift. | Two nails per slate, head- or centre-nailed depending on size and exposure, with the choice affecting how the slate behaves in wind. |
| Reusing the old ones | Rarely worth it — profiles change and the bedding usually breaks them on removal. | Often worth it, with care. Sound reclaimed slate is a real material, and the limiting factors are existing nail holes and whether enough of a batch survives sorting. |
Which one, and when
Choose clay tile when…
- The pitch is at the lower end of what a lapped covering tolerates, where an interlocking profile earns its side channel.
- The local vernacular is tile, which usually means the detailing, the fittings and the trade are all set up for it.
- The climate is mild enough that frost is not the governing durability question, or a high frost class is specified and available.
- Broken units need to be replaceable by anyone, without a tingle and without a specialist.
Choose natural slate when…
- The pitch is steep and the roof is simple enough that headlap is not driving the quantity up.
- The building is listed or in a conservation area where slate is what the context requires.
- The intended life is measured in generations, and a high-grade slate is available and affordable.
- The climate is severe: a good slate is indifferent to freeze-thaw in a way clay has to be specified for.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Will my roof carry it?
- That is the first question and it is answered by a structural check rather than by a rule of thumb. Both coverings weigh several times what asphalt shingles weigh, so a roof framed for a light covering has to be assessed for the new dead load — rafters, purlins, wall plates and the load path all the way down. A roof that already carried slate or tile is usually fine to carry it again, which is why a like-for-like re-roof is straightforward and an upgrade from shingles is not. If the answer is that it will not carry, the choices are strengthening or a lighter covering, and the second is not a defeat: a good lightweight system on a sound structure outperforms a heavy one on a strained one.
- How does pitch change the quantity?
- Through the lap, and the effect is larger than people expect. A lapped covering weathers by the overlap between courses, and the shallower the pitch the further wind-driven rain can travel up that overlap — so the required headlap increases as the pitch falls. More lap means closer batten spacing, more courses over the same rafter length, and more units per square metre. A roof at the low end of a covering's permitted range is therefore materially more expensive per square metre than the same covering on a steep roof, before anyone has quoted a rate. The calculators here take the lap as an input for exactly this reason: it is not a constant, and treating it as one under-orders the roof.
- What is a tingle, and why can't a slate just be nailed back?
- Because the nail holes of any individual slate sit under the two courses above it, and reaching them means stripping those courses. A tingle is a narrow metal strap — traditionally lead, now usually copper or stainless — nailed into the batten below the gap, with the replacement slate slid up into place and the strap bent up over its tail to hold it. It is a proper repair, accepted everywhere, and visible if you look for it. A roof with a scattering of tingles has been maintained; a roof with dozens is telling you the fixings are failing generally and that a strip is due, which is the more useful thing to read from them.
- Are reclaimed slates a good idea?
- Often yes, with two caveats that decide it. The first is holes: a reclaimed slate already has nail holes, and unless they can be covered by the lap in the new coursing the slate has to be re-holed, which needs enough sound material. The second is sorting — a reclaimed batch contains slates at very different stages of life, and the value is entirely in whether somebody has graded them. A sorted batch from a known roof is a real material with decades left; an unsorted pallet is a lottery in which the cheap ones are cheap for a reason. Test a sample the traditional way: hold it and tap it, and listen for a ring rather than a dull note.
- What does frost resistance actually mean on a clay tile?
- That the tile has been tested through freeze-thaw cycles while saturated and has not spalled. It matters because clay's vulnerability is physical rather than chemical: water absorbed into the body expands as it freezes, and if the pore structure cannot accommodate that, the face flakes away. An underfired tile absorbs more and survives less. Classification schemes grade tiles for this, and in a climate with repeated freezing the class is the specification decision that matters most — more than profile, more than colour. It is also why a tile that performs well in a mild maritime climate can fail in a continental one, and why local vernacular is often a better guide than a datasheet from elsewhere.
- Does the underlay matter, given the covering sheds water?
- It matters a great deal, and it is the layer most often downgraded to save money. Neither covering is waterproof: both shed the bulk of the water and both admit some, particularly under wind-driven rain, which is exactly what the underlay is there to catch and drain to the eaves. Its second job is to keep the roof space from being ventilated by the gaps between units, and its third — for breathable types — is to let moisture out of the structure. The choice between a breathable membrane and a traditional impermeable one is not cosmetic: it changes whether the roof space needs separate ventilation, and getting that combination wrong produces condensation on the underside of a covering that is working perfectly.