How Much Does a New Slate Roof Cost in Chester?

“Slate costs more than tile” is true and unhelpful. The interesting question is why, because the answer tells you which decisions on your roof actually move the figure — and several of them are not the ones people focus on.

The Units-Per-Square-Metre Problem

A large concrete interlocking tile covers a substantial area in one piece and is single-lapped: each course overlaps only the one below. A slate is much smaller and is double-lapped, meaning each course overlaps the two courses below it.

The consequence is that a slated roof takes several times as many units per square metre as an interlocking tiled roof of the same area, and every one of them is lifted, positioned, aligned and nailed by hand.

That is the cost gap, and it is a labour gap rather than purely a materials one. Natural slate is also the more expensive material, but on many roofs the labour difference is the larger part.

What Determines the Number of Pieces

Slate size

Slate comes in a range of sizes. A larger slate covers more area per unit, so fewer units, fewer nails and less handling — which generally makes a larger slate cheaper per square metre to lay, even though each slate costs more. It is not a free choice, though, because size interacts with the next two factors.

Headlap

Headlap is the amount by which each slate overlaps the slate two courses below it. Increase it and each slate covers less exposed area, so you need more of them — more material and more labour for the same roof.

Headlap is not arbitrary. It is set by the pitch of the roof, the size of the slate and the exposure of the site, working to BS 5534. The principle is straightforward: the lower the pitch, the greater the lap, because water moves more slowly down a shallow roof and wind can drive it further back up under the slates.

Pitch

Which is why two roofs of the same measured area, covered in the same slate, can need noticeably different numbers of slates. A shallow, exposed roof needs more headlap, which needs more slates.

How Natural Slate Is Graded

Slate is the covering where quotations are hardest to compare, because “natural slate” is not a specification — it is a category containing products that differ enormously in quality, price and service life, and the difference is not visible on a sample in a merchant's yard.

There is a published way to tell them apart. BS EN 12326-1 requires producers to declare how their slate performs in three tests, each expressed as a letter and a number. In every case, 1 is the best grade.

W — water absorption

• W1 — absorption at or below 0.6%. The better result, and no further testing required.
• W2 — absorption above 0.6%. The slate must then pass a 100-cycle freeze/thaw test before it can be used at all.

T — thermal cycling

How the slate behaves through repeated wetting and drying, which is really a test of how the metallic inclusions in the stone oxidise over time.

• T1 — no change, or only a surface colour change that neither affects the structure nor produces runs of discolouration.
• T2 — oxidation producing runs of rust staining down the roof, but no structural change.
• T3 — oxidation penetrating the slate, with the risk of holes forming.

S — sulphur dioxide resistance

• S1 — unaffected by strong exposure. Acceptable in all conditions.
• S2 — measurable surface softening; thickness has to be increased to compensate.
• S3 — affected by weak exposure; a minimum thickness and further adjustment are required.

The Four Things Called “Slate”

Read the last row carefully. A slate-effect concrete tile is not a lighter covering and it is not slate: it is heavier than the material it imitates, the units are much larger, and at close range the thick edges give it away. If the reason for choosing slate is appearance, it defeats the purpose.

Why Slate Roofs Need Replacing — and the Third Option

Slate itself is extremely durable. What fails is the fixings: every nail on the roof went in at the same time and has sat in the same conditions since, so they corrode together. The symptom is slates slipping in unrelated places over several years with no storm to explain them, and once that is general there is no repair for it — the slates either side of the one you refix are on the same clock.

That has a commercial consequence worth knowing. A failing slate roof often has a perfectly sound covering on it. Stripping the roof, renewing the battens, underlay, fixings and lead, and relaying the original slates is a genuine third option between patching and buying a whole new covering.

It is not cheap — everything is handled twice, sorted, graded and re-holed, and you buy in material to cover the losses. But against a full roof of new Welsh slate it frequently comes out ahead, and the building looks exactly as it did. Worth having priced alongside a new covering rather than assumed either way.

Where the Rest of the Labour Goes

• Cutting. Slate is cut around hips, valleys, dormers and chimneys, and cutting slate is slower and more careful than cutting tile.
• Junctions. Older slated property typically carries more abutment and valley detailing than a modern tiled roof, and every one needs lead.
• Sorting. Where slates are being reclaimed, everything is handled twice.
• Battens and underlay. Renewed as standard on any proper re-slate.
• Access. Scaffolding, and a footway licence where one is needed.

The Regulations, in Brief

Two systems, and they are separate. Planning: a like-for-like re-roof does not materially affect the external appearance of a building, so under section 55 of the Town and Country Planning Act 1990 it is not development at all and needs no permission. A visible change of covering can be a different matter, particularly inside a conservation area.

Building regulations: where more than 50% of the roof's surface area is renewed, or the work forms part of a major renovation, regulation 23 of the Building Regulations 2010 requires the roof to be brought up to current thermal standards so far as that is technically, functionally and economically feasible. A patch repair does not trigger it, and neither does a roof over unheated space such as a detached garage. The work is signed off through building control or by a roofer registered under a competent person scheme, and you should receive a certificate.

More New Roof Guides

All new roof guides →
New Roof Installations Chester →