Does a Roof Window Work in a Slate Roof?

Natural and reconstituted slate both lie thinner than a profiled tile, with a flat, close-fitting surface rather than a raised interlock, and that flatness is an advantage once a roof window goes in rather than an obstacle.

A tile flashing kit has to be pressed to clear a raised waterway without crushing it flat; a slate kit does not, because there is no waterway to clear in the first place. The step flashings slide in under each course rather than over a moulded edge, and the apron above the frame can sit closer to the roof plane than it would on a deep interlocking tile. That is one reason a roof window reads as a genuinely clean junction on a slate roof, with a slim, flat run of lead either side of the frame rather than a bulkier stepped one. Our guide to flashing kits for slate, tile and plain tile roofs goes through how each covering is met in full; this page is about whether the window itself sits comfortably in a slate roof at all.

Natural slate is heavier per square metre than most tiles, and that weight is carried by the roof structure as a whole rather than by anything specific to the window, so it rarely changes what size of roof window can go in.

What it does change is how the covering behaves at the cut edge. A slate is holed and nailed or hooked in a fixed pattern set by its thickness and the batten gauge beneath it, and a slate trimmed to meet the frame is cut and re-holed to keep that same fixing, rather than left loose on a cut corner the way a snapped tile sometimes is. Reconstituted slate, a fibre cement product cut to a standard size and thickness, behaves more predictably at the cut edge than natural stone does, since every unit in a batch is the same thickness and takes the same fixing; natural slate varies piece to piece, which is one reason the actual roof is measured on survey before a flashing kit is ordered rather than assumed from the property type.

Slate roofs are laid across a wide range of pitches, and a roof window's manufacturer states its own minimum pitch for slate, usually shallower than the minimum for a plain tile, because a flatter, closely lapped covering sheds water more predictably at a lower angle than a deep profiled one does.

A roof already close to a covering's minimum before a window goes in is worth checking early, since the window's own flashing has a stated pitch range too, and the two figures have to agree. Where a roof sits below the usual pitch for either the covering or the window, our guide to roof windows below the usual pitch sets out what changes in the specification.

Collyweston stone slate, the limestone slate that roofs the villages around Stamford and the Nene valley, is a different proposition again: it is laid by hand in diminishing courses of random widths rather than a uniform gauge, and it is not one of the coverings a manufacturer's catalogue flashing kit is built around.

A roof window still fits this covering, but the lead soakers either side of the frame are cut and dressed to the actual width and thickness of the stone in that particular course, agreed on survey rather than ordered to a standard size. Where the roof sits in a conservation area, that is a position we check for the address before any specification is written, since it can affect what the finished flashing and any visible frame are allowed to look like once the job is complete.

Many roof windows on slate roofs across Peterborough and the surrounding villages were fitted decades ago, and a straight swap for a new unit is usually simpler than a first-time installation, since the opening and the frame size are already established.

What still has to be checked is the slate itself: whether the courses around the old frame were disturbed or damaged when it was first cut in, whether the slate has weathered evenly with the rest of the roof, and whether the covering has been renewed at all since the original window went in. A frame swapped into slate that has moved slightly out of true, or lost a fixing over the years, needs that addressed at the same time rather than flashed over. Our guide to replacing an old roof window in a tiled roof covers the same survey steps for a slate roof as for a tiled one.

None of this changes what the window itself offers once it is in: the same sightline planning, from the bed, the sofa or the desk, that decides frame size, opening type and reveal shape applies whatever the covering is outside. A flat slate roof does, if anything, make the frame edge read more cleanly against the sky, since there is no deep tile shadow line competing with it.

Our guide to setting the sightline for sitting and standing and our page on lining a roof window so the opening widens towards the room both apply directly to a slate roof, since the covering decides the flashing rather than the view.

Our roof window installation service surveys the actual covering, its gauge and its pitch before any kit is specified, and every quote comes with a sightline sketch showing the view the finished window will give from where you actually sit. See our page on what affects the cost of a rooflight for how the covering feeds into a quote alongside the window itself.

Does a slate roof need a different roof window to a tiled one?

No. The window itself is the same unit; it is the flashing kit and the minimum pitch that are matched to slate rather than to a tile.

Is natural slate harder to work with than reconstituted slate?

It takes more care rather than being harder in principle. Natural stone slate varies piece to piece in thickness, so each cut edge around the frame is measured and fixed to suit, where a reconstituted unit is a consistent size throughout.

Can a roof window go into Collyweston stone slate?

Yes, but the flashing either side of the frame is dressed by hand to the actual stone in that course rather than ordered as a standard kit, and any conservation position is checked for the address first.

Will a roof window look different set into slate compared with tile?

The frame and glass are identical; what differs is the covering around them. Slate's flatter surface tends to give a cleaner, less shadowed line at the frame edge than a deep profiled tile does.