How the two differ in kind
The two coverings are not two grades of the same idea. A slate is a piece of metamorphic rock split along its cleavage plane, holed, and hung on two nails; it has no volatiles to lose and no coating to fail, and there is no clock running in the material itself — which is not the same as saying nothing can touch it, since the very tests the standards apply to slate exist because some stone does weather. An asphalt shingle is a manufactured composite — a fibreglass mat saturated in bitumen and surfaced with mineral granules — and every part of that description is a clock. The granules are the sunscreen for the bitumen beneath them, the bitumen stiffens as its volatiles leave, and the assembly ages from the day the sun first reaches it. That is not a criticism of the product; it is what the product is, and it is priced accordingly.
They also fail on completely different schedules, which matters more than the headline lifespan. An asphalt roof is a cohort: one production run, one week of laying, one identical exposure history, so it arrives at the end of its life across the whole plane at once and the roof is replaced as a roof. A slate roof fails one unit at a time and gets repaired one unit at a time, for generations, and when the whole-roof event finally comes it is usually not the stone that has gone — it is the nails. Ferrous fixings corrode in the batten zone and the slates start slipping while remaining perfectly good slates, which is why a century-old roof is so often stripped, sorted and re-laid using most of the material that came off it. Nobody has ever re-laid an asphalt shingle.
For a period house, the argument that decides it is rarely the price of one roof. Slate is dearer on three axes simultaneously — a quarried material, a trade with far fewer hands in it, and fixings and flashings specified to outlast the stone rather than merely to match it — and possibly a fourth, if the structure has to be strengthened to carry the load. Asphalt is inexpensive on all of them. What flips the sum, when it flips, is that a tall period house pays a large fixed cost every time the roof is opened at all: the scaffold, the access, the fortnight of disruption, the skip. That cost attaches to the event, not to the area, and one side of this comparison expects to pay it once in your ownership while the other expects to pay it more than once. So the honest sequence of questions is: was this house framed for slate, and is that frame still sound? Is the covering controlled by somebody else — a landmark or listed designation, a historic district or conservation area, a planning condition or a covenant — because that can end the discussion before it starts. Is the roof a simple plane or a warren of dormers and valleys, since slate's cost concentrates in exactly those details? And how long is this house going to be yours? Answer those four honestly and the roof has chosen itself.
The factors that actually differ
Show
| Natural slate | Asphalt shingles | |
|---|---|---|
| What keeps the water out | Geometry, and nothing else. Each slate seals to nothing at all; dryness is headlap over the course two below plus the side lap set by breaking the joints. Lower the pitch or raise the exposure and the geometry must be re-worked or the roof leaks — there is no adhesive anywhere to compensate. | Geometry plus adhesion. The self-seal strip bonds each course to the one beneath as the roof warms, turning the courses into a quasi-continuous mat that also resists uplift. The assembly's weather performance assumes those strips have actually sealed. |
| What sets the quantity | Exposure, which pitch and site exposure set through the required headlap. The course count is the roof run divided by whatever exposure your slate size is permitted at your pitch — so the same area of roof takes more slate on a shallower or more exposed plane. Area alone cannot produce an order. | Area, with pitch as a multiplier and nothing more. Footprint times the slope factor gives the true roof surface, that surface gives squares, squares give bundles. The shingle's own exposure is manufactured in, so two roofs of equal surface take the same shingles whatever their pitch or site exposure — only the waste allowance moves. |
| What it is fixed to | Whatever holds a nail for a century — and that is regional. British and Irish practice hangs it on battens over underlay or over boarded sarking, as a drained covering; North American practice nails it through underlayment straight into a solid deck of sawn boards. What does not change is the nail-holding demand: the substrate has to still be gripping non-ferrous nails long after this roof's warranty language would have expired. | A continuous deck, always — the shingle spans nothing, so battens alone are never an option. But the deck itself is undemanding: sheet goods are the normal substrate and the nail only has to hold for the life of the mat above it. Putting asphalt on a battened roof means buying and fixing a whole deck first, at full area, before a single shingle is laid. |
| Load on the structure | Among the heaviest coverings in ordinary use — enough that rafter section, span and spacing, the wall plate and the walls beneath all have to have been designed for it or be checked by somebody qualified to check them. | Among the lightest. It asks essentially nothing new of any structure that has ever held a roof, which is precisely why so many slate houses were re-covered in it during the twentieth century. |
