Rooflights in natural slate roofs

Why it is easy

Flat, so soakers sit tight with no roll to bridge. A flat profile flashing kit suits it, and a flush conservation unit beds in cleanly.

Slate also cuts cleanly with the right tool, so slates closing against the unit are dressed accurately rather than approximately.

Why it is unforgiving

The courses are dead regular and the lines read from a long way off. A unit half a course out, or a few millimetres off parallel, is obvious from the ground in a way it never would be on an irregular clay roof.

Setting out follows the slate courses, agreed with a tape on the roof rather than from the ceiling below.

Broken slates

Old slate is brittle at the nail holes and breakages are normal when lifting. Matching matters: new Welsh slate beside weathered Welsh slate is visibly different for years.

Reclaimed slate of the right size and thickness is worth sourcing, and we check availability at survey.

Nail sickness

On very old slate roofs the fixings corrode before the slate itself fails, so slates slip. Where we find it, the honest position is that the roof needs attention beyond the rooflight, and we will say so rather than fitting into a covering that is failing.

Slate on a period property

Frequently in a conservation area. A flush conservation rooflight suits slate particularly well because the flat covering allows a genuinely flush setting, which is what makes it acceptable on a visible slope.

The easiest covering to work with

Slate is the most cooperative roof surface in this trade. It is flat, so the flashing arrangement is a straightforward soaker and apron rather than a deep formed profile, and individual slates can be lifted and replaced without disturbing their neighbours if the work is done carefully.

Slate roofs are also generally steep, usually 40 degrees or more, because slate needs the pitch. That is good news: water sheds fast, gravity does much of the weathering work, and the headroom gain from a roof window is generous.

How slates are held

Either nailed to battens or, on older roofs, hung on pegs or torched with mortar underneath. Removing a slate means releasing or cutting the fixing, and replacing one in the middle of a slope means a slate hook or a tingle rather than a nail, because you cannot get at the head of the slate to nail it.

That is normal work and it is worth knowing, because a slate reinstated with a visible metal tingle is what a proper repair looks like rather than a shortcut.

The flashing

A flat profile kit, matched to the window size. Soakers interleave with the slate courses up each side, an apron sits at the sill, and a head flashing goes at the top with the slates above lapping over it.

The sequence is what makes it work: underlay collar first, then flashing, then the slates reinstated over the flashing rather than under it.

What varies between slate roofs

Welsh slate is hard, thin, regular and long lived. Century old slate is frequently still sound and can be reused.

Imported and reclaimed slate varies in thickness and quality. Some delaminates with age, and on those roofs a higher proportion breaks on removal.

Fibre cement slates look similar from the ground and behave differently. They are more brittle at the edges and matching a weathered one is harder than people expect.

What is underneath

On a Victorian or Edwardian roof that has never been stripped, frequently no underlay at all: slates on battens with the rafters visible from inside.

Modern rooflight weathering assumes a membrane to lap into. Where there is none, the detail is built around the opening instead. That is entirely normal on a period property, it takes longer, and it should be in the quotation rather than discovered on the day.

Conservation contexts

Slate roofs are common on listed and conservation area property, and a conservation rooflight sits particularly well in slate because the low profile reads correctly against a flat covering.

Why slate is the easiest covering to work with

Slate is thin, flat and regular, and it is laid to steep pitches. All three help.

Because the covering is flat, a flashing lies close against it with no voids underneath, and a low profile unit reads as genuinely low rather than merely low relative to the battens. On a deep profile tile the same unit stands proud.

Because the pitch is steep, water sheds fast and the flashing is doing less work than it would on a shallow roof.

And because individual slates can be lifted and replaced without disturbing their neighbours, the covering goes back cleanly.

The correct fixing for a replacement slate

A slate in the middle of a slope cannot be nailed, because its head is under the courses above. A replacement is held by a slate hook or a tingle: a metal strip fixed to the batten below and turned up over the tail of the slate.

A visible tingle is what a correct repair looks like. A roofer who has nailed through the face of a slate to avoid one has put a hole in the covering.

What varies

Welsh slate is hard, thin and extremely long lived, and century old slate is frequently still sound enough to reuse. Reclaimed and imported slate varies, and some delaminates with age, so breakage rates on removal differ considerably between roofs.

Where an exact match cannot be found, the answer is to take sound slates from a hidden slope for use around the rooflight and put new ones where they will not be seen.

What lies below the slate

On an unstripped Victorian roof, frequently no underlay at all. That is normal, it means the detail is built around the opening rather than lapped into a membrane, and it should be identified at survey rather than discovered on the day.