Can you fit a roof window without removing tiles?

A roof window is not laid on top of the covering. It sits down in the plane of the roof, with its frame supported by the timber underneath and its edges sealed by a flashing kit that tucks under the tiles above and laps over the tiles below. For any of that to happen, the covering in the opening zone has to be lifted first.

The sequence on a pitched roof runs in a set order. The tiles or slates across the opening and a course or two either side come off and are set aside. The felt or breather membrane is cut back and later dressed to the frame so water is carried around the window rather than into the roof. One or more battens are removed, and depending on the width of the window a rafter may be cut and trimmed with new timber to carry the load around the hole. The window goes in, the flashing is fitted, and then the covering is reinstated up to the edges of the flashing.

So tiles move on every genuine roof-window installation. The honest version of the question is not whether any come off, but how far the disturbance spreads, and the answer is: not far. The roof is opened where the window goes and put back everywhere else.

The size of the patch depends more on the type of covering than on the size of the window. Some coverings lift and relay cleanly, others fight you.

• Interlocking concrete tiles. These are the large tiles on a great many post-war and 1930s roofs. They clip and overlap in a way that lets you lift a defined block, fit the flashing, and clip them back down. Disturbance stays close to the window.
• Plain clay or concrete tiles. Small, double-lapped tiles laid in a bond pattern. More of them come off around the opening because the bond has to be kept true, and the flashing kit for a plain-tile roof carries a wider apron and soakers to suit the shallower profile.
• Natural or fibre-cement slate. Slates are nailed, so the ones directly around the window are usually cut and refixed with clips or hooks rather than simply relaid. Expect a modest number that cannot be reused and are replaced with matching slates.
• Pantiles and deep-profile tiles. The strong S-curve means the flashing has to be dressed into a moving surface, so the working margin is a little wider again to get a clean, watertight line.

Across all of them the principle holds. You are opening a rectangle a bit larger than the window and reinstating the rest. A single 780 by 980 mm roof window in an interlocking-tile roof is a contained job. A wider window, a plain-tile or slate covering, or a rafter that has to be cut will each widen the working area, but none of them turns into re-covering the roof.

There are two situations where no tiles are involved, and it is worth knowing which one you are in before you picture a stripped roof.

The first is a flat or very low roof. A rear kitchen extension with a flat roof has no tiles at all, so a rooflight or a lantern drops into a timber upstand and is sealed with the roof membrane rather than with a tiled flashing. That is a different trade of work entirely, and it is covered under flat roof skylight installation rather than here. If your roof is flat, the tile question does not arise.

The second is a smaller top-light such as a sun tunnel or light tube, which brings daylight down through a reflective shaft from a small dome on the roof. It still lifts a tile or two to get the collar through the covering, so it is not a no-disturbance option, but the opening is far smaller than a full window and no rafter is cut. It is a sensible route where a room needs daylight but the roof structure or the layout inside makes a full window awkward. It brings in light, not a view or ventilation, so it suits landings, bathrooms and box rooms rather than a room you sit and look out of.

If the roof you are opening faces south across the estuary, the tiles come off and go back in a day, and that is not where the outcome is won or lost. The glass is. A south pitch collects sun on the glazing from mid-morning until the sun swings west, and along the seafront there is an open horizon and light bouncing off the water on top of that, so the room can gain a lot of heat through a window that was fitted with no thought given to it.

This is the point where the covering stops mattering and the specification starts. On a south or west pitch, ask for solar-control glazing with a G-value around 0.3 or below, which limits the share of the sun’s energy that comes through as heat while still letting the room feel bright. On a north pitch, standard clear glazing nearer 0.6 is the right call and the winter warmth is welcome. The tile type sets how the window meets the roof; the orientation sets what the glass has to do once the sun is on it, which is the whole argument behind our energy-efficient skylight installation work.

Choosing the window, the flashing to suit your covering and the glass to suit the orientation is the substance of a pitched roof skylight installation, and it is worth settling all three before anyone lifts a single tile. A new or enlarged rooflight is also notifiable under Building Regulations, with Part L setting a ceiling on the U-value and Part K covering the safety glazing, so the specification is not only a comfort question.

Start by working out what your roof actually is. Note whether it is pitched or flat, and if it is pitched, look at the covering: large interlocking tiles, small plain tiles, slate, or a deep pantile. Then note which way the roof slopes, because that decides the glass more than anything else does.

If you want a full window and a view, plan for a contained opening and a flashing kit matched to your tiles. If you only need daylight in an awkward spot, a sun tunnel lifts far less of the roof. If the roof is flat, the tile question falls away and the job runs a different way. Either way, replacing a tired existing rooflight is the moment to improve the glass, because the opening is already there and the only real choice left is what goes in it. A roof window installation in an existing pitch is well-trodden ground.

If you would rather we sized it and specified the glass for your orientation, tell us the covering, the pitch and roughly where the room faces. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the tiles that have to move.