| The fixings | Specified to outlast the stone: copper, silicon bronze or stainless, with head or centre nailing chosen for the slate length and the wind exposure. This is not a detail to value-engineer — corroded nails, not worn slate, are what ends most slate roofs. | Galvanised roofing nails that only have to survive as long as the shingles above them, which they do comfortably. The fixing is a consumable in a consumable system and is priced as one. |
| How the roof reaches its end | Slate by slate, indefinitely. The whole-roof event, when it comes, is a strip and re-lay on new battens and new nails — and a large share of the material that comes down goes straight back up. | All at once, as a plane. One batch, one exposure history, one end date, and the material that comes off is landfill or road-base feedstock rather than stock for the next roof. |
| Where a complicated roof costs you | In the details. Verges, eaves and top courses, mitred hips over soakers, valleys swept or laid in lead, every abutment and every chimney — all specialist hours that multiply with the number of features, not with the area between them. | Mostly in area still. Hips and valleys are cut shingles and a roll of valley material; complexity costs something, but a cut-up roof barely bends the estimate the way it bends a slater's. |
| Repairs and everyone who comes after | A cracked slate is ripped out and hooked back on a copper tingle in minutes, and the repair is invisible. But nobody walks on slate: every aerial, chimney, gutter and solar job needs boards and an access plan, or it leaves behind more broken slates than it fixed. | Walkable in mild weather by any trade, and a replacement shingle takes minutes — but it will read as newer than everything around it, because the roof has weathered and the patch has not. Colour lots do not stay available for long. |
| What the elevation reads as | The surface the house was designed around: a thin stone arris at the verge, courses that can diminish up the slope, random widths, and a colour that changes completely in the rain. | A granular texture that works at a distance and reads unmistakably as asphalt at the eaves. A slate-look laminate can borrow the colour and the shadow line; it cannot borrow the edge. |
Which one, and when
Choose natural slate when…
- The house was built with slate and is still on its original frame — the expensive prerequisite, a structure sized for stone, is already bought and paid for, and putting slate back is restoration rather than upgrade. The battens are a consumable and will be renewed whatever you choose; the rafters are the part you cannot buy again cheaply.
- Somebody else controls the covering. A landmark or listed designation, a historic district or conservation area, a planning condition or a covenant can each make like-for-like a requirement rather than a preference, and finding that out after the scaffold is up is the most expensive way to learn it.
- You will hold the house long enough — or hand it on deliberately enough — that the number of times the roof gets opened matters more than the price of opening it once.
- The roof is a simple plane or two, so slate's cost stays in area rather than draining into mitred hips, swept valleys and a dozen abutments.
- The elevation is the point. The roof is the largest single surface on most period houses, and its edge is visible from the street on every one of them.
Choose asphalt shingles when…
- The house was never framed for stone, and the strengthening would be real structural work buying you nothing anybody can see from the ground.
- The existing deck is sheet goods rather than sawn boards. Slate can be nailed to a solid deck, but it needs one that will still be gripping nails in fifty years, so the honest options are re-decking or counter-battening over it — either way an added layer at full roof area that asphalt does not ask for.
- The roof is a warren of dormers, valleys, hips and abutments, where slate's specialist hours multiply feature by feature and asphalt hardly notices.
- The ownership horizon is short, or the house has more urgent claims on the money than the covering does — period houses usually do.
- You want the roof to stay inside the competence of any local crew for the rest of the building's life, with no sourcing problem and no specialist to wait 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
- Does slate really last a hundred years?
- Good slate does, and 'natural slate' is not one product. Both major standards exist precisely because quarried stone varies: EN 12326 codes it on water absorption, thermal cycling and sulphur dioxide resistance, and ASTM C406 grades it S1 to S3 by expected service life. The top grade is where the century reputation actually lives; the lower grades are rated in decades and sit far closer to the asphalt end of this argument than their appearance on a pallet suggests. Ask for the grade and the test data for the specific slate being quoted, not the country it came from — reputation attaches to quarries, not to borders. The second half of the honest answer is that the roof will very likely outlast its own nails. Plan for a mid-life strip and re-lay on new battens and non-ferrous fixings, reusing most of the stone, rather than for a replacement roof — it is a different and much smaller event, but it is not nothing and it belongs in the ownership plan.
