Roof type guide: slate, clay, concrete tile and metal

By covering

Slate. Routine, but less forgiving. Slates are brittle and individually more likely to crack than tiles, and the fixing usually involves slate hooks or lifting and re-dressing slates around the bracket. Ask how the installer fixes into slate, and how breakages are handled — a competent one will expect the question and carry spares.

Clay tile. Generally straightforward. Older handmade clay tiles are more variable in size and more brittle than modern machine-made ones, and on a historic roof the tiles themselves may have heritage value — which matters on a listed building.

Concrete tile. The easiest to fix to, and the one where weight deserves attention. Concrete is heavier than slate, so a roof already carrying it has less headroom. If the roof was originally slate and was re-covered in concrete tile, that is a “change of roof covering to a heavier material” — one of the five conditions MIS 3002 treats as making a roof unusual and requiring a structural engineer. See structural surveys.

Metal — standing seam. Often the best case. Clamps grip the raised seam without piercing the roof at all, so the principal weathertightness risk simply does not arise. Confirm the clamp is specified for that seam profile.

Metal — profiled or corrugated sheet. Fixings penetrate, so the sealing detail and the condition of the sheet matter. Also common on outbuildings, which are Class A permitted development in their own right and often a better proposition than the house roof.

Flat roofs. Normally ballasted or framed rather than penetrating the waterproofing. Two things follow: the structural load includes the ballast, and permitted development applies a separate limit — since 21 December 2023 the array must not sit more than 0.6 metres above the highest part of the roof. On article 2(3) land a flat roof installation also requires an application for a determination as to whether prior approval is needed.

In-roof systems, on any covering, replace the covering rather than sitting on it. That changes the weathertightness question, the fire performance of the roof build-up, and the structural detail — and it makes most sense when the roof is being recovered anyway.

What to ask, whatever the covering

1. How will you fix to this covering, and with what hardware?
2. What happens if tiles or slates break during installation, and who pays?
3. How is weathertightness maintained at each penetration, and does it affect any existing roof warranty? MIS 3002 requires the contractor to be able to evidence this.
4. What is the remaining life of the covering? If it is short, do the roof first — see below.
5. Has the structure been checked, and by whom?
6. What is the mounting system’s height above the roof surface? It decides whether you stay inside the 0.2 metre limit.

The sequencing question

This one is worth more than the covering choice.

If the roof covering is near the end of its life, re-roof first. Installing over a covering that needs replacing in a few years means paying to remove the array, recover the roof, and refit it — with scaffolding twice.

Signs it should come first: widespread slipped or cracked units, failed underfelt visible from the loft, repeated leaks, or a covering at the end of its typical service life.

The opportunity runs the other way too. If you are re-roofing anyway, the scaffolding is up, the covering is off, and an in-roof system becomes a realistic option.