Conservation rooflights on slate roofs

Plain clay tiles and concrete interlocking tiles are forgiving. Concrete tiles come in regular sizes with a deep interlock, so a window kit is designed around them and the flashing sits in the profile. Slate is flat, thin and laid in overlapping courses with nothing to interlock, so water is kept out by the overlap and by the lead that finishes every edge.

That has two consequences for a rooflight. First, the flashing is not a pressed kit that clips into a profile. It is lead, cut and dressed on the roof to suit the courses you have, and a good conservation rooflight is designed to take exactly that. Second, the visible result depends on the slate work as much as on the glass. A beautiful steel rooflight with its slates relaid unevenly around it will always look like a modification, while the same unit with tidy courses running into it looks as if it has been there for a century.

Slate also ages in ways you can read. A Victorian terrace roof in St Leonards or a Georgian one in Southernhay will have slates that have been replaced in patches over the years, with different colours, thicknesses and sizes.

The survey for a rooflight on slate starts with the roof surface, not the ceiling. We look at several things, and you can look at the same ones from the garden with a pair of binoculars.

Slate size and thickness

Roofs built in the nineteenth century commonly use slate in graded sizes, larger at the eaves and smaller towards the ridge. A rooflight set high on the slope meets small slates and a tight gauge, while one set low meets larger, wider-spaced courses. That affects the flashing and how the unit lines up with the lines of the roof.

Condition and fixings

Old slates are held by nails, and the nails can corrode long before the slate does. Slipped slates, a dished surface and nail rust stains on the surface of the slate are all signs that the courses near the opening may not survive being lifted. If they will not, new slates have to be found, and that is better known at the survey than discovered on the roof on the day.

Matching slate

Salvaged slate of the right colour and size is usually the better match for a period roof, and there is a limit to what a roof can spare. Slate for the work is lifted carefully, stacked in order and relaid, with any breakages replaced from reclaimed stock or from a close new match. Welsh, Cornish and imported slates differ in colour and in how they weather, so a mismatch on a visible slope is something we point out in writing, not something we hope you will not notice.

Position

The rooflight is best set between existing rafters, in line with the courses and, on a street-facing roof, towards the rear slope where the roof allows it. Where planning is in play, the position can also be what decides whether a rooflight is accepted. We cover that on the pages for conservation areas and for listed buildings.

A conservation rooflight is slim on purpose. It is made to sit close to the roof plane, with a narrow frame and a single flat pane, so that from the street it reads as a glazed panel in the roof rather than as a box on it. Permitted development for rooflights allows a unit to stand 150mm clear of the roof surface at most. A conservation unit typically sits well inside that, which is part of why it suits a slate roof.

The weathering is where slate matters most. The usual arrangement has a lead flashing around the frame, with the lead sections laid so that water always runs over the next piece and never under it:

• the sill apron at the lower edge of the unit runs out over the slates below it;
• the side flashings run up each side of the frame, usually with soakers or stepped pieces working into the slate courses;
• the head at the top of the unit is tucked beneath the slates above it, so water on the roof runs onto the glass and out over the apron.

Lead is the traditional material for this work and suits an old roof, partly because it can be dressed tightly to the surface of each slate and partly because it ages to a grey that sits quietly against slate. Some manufacturers supply an aluminium or lead-effect flashing system with the unit. Where a unit comes with its own flashing, we follow the maker's instructions and check how it works with your actual slate before choosing it.

Slate on a house of the Georgian or Victorian period is nailed to timber battens, which are fixed across the rafters. On many older roofs there is no modern underlay beneath, and the underside of the slates may be lime-torched, which means bedded with a lime mortar, or simply open to the loft. A newer roof, or one that has been re-covered, will have a breathable membrane under the battens.

What a rooflight needs is a clean opening framed in timber. The rafters either side of the opening carry the roof, so new timbers go in at the head and sill to make a trimmed frame. How that is done depends on the roof you have:

• on a common rafter roof, a pair of trimmers is fixed between two rafters, which is why we try to choose a unit that fits between rafters rather than cutting one;
• on a roof with larger spacing, as in some cob and stone houses where rafters are irregular and hand-sawn, we measure the actual gaps, because they are rarely the same from one bay to the next;
• on a roof with purlins and wide-spaced principal rafters, the position of the unit is set by the structure, and sometimes a steel or timber trimming detail is specified by a structural engineer.