- Is my roof strong enough for slate?
- This is the question this page cannot answer and no rule of thumb should. Slate is among the heaviest coverings in common use and asphalt among the lightest, and the gap is wide enough to change what rafter section and spacing are adequate. There are three situations and they need different work. If the house was built with slate and still stands on its original frame, the design load was there from the start and your question is condition, not capacity — rot at the rafter feet, historic spread or sag, and whatever previous trades notched out of it. If it was built with slate but re-covered lighter, the capacity was there once; find out what was removed, cut or replaced when the deck went on. If it never carried slate at all, this is a new structural question for an engineer. In every case the sequence is the same: choose the slate first, get its actual weight from the supplier, add battens and any underlay or boarding, and put that against the real rafter section, span and spacing. Price the roof last, when you know it can go on.
- Which costs more, and where exactly does the money go?
- Slate, comfortably — but the shape of the two costs matters more than the gap, which is why this site publishes no figures for either. Asphalt's cost is close to pure area: one manufactured product, one operation, a deep and competitive pool of crews quoting against each other. Slate's splits four ways — a quarried material, a much smaller trade, fixings and flashings specified to outlast the stone, and a large share sitting in details that scale with perimeter and feature count rather than with area. Then the two lines people leave out on a tall period house. Access is a per-event cost, not a per-area one: the scaffold on a three-storey house costs what it costs whether the roof is opened once this century or three times, so the number of events is the comparison, not the price of one. And disposal runs in opposite directions — sound reclaimed slate has a resale value and a market waiting for it, while worn asphalt is a skip you pay to fill. Run both calculators — the shingle one gives you squares and bundles, the slate one gives you the course count your order gets built from — and then put your own local quotes against those quantities rather than against anybody's published rate.
- Why does the slate calculator ask about exposure when the shingle one just wants dimensions?
- Because the two coverings are genuinely quantified by different methods, and the tools follow the trades rather than each other. Asphalt is an area calculation from end to end: measure the footprint, apply the slope multiplier for the pitch to get the true roof surface, convert that surface to squares, convert squares to bundles. Pitch enters once, as a multiplier, and then stops mattering. Slate is set out course by course. The pitch and the site's exposure determine the headlap the slate must carry, the headlap determines how much of each slate is left visible to weather, and the course count is simply the roof run divided by that exposure — so a shallower or more exposed roof consumes more slate and more courses over exactly the same area. That is why the slate order starts with the slate's size and the supplier's exposure table for your pitch, and only becomes a quantity afterwards. Note also what the course figure covers: field courses from eave to ridge. The starter course, the ridge closure, hips and valleys are separate work and a separate order.
- Can I reuse the slates already on the roof?
- Often a good proportion of them, and this is exactly why the comparison is not a straight price fight. When a roof is coming off because the nails have failed rather than because the stone has, much of what comes down is still sound: it gets sorted on the ground, the delaminating and cracked pieces are set aside, thicker slates are put back at the eaves where tradition puts them, and the shortfall is made up from reclaimed stock or new to match. Judge them by the ring when tapped, by flaking and softness at the edges, by visible delamination at the head, and by how many break during the strip. Two consequences for the decision: it materially changes the material line on the slate side of your estimate, and it is frequently the only way to match a roof whose original slate is no longer quarried at all. Get the slater to assess salvage before you price anything, because that assessment moves the number more than any other single variable on this page.
- Am I allowed to change the covering, and will asphalt look wrong?
- Two questions, and the first can end the project. Listed buildings, conservation areas and specific planning conditions all control roof coverings in various jurisdictions, and 'like for like' is regularly a requirement rather than an aesthetic opinion — ask your local authority in writing before you price anything, because a refusal after the scaffold is up is the worst possible moment to find out. On appearance: at a distance, a dark architectural laminate on a period house reads perfectly acceptably, and anyone claiming otherwise is arguing from a photograph taken at ten paces. The difference lives at the edges. At the verge and along the eaves the covering shows its thickness against the stone or brick, and a thin cleaved arris and a thick laminated one do not look alike from the pavement. If the house sits in a street of slate roofs and you intend to be there a long time, that edge is the thing you will keep noticing.