Skylights and roof windows: what they cost and how they leak

A roof window is one of the better things you can do to a dark room. A loft, a back bedroom on a terrace, a landing that has never had daylight in it. The room changes completely and it does so for a fraction of what any structural work would cost.

It is also a job with a very binary outcome. Fitted correctly a roof window lasts twenty years and you forget it is there. Fitted badly it leaks within two winters, and by the time you notice, water has been getting into the roof structure for a while. There is very little in between.

What it costs

Market figures, not my price list.

For a standard sized window in an existing pitched roof, supplied and fitted, £1,000 to £2,000 covers most jobs. The window itself is usually £300 to £700 of that depending on size, glazing and whether it opens manually or electrically.

What moves it:

• Size. Bigger windows need more of the rafter cut away and properly trimmed.
• Whether a rafter has to be cut. If the opening fits between existing rafters it is straightforward. If a rafter has to be cut, the load has to be carried by trimmers, and that is structural work.
• The tile type. Every roof covering takes a different flashing kit, and the ones for slate and for deep-profile tiles cost more and take longer to dress in.
• Internal finishing. Plasterboarded reveals ready to decorate is the normal expectation. Vaulting the ceiling into the window costs more.
• Access and scaffold. Two storeys and up will usually need it.
• Electric opening, blinds, rain sensors. All extra, all optional.

A roof lantern on a flat roof is a different and larger job. Those are usually £2,500 to £6,000 depending on size, because there is a structural kerb to build and the flat roof around it has to be detailed correctly.

The flashing kit is the whole job

Here is the part worth knowing before you take any quotes.

A roof window comes with a frame. What keeps water out is not the frame, it is the flashing kit that sits around it and ties it into the roof covering. That kit is specific to the covering it is going into:

• Slate and flat tiles take a low profile kit and the window sits down into the plane of the roof
• Interlocking concrete tiles take a deep profile kit that steps over the tile shape
• Deep pantiles take a different one again
• Combination kits exist for windows fitted in groups

Using the wrong kit, or no kit at all, is the single most common reason I get called to somebody else’s roof window. The signature of a bad fit is silicone. Mastic run round the edge of the window where the flashing should be doing the work. It looks fine, it holds for a couple of seasons, then it fails and water goes into the roof.

The question to ask any quote: which flashing kit are you using for my roof covering? Somebody who fits these regularly will answer it immediately. Somebody who cannot is planning to use sealant.

The other things that go wrong

The opening is not trimmed properly. A rafter is cut and the load is not carried by anything. This is structural and it is not visible once the plasterboard goes up.

No insulation collar. There should be an insulation collar around the frame within the roof build-up. Without it the frame is a cold bridge, and you get condensation running off the inside of the window every winter. People then complain the window leaks. It does not; it is cold.

Fitted too low on the slope. Below about 15 degrees pitch most roof windows are outside their designed range and will not shed water reliably.

The reveals are wrong. This is an aesthetic point rather than a leak, but it matters. The reveal below the window should be vertical and the one above should be horizontal. That throws light down into the room and back across the ceiling. Square reveals all round give you a much darker room from the same window.

Planning permission and conservation areas

For most houses in Leeds, fitting a roof window is permitted development and needs no planning application. The usual conditions apply: it must not project more than 150mm beyond the plane of the roof, and it must not be higher than the highest part of the roof.

The exceptions matter here, though, more than in most cities.

Conservation areas. Headingley Hill, parts of Chapel Allerton, parts of Roundhay and a number of others. In a conservation area, a window on a roof slope facing a highway generally needs permission, and even where it does not, the council will usually expect rooflights on the rear slope. A conservation-style window, which sits flush with a flat external profile and often has a central glazing bar, is frequently the difference between approval and refusal.

Listed buildings. Listed building consent is required regardless of permitted development, for anything.

Flats and maisonettes. Permitted development rights do not apply. You need permission.

The practical advice is to ring Leeds City Council planning and ask before you order anything. It is a five minute phone call and it is a lot cheaper than fitting a window that has to come out.

Where they go on Leeds housing

On the slate terraces through Headingley, Hyde Park, Woodhouse, Burley and Kirkstall, roof windows are how most loft conversions get their light. That means a slate flashing kit and a recessed fit sitting down in the plane of the roof. Using a raised profile kit designed for concrete tile on a slate roof is the most common mistake I see round here, and it leaves a gap you can see from the street as well as one water gets into.

On the interwar semis at Adel, Cookridge, Weetwood, Moortown and Horsforth, the roofs are interlocking concrete tile and the deep profile kit is right. Those roofs are also often steeper and easier to fit into, and the jobs tend to be straightforward.

On the stone slate out at Bramhope, Otley and toward Ilkley, the stone is thick, heavy and laid in diminishing courses. The opening has to be formed carefully and the flashing dressed to a much deeper unit. It is a slower fit and it should be quoted as one. Anybody pricing it the same as a window in a concrete tile has not looked at the roof.

Replacing an old one

A lot of the calls I get are about roof windows fitted in the eighties and nineties that have reached the end. The signs: misting between the panes on a sealed unit, a frame that has gone soft where it is timber, draughts, or a leak that has started after twenty good years.

Replacing like for like in the same opening is usually straightforward, and modern units are dramatically better on insulation than anything from that era. If the opening size has changed, or you want to go bigger, you are back into trimming rafters.

Worth knowing: a window that has been leaking for a while may have caused rot in the timber around the opening. That gets found when the old one comes out, and it is worth asking a quote to cover the possibility rather than being surprised by it.

The summary

Get the flashing kit right for your covering, get the opening trimmed properly, get an insulation collar in, and check the conservation area position before you order anything.

Do those four and a roof window is one of the best value improvements available to a house. Skip any of them and it is a leak with a nice view.