Two identical roofs can need noticeably different quantities of slate, and exposure is one of the reasons. It is worth understanding before you compare quotations.
Slate is the covering where quotations are hardest to compare, because “natural slate” is a category rather than a specification. On the Wirral there is an additional variable that inland quotations do not have to think about, and it changes the quantity rather than the quality.
Headlap is the amount by which each slate overlaps the slate two courses below it — slate is double-lapped, so each course covers the two beneath. Increase the lap and each slate covers less exposed area, so the roof takes more of them.
The lap is not chosen. It is set by three things:
So two roofs of identical area and pitch, one sheltered and one facing the estuary, genuinely need different numbers of slates. That is a real cost difference and not a contractor being awkward.
Slates are held on by nails, or in some situations by hooks. Whichever is used, the pattern comes from a calculation under BS 5534 that takes the exposure of the site as one of its inputs, alongside altitude, terrain, building height and pitch.
There is a second consideration near the sea. Salt-laden air is harder on fixings than inland air is, which points towards appropriate fixing metallurgy for the covering rather than the cheapest available — and towards renewing the lead at every junction during a re-roof rather than re-using what is there.
Natural slate is graded, and the grading is published. Producers declare performance under BS EN 12326-1 in three tests — water absorption, behaviour under thermal cycling, and resistance to sulphur dioxide — and in each case grade 1 is the best available. Ask which slate is being quoted, by producer and product name, and ask for its declaration of performance.
| Material cost | Laying speed | Weight | Service life | |
|---|---|---|---|---|
| Natural slate, new | Highest | Slow | Light | Very long |
| Reclaimed slate | High | Slow — sorting and re-holing | Light | Unknown remainder |
| Fibre cement slate | Moderate | Faster — uniform units | Lightest | Shorter |
| Slate-effect tile | Lowest | Fastest | Heaviest | Moderate |
The bottom row is worth reading carefully. A slate-effect concrete tile is heavier than the material it imitates, the units are much larger, and at close range the thick edges give it away. On an exposed roof it also brings the weight question back into play on a structure that was framed for slate.
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.
That distinction matters more here than inland, because on an exposed peninsula there usually is a storm to point at. Being able to tell storm damage from nail fatigue is the difference between a sensible repair and a treadmill, and it is covered in more detail in our guide on repair versus replacement.
Where the fixings have gone but the slates are sound, there is a third option: strip the roof, renew the battens, underlay, fixings and lead, and relay the original slates. Everything is handled twice so it is not cheap, but against a full roof of new slate it often comes out ahead and the building looks unchanged.
Two separate systems. 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, where the test is applied more tightly.
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 — 0.16 W/(m²K) for a roof in an existing dwelling. A patch repair does not trigger it, and neither does a roof over unheated space such as a detached garage.
We will tell you the slate, the size, the headlap and what the exposure of your site does to all three.
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