Cutting or trimming rafters is a structural alteration, and a new rooflight in an existing roof is building work that needs Building Control approval. We handle that notification. On an older house the assessment of the roof timbers is part of it, because decay or insect damage near an opening is a reason to change the plan.

A rooflight newly cut into a slate roof has to reach the U-value limits given in the 2021 Volume 1 of Approved Document L. In practice that means the glass, the frame and the insulation around the unit all have to do their share. Conservation rooflights usually have double glazing in a slim, warm-edge sealed unit, and that is a good option that keeps the profile modest while meeting the standard.

The roof around the opening matters just as much. Insulation has to wrap the frame and continue down the lining, with a vapour control layer lapped onto the frame so that warm, damp air from the room cannot reach the cold side of the roof build. On a slate roof with an open underside, that detail needs thought: you do not want to close off the ventilation the roof has always had, and you do not want a cold bridge at the opening. The steel or aluminium comparison shows how the frame material changes the thermal picture.

This is the part that gets the least attention in most quotes, and the part you see every day. On a slate roof the ceiling beneath the opening is often lath and plaster, either on the rafters in a roof room or on the ceiling joists with a cold loft above. Lath and plaster is made of thin timber strips with lime or gypsum plaster pushed through the gaps to form keys, and it cracks when it is cut carelessly.

So the ceiling is supported before the opening is cut. Battens are fixed either side to hold the plaster in place, the lath is cut cleanly rather than torn, and the edge is formed with a bead so that it meets the lining in a straight line. Where the existing plaster is lime, we make good in lime, because gypsum is harder and less breathable and can crack away from an old ceiling.

The lining between the frame and the ceiling is formed to suit the depth of the roof. Under a sloping loft ceiling it is short and mostly a question of edges. Through a cold loft above a flat ceiling, it becomes a lightwell and the question of splay arises: angling the sides outward so the opening is wider at the ceiling. There is more on that in how a rooflight is finished inside.

All of that is written into the finish schedule that comes with the quote: the ceiling and roof build, how the lining will be formed, the finish material, whether the room is handed over bare or painted, and how dust will be managed.

Class C of the General Permitted Development Order (Schedule 2, Part 1) is the right that usually covers a house rooflight, subject to a 150mm limit on projection from the roof surface and a bar on rising above the highest point of the roof. Conservation areas allow Class C, though an Article 4 direction may take it away, and a listed building must have listed building consent for any change to its exterior. Dartmoor and Exmoor are national parks and have some restricted rights. What applies to your address varies, so ask the planning authority before you commit.

Slate is often the reason a planning officer cares about a rooflight in the first place. On a street of slate-roofed Victorian houses, the thing that is noticed is how the unit sits on the roof. A slim frame, a flat pane, a dark finish, no trickle vents on the outer face and tidy lead flashing are details that planning officers look at, and they are the details conservation rooflights are built to get right. Our planning answer for conservation rooflights goes through the rules in more detail.

A rooflight on slate is a careful job, and it goes in a set order.

1. The slates in the working area are lifted by hand, stacked in sequence and protected.
2. The battens are cut back and the rafters trimmed, with the ceiling below supported from inside before any rafter is cut.
3. The unit is fitted to the trimmed opening and fixed to the rafters, then levelled to the plane of the slates.
4. The lead flashing is cut and dressed, starting with the sill apron and working up the sides to the head.
5. The slates are relaid in their courses against the lead, with any broken ones replaced from the matched stock.
6. Inside, the lining is formed, insulated, boarded or plastered, and edged.

The slates are done first and the lining last, because plaster dust and wet work should not happen over a roof that is still open. Dust sheets and a protected route through the house are set up before the first slate moves, and the room is left clean.

These are the questions that separate a good quote on a slate roof from a vague one. A written answer to each should be in the quote and in your finish schedule.

• What are the slates, and will the ones around the opening be relaid or replaced?
• What is the flashing made from, and how is it dressed to the slate?
• How far does the unit sit above the roof plane?
• How wide is the bay between rafters, and does the unit fit without cutting one?
• What is the ceiling, how will it be supported, and what will the edge be made good in?
• Who handles the Building Control notification?

We answer the last one ourselves: for any notifiable work the Building Control notice is ours to send, and the installation carries our 10-year workmanship guarantee. If you would like a survey of your slate roof and ceiling, the quote form is the place to start, and a few photographs of the roof and the room help us a great deal.