Do Conservation Rooflights Suit Slate Roofs?

Slate roofs are thin, flat and regular, and they have a clean, dark look. A bulky unit with a pale frame breaks that look at once. A conservation rooflight keeps to it. The frame is dark and slim and it lies close to the plane of the slates, so from the ground the roof still reads as slate with a thin line of sky in it.

The two also share a history. Cast-iron skylights were set into slate roofs long before modern roof windows existed, and the conservation type takes its proportions from them. That makes it a natural match on Victorian terraces, chapel roofs, mill conversions and outbuildings in Derby and the towns around it.

The slate roof skylight you choose should also follow the roof's lines. Aligned with the courses and the rafters, it looks as though it was always there. Off the grid, it looks like an afterthought. We set the opening out against the roof before cutting.

Slate is brittle and the slates on an older roof are often held by nails that have aged, so the work is done by lifting the slates in a careful order. The slates in the opening and around it are taken off, set aside and kept in sequence. Battens are trimmed or replaced to suit the opening. The rooflight is fixed and flashed, and the slates are relaid round it.

The flashing is where a slate roof is won or lost. Water runs down the slate face and across the lap, so the flashing is stepped and tucked beneath the slates above and over those below, following the lap. A flashing that ignores the lap lets water track in. For a slate roof the flashing is made to suit the slate gauge and bonding, not taken off a stock sheet.

Slates that crack when lifted are replaced with ones that match. Reclaimed slates are often the best match for colour and thickness. The aim is a roof where the opening is the only thing that has changed.

Where the opening needs a rafter cut or moved, the load is carried by trimmers to the neighbouring rafters, and because that changes the roof structure it is notifiable under the Building Regulations. The firm handles the notification with the local authority or a registered building control approver.

Start with the state of the slate. If the slates are sound, the covering can usually be opened and relaid. If they are slipping, delaminating or held by rusted nails, the survey will say so, and it may make sense to relay a larger area while the roof is open. A slate roof that has little life left is a conversation to have before choosing a rooflight position.

Check the pitch. Slate roofs are normally steep enough for the type, but a shallow pitch and a poorly detailed flashing do not go together. The survey records the angle and the detail suited to it.

Then check the slope and its visibility. If the roof sits in a protected area or on a listed building, the council has a say, and the rear slope is the safer candidate. For listed buildings, consent is needed first, and the process is described in the listed building guide. Planning for ordinary houses is linked from our planning guide.

Finally, check the spot inside. A rooflight on slate serves a landing, a desk, a bath or a bed best where the opening is not too far from the place that needs light. The survey marks the spot on the floor, then finds the position on the slope that serves it and the roof together.

The free survey starts with the room, then the spot in it, then the slate, before any price is discussed. A fixed written quote follows, and the installation carries a 10-year workmanship guarantee in writing. On the roof, a straightforward single rooflight tends to need a day or two.

Book through the free survey form. For the wider picture of this work, see conservation rooflights, or read about the difference between the types in choosing between a conservation unit and a standard window.