Rooflights in Welsh Slate Roofs

Thin, regular, split by hand

Welsh slate is a metamorphic rock with a near-perfect cleavage, which means it splits along one plane far more readily than in any other direction. A skilled splitter can take a block down to sheets thin enough to roof with, flat, of even thickness, and strong enough to carry their own span between battens. Nothing else available in Britain does this as well.

Once split, the sheets are dressed to rectangular sizes and holed. Historically the sizes carried their own names, which is why old specifications and old slaters talk about a roof in terms that mean nothing to a modern merchant’s catalogue. The important point is that within one size, every slate is effectively interchangeable with every other one.

Colour comes from the quarry district rather than from any treatment. The familiar blue-grey is one; there are purples, heathers and greens from other beds, and a roof laid from a single delivery has a consistency of tone that is very hard to reproduce later.

Slate does not rot and does not absorb much water, so the material itself commonly outlives the fixings holding it up. A slate lifted off a roof after generations is frequently sound, still ringing when tapped, and perfectly good to lay again. That fact does a great deal of work later on this page.

Why Victorian builders chose it

Before the railways, a roof was covered in whatever came from within carting distance, which is why the older building stock of any English county is a map of its local geology. The railway network broke that. Slate from north Wales could be loaded at the quarry and set down almost anywhere, and it arrived cheaper than the local alternative in most places.

It also arrived lighter. A slate roof weighs a fraction of a stone one, which meant thinner rafters, smaller purlins, less masonry under the wall plate, and pitches that could be laid shallower than a heavy covering allows. For a builder putting up a long terrace to a budget, that combination was decisive. Speed came into it too: a covering where every unit is identical is quick to lay and quick to price.

The result is the housing most people picture when they picture a Victorian street. Terraces, villas, rear outriggers, station buildings and the extensions added to older houses through the same decades are all overwhelmingly slated, and in Oxfordshire that stock sits alongside much older stone and tile rather than replacing it.

For our purposes the inheritance is a covering that is standardised in a way nothing older is. That is why the roof window industry designs its products around it, and why a slate flashing kit is the one every manufacturer makes.

Regularity makes the geometry simple

A slate roof has one gauge. Every course sits the same distance above the one below it, from eaves to ridge, and every slate in a course is the same width. That means the coursing can be read from the outside, before a single slate is lifted, and the position of an opening can be worked out on paper with confidence that the roof will agree.

The head of the opening lands on a course line. The sill lands on a course line. The number of courses to be taken out is known in advance, the width in whole slates and half slates is known, and the trimming below can be planned against actual rafter positions established at survey rather than guessed at. Compared with a covering laid to a changing gauge, this is arithmetic rather than judgement.

It also means the manufactured flashing kits do what they are supposed to do. A slate kit is designed to sit against a flat plane of even-thickness material, interleaved into regular courses, and a slate roof is exactly that. The kit is still ordered against the unit, the covering and the pitch rather than by covering alone, but the match is a genuine one.

The honest summary is that on slate, the geometry of the opening is the straightforward part of the job. Nearly all of the risk has moved somewhere else.

Breakage, and allowing for it

Where it has moved to is handling. A slate that has been on a roof for a hundred years is still hard, but it has become brittle at the edges and around the nail holes, and it is fixed to a batten by nails that are no longer what they were.

Opening a slate roof means working upwards from the intended sill line, releasing each slate by cutting or drawing its nails, and lifting the slates above so the courses can be taken back cleanly. Every one of those movements loads a thin plate of stone in a way it has not been loaded since it was laid. Some will break. Slates two or three courses beyond the opening, which are not being removed at all but which have to be lifted to release the ones below, are the ones most likely to go.

The way it is managed is provision. A stock of matching salvage is on site before the covering is touched, sorted and sized, so that a broken slate is replaced from the pile rather than becoming a problem at four in the afternoon. The older and more weathered the roof, the larger that allowance needs to be, and an installer who tells you nothing will break has either not done many or is not telling you the whole picture.

Matching salvage across quarries

Because slates from one delivery are consistent, a roof laid from one delivery is consistent, and anything introduced later has to live up to that. This is where matching gets harder than it first appears.

