Roof window installation in SouthamptonSized to your rafters, set with the slates.

A roof window is a glazed sash and frame made to sit in the plane of a sloping roof. Unlike a skylight added as an afterthought, it is a complete window: it opens, it has its own weatherproofing and it comes with a flashing kit that ties it into the tiles or slate around it. A pitched roof skylight in Southampton usually means exactly this: a window that lies flat to the slope and does not stick up above it.

The reason it works well in a pitched roof is that the roof already does half the job. Rafters carry the weight, the battens hold the covering and the slope sheds water, so a roof window needs only a trimmed opening and a flashing kit to join it to that system. The glass sits low in the slope, close to the covering, which keeps it quiet from the street and rarely changes the look of the house.

Houses that take them well

Roof windows suit a loft with enough height to stand in, a landing that has no window of its own, a pitched kitchen or garden room with sloping rafters, and a chalet bungalow whose upstairs rooms are in the roof. They are less obvious on a roof with very little pitch or a covering that cannot take a standard flashing kit, and those cases are worth raising at survey before anyone orders a unit.

What changes the plan is when the window goes in. In a loft conversion the rafters are exposed and the window is part of the structure. In a house with a finished loft room, the plasterboard, insulation and sometimes the tiles have to be opened up. The earlier it is decided, the less is undone, which is why we ask to see the plans at the start of a conversion and not at the end.

The flashing kit is what makes a roof window watertight. It is a set of folded metal pieces, usually aluminium or lead-coated, that wrap round the frame and tuck under the tiles at the sides and top, then lie over the covering at the bottom. A kit is made for a particular type of covering, so the window frame and the roof must be matched before the order is placed.

Plain clay or concrete tiles need a kit with a low-profile apron that sits snug against the tile. Interlocking tiles and pantiles, which have deeper profiles, need a kit with a deeper apron that rises over the roll of the tile. Natural slate and fibre-cement slate need a flatter kit with soakers that can be slid beneath each course. Choosing the wrong one means gaps at the tile edge, and no amount of sealant makes up for it.

Who does what at the slope

On a new loft, the roofer strips the covering back to the battens round the opening, lays the underlay in a tray around the frame and then fits the flashing kit as the covering goes back. On an existing roof, the same steps are done on a smaller scale. We supply the kit, set the window, and the roofer, or one of our fitters, dresses the flashing to the covering.

Because the flashing is built up in stages, the roofer needs to be there at the right moment, not a week afterwards. That is why we ask for the covering type at the survey and fix the date for the window against the roofer's visit. The sequence is to batten, then felt, then flashing, then covering, then the lining inside.

The detail changes with the covering, and our guide to flashing kits for tiles and slate covers each type in turn. On older roofs with uneven or nailed-down slates, we also look at the pitch of the slope and the state of the surrounding courses, since new flashing cannot sit well on a tired slope.

The kit is chosen for the covering, and the covering is chosen by the roofer. Neither choice is difficult, but both belong on paper before the first tile comes off.

Part L of the Building Regulations covers the thermal performance of new and replacement windows. In existing homes, a roof window is treated as a window and not as a rooflight, which matters because the limit is tighter. Table 4.2 of Part L, as revised in 2021, sets roof windows against the window figure, 1.4 W/m²K, with Window Energy Rating Band B as the other route. Rooflights, which is a separate category, take 2.2 W/m²K.

Southampton's housing gives roof windows plenty to do. The inter-war semis that fill much of the outer city are often hipped, and a hip-to-gable conversion turns the sloping end of the roof into a vertical wall, giving a loft a full-height room. The new gable wall can take a normal window, and the rear slope takes roof windows for the remaining light. The result is a roof with two kinds of opening that are planned together.

The Victorian and Edwardian terraces nearer the centre are different. Houses of that age often have a rear outrigger, a lower roof that extends back from the main house, and a main roof with a steep pitch. Slate was the usual covering, and some roofs have been recovered in modern slate or tile since. A roof window in a rear slope on these houses is normally out of sight from the street, which helps with appearance, and the flashing kit is chosen to suit the covering that is actually up there.