Installing a new roof window in a pitched roof

This page is about creating an opening that does not exist yet. Replacing an old roof window in the same opening is a simpler job, usually done by window installers, and is covered on our page about replacing roof windows. Enlarging an existing opening, or joining two windows into one, counts as new work because the structure changes.

Roof windows come as centre-pivot and top-hung designs, with top-hung better where there is furniture below or where the window doubles as an escape route. Conservation-style rooflights sit lower in the roof and have slim glazing bars, and are often preferred on older houses. For dark corridors, a sun tunnel brings light through the roof with a much smaller opening.

Rafters normally have to be cut to make the opening. The Planning Portal explains that the cut ends need new support from doubled timbers, called trimmers, across the top and bottom of the opening, and that the rafters on either side may need strengthening because they now carry extra load. A wide window, or several windows side by side, may need a structural engineer to check the design.

On the outside, the roofer strips back the covering around the opening, fits the frame, and weatherproofs the joint with a flashing kit matched to the covering: a different kit for thin slates than for deep profiled tiles. Inside, the lining, insulation around the frame and a vapour control collar prevent cold spots and condensation. Getting the window high enough for a view but not so high it cannot be reached is a decision to make with the installer before anything is cut.

On a house in England, adding a roof window is usually permitted development under the rules for other alterations to a roof. The Planning Portal sets the limits: the window must not project more than 150 millimetres from the plane of the roof slope, must not be higher than the highest part of the roof, and side-facing windows must be obscure-glazed and, if they open, only above 1.7 metres from the floor. These rights do not apply to flats, and the council may have removed them in a conservation area. Listed buildings need listed building consent.

Building Regulations approval is generally needed for a new roof window, because the structure is altered, the roof must carry the load, the window has to meet energy efficiency standards, and fire performance matters close to a boundary. Replacement window schemes such as FENSA cover replacement roof windows, not new openings, so the installer will normally involve Building Control.

If the window serves a room in a loft conversion, it may also have to work as an escape window, which sets minimum opening sizes and a position that can be reached. That depends on the fire strategy for the whole loft.

Roof windows on a south-facing slope can overheat a room in summer. External awnings or blinds work better than internal ones, and it is easier to order them with the window than to add them later. Windows placed without checking the rafter spacing can end up oddly positioned or require more structural work than expected. And a roof window above a bed or a bath needs thought about access for cleaning and opening, especially if it is out of reach.

• window type, size, glazing and opening mechanism, and the flashing kit
• structural work: trimming, strengthening, and any engineer's design
• internal lining, insulation, vapour control and making good
• blinds, awnings or remote-operated openers if wanted
• scaffolding or access, and protection of the room below
• Building Control application and fees, or who arranges them

Your request is free. Roofers who cover your postcode see what you want built, without knowing who you are. The first to buy it pays Offrano, and the request then closes. We email you the roofer's name, address and phone number before it gets your contact details.

There is no promise that a roofer takes up the request. After 21 days without a buyer it is withdrawn, and you are never bound to accept the quote. Offrano does not inspect workmanship, so ask the roofer how it will handle the Building Regulations sign-off.