Two differences bite. The first is source. A blue-grey slate from one quarry district and a blue-grey slate from another are both blue-grey until they are on the same roof in flat light, at which point one of them is a shade off. Size can match exactly and thickness can match closely while the tone does not, and slate does not blend in over time the way some coverings do.

The second is weathering. An exposed slate face acquires a surface: lichen on the sheltered parts, a dulled and slightly roughened texture elsewhere, and a tonal variation across the slope that comes from decades of water running the same routes. A clean salvaged slate from a demolition site has its own history rather than this roof’s history, and a genuinely new slate has none at all.

The most reliable stock is the building itself. Where a house has a hidden slope, a rear outshot, a lean-to, an elevation facing a blank wall, slates can be taken from there to close the visible work, and the newer or imperfectly matched material goes back where nobody looks at it. That is a slater’s habit rather than a clever trick, and it is worth asking for by name.

Nail sickness and roof condition

Nail sickness is the ordinary way an old slate roof reaches the end of its life, and it has nothing to do with the slate. The nails corrode where they pass through the batten and the slate, thin down, and eventually let go. The slate then slides, sound and undamaged, and lands in the gutter or on the path.

The signs are worth knowing because they change the advice. Slipped slates appearing in ones and twos across a slope rather than in a line after a gale. Nail heads visible on the surface. Slates tapped back with a hook here and there over the years. Underlay visible from inside, or no underlay at all, which is common on roofs of that age. Any of those suggest the covering is at or near the point where lifting it locally will start something.

This matters directly. Opening a roof in that condition disturbs a covering with very little life left, and the sensible conversation is about sequence rather than about the rooflight. Sometimes the answer is that the covering is renewed first and the rooflight is set into a fresh slate roof, which is both easier and better value than doing it the other way round. Sometimes the two are coordinated so the scaffold serves both.

We install rooflights rather than re-cover roofs, so where a slope has reached that point we say so at survey and let you decide the order of work. That is a more useful answer than proceeding and hoping.

Why flush detailing shows best

Slate is the flattest and most uniform covering there is, which is exactly why the appearance of a rooflight matters more on it than on anything else.

A stone or handmade tile roof has texture. Its surface is broken up with shadow, variation and irregular edges, and a small addition has something to hide in. A slate slope has none of that. It is a single dark plane, read from the street as one shape, and anything standing above it casts a hard shadow along its head and down its sides. That shadow is what the eye finds first, from a distance where the finish of the frame is no longer distinguishable.

The same flatness works the other way. A unit set down into the plane, finished dark externally, on a covering that is already dark blue-grey, has almost nothing to give it away. It becomes a rectangle of a slightly different dark against a dark roof, and on a wet day it is very nearly invisible. No other covering gives a flush unit that much cover.

That is why the flush profile is so often the specification on visible slate elevations, and why the difference between a flush unit and a low profile one is more obvious here than on a textured roof. It is also why getting the unit down into the plane, which is a fitting decision about battens and trimming rather than a product decision, is worth the extra labour on this covering in particular.

Head, side and apron flashings

The perimeter of a rooflight in a slate roof is made of three distinct parts, and they fail for different reasons, so it is worth understanding them separately.

The head is above the unit, where water arriving from the whole slope above has to be diverted round the opening rather than allowed to reach the frame. This is the part carrying the most water and the part where a shortfall shows soonest. The sides are interleaved into the coursing: soakers, or a formed side section, worked in course by course as the slates are laid back, so that water crossing sideways in wind is carried out onto the slate rather than in behind it. The apron is below, taking everything shed by the glass and the frame and delivering it back onto the covering below.

Each part laps over the one downstream of it in the direction water travels, never the other way round, and each lap is generous rather than minimal. The reason is that the design case is not rainfall. It is driving rain, where water arrives across the slope and is pushed up it by wind, so a lap sized for water running quietly downhill is the wrong lap.

Exposure changes the sums. A slope facing the prevailing weather with nothing in front of it wants more side cover than a sheltered rear slope, and that is a specification decision made at survey rather than a default taken from a box.