Question answered
Fitting a roof window in a slate roof
The short answer
Yes, a roof window can be fitted in a slate roof, using a flashing kit made for slate, a frame fixed between the rafters and the slates cut and re-hung around it. Slate is brittle and often old, so the job is slower and slates are salvaged. In a conservation area or on a listed building, check consent first, and where a local Article 4 direction applies, as in parts of Shoreham and Worthing, street-facing roof alterations may need planning permission.
A roof window can go into a slate roof, but slate asks more of the job than tile does. The kit, the handling and the consent questions all differ, and the roof's age and listing status come first.
A flashing kit made for slate
A flashing kit is the metal surround supplied with a roof window; it ties the frame into the covering and steers rainwater round it. Kits are made for particular coverings, because the way the metal meets the roof is different for a flat-profile tile, a profiled tile and a slate. Slate is thin and sits in tight overlapping courses, so the kit has to be shaped to dress down beneath the slates above and over the ones at the sides.
Around the sides, a slate roof relies on soakers, small sheets of lead or other metal that sit under the slate edge and turn up against the frame. A kit for slate is meant to be built up with the slates course by course, not laid over them. For more on the kits themselves, see our page on roof window flashing kits.
Using a tile kit on a slate roof is a mistake that tends to show as a leak at the corners within a season or two.
Handling slate without breaking it
Slate splits, and a slate that has lasted decades can crack as it is lifted. MyBuilder's cost guide puts the life of good slate at 80 to 100 years or more, so an old roof has had a long time for nail holes to wear and edges to flake. The sensible approach is to remove the slates around the opening carefully, keep every sound one, and allow spares for those that do not survive.
Matching is the next problem. Replacement slates need to be close in size, thickness, colour and finish, and a clean new slate beside weathered ones will show. Salvaged slates from the opening can be used on the visible side, with the odd new one tucked in where it will not stand out.
Walking on slate is itself a cause of damage, so the access method matters. Our answer on fitting from inside describes how the roof is reached.
Pitch, headlap and fixings around the opening
The opening should not make the slate weaker than it was. Cupa Pizarras reads BS 5534 as giving natural slate a lowest pitch of 20 degrees, at moderate exposure and with a 115 mm overlap between courses, and hook fixings are not used on any pitch below 25 degrees. The overlap is the headlap, the distance each course of slate covers the course two below it.
The slates just beside and above a window take extra load from the water diverted past the frame. They should be fixed properly, with the nails or clips the pitch and exposure call for, and not left loose. On a coast exposed to gales and driving rain, a poorly held slate beside an opening is an early failure point.
Where the roof is old and tired as a whole, an opening is also a chance to look at the battens and underlay beneath it. We would tell you if the slope needs more than a window.
Old slate streets: consent and local directions
Slate roofs are often in older streets, so check the status of the building before choosing a window. A listed building needs listed building consent for work altering its special character, and a new roof opening often does. In a conservation area, Class C permitted development for roof windows survives the designation itself, but an Article 4 direction can withdraw it.
Adur & Worthing Councils hold directions of that kind. For Old Shoreham and Kingston Buci, Class C alterations to the roof, rooflights included, need permission where the roof faces a road or public right of way. In Worthing, Warwick Gardens and Warwick Place have directions touching roof alterations on the street side, and Western Row is annotated for all dwellinghouse rights.
Check with the planning team before ordering anything. Our page on roof window Building Regulations deals with the separate approval side.
Glazing, security and what we check
The window itself also has to meet current standards. Table 4.2 of Approved Document L allows a roof window at 1.4 W/(m2.K) at most, or at Band B on the energy rating scale; see our answer on roof window U-values. Old roofs in conservation settings sometimes suit a low-profile, flush-fitting style, and our answer on walk-on rooflights shows how flat glass is built for other uses. A second concern is break-in, covered in roof window security.
Our survey examines the slates, battens, pitch and rafter positions, and we photograph each for you. A written quote follows as one total figure. The wider skylights and roof windows page covers the full range of work we do.
If the slates are original and in good order, we will often suggest keeping as many as possible and limiting the work to the area around the opening, rather than reroofing the slope for the sake of one window.
Slate and roof windows: two more questions
Will new slates look different from my old ones?
They can, at first. New slate is darker and cleaner than weathered slate, and a patch may show for a year or two. We reuse sound slates from the opening wherever possible, so that the new work blends in.
Can I use a standard tile flashing kit on a slate roof?
No. Tile kits are shaped for a different covering, and on slate they tend to leak at the corners. A kit made for slate, built up with the courses, is the right choice.
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