Where is the best place for a roof window?

Glass in the slope collects much more sky light than a pane in a wall, since it looks up at open sky and not across a garden and a fence. The direction of the slope changes the character of that light. A north-facing slope gives steady, even light that suits a study, a workroom or an artist's corner and rarely makes the room hot. A south-facing slope is bright and warm, lovely in winter and likely to overheat in summer unless it has shading, which we deal with on our page about skylights that make a room too hot.

East slopes bring morning light, which suits bedrooms and kitchens, and west slopes bring the low evening sun. Because daylight from above reaches far into a room, one well-placed window can often do the work of two on a wall, so there is a real case for fitting one larger unit instead of several small ones, provided the rafters allow it.

People forget to stand in the room before choosing a position. The height of the sill decides what you see. A window that starts at floor level on a steep slope gives a view of the ground and the neighbours' windows. One set higher gives sky, treetops and rooftops. If you want to see out of the window while seated, in bed or at a desk, the lower edge needs to sit lower than a wall window's would, and the top edge of a pivot window needs enough clear floor in front to swing the sash.

Headroom is the other check. In a loft room the ceiling slopes down towards the eaves, so a window placed too low can leave you stooping to reach the handle or bumping into the frame. Put a tape on the floor of the room and mark where you would stand to open the window. If that spot is under the slope, a different position or a window with a top-hung handle may suit better.

The roof has its own geometry. A window has to fit between rafters, or the rafters have to be reframed around it, as our page on trimming rafters explains. It should keep clear of a chimney stack, since the flashings of the two would crowd each other and the water coming off the chimney would meet the window head. It should stay away from valleys and hips, where the covering is complicated and the water load is greatest, and from purlins, which are the horizontal beams that carry the rafters partway along the slope.

Two further limits are practical. A window too close to the ridge is hard to flash, and one too close to the eaves may sit behind the gutter line. Rooflights, windows and lanterns are different units and suit different roofs, so our guide to types of rooflight is worth reading if you are not sure which you need.

A roof window is an alteration to the roof, and Class C of the permitted development rules is the part that deals with it. Under Class C the window may not protrude more than 0.15 metres from the original slope surface, and it cannot sit above the highest point of the old roof. There is also a Class C condition for side slopes, meaning the ones that face a neighbour or the side of the plot: each window there must be obscured glass, and it has to stay closed unless the parts that open are over 1.7 metres up from the room floor.

The rules can be removed by an Article 4 direction or a planning condition, and they do not replace listed building consent. In a conservation area or a National Landscape, Class C can still apply, but speak to Swindon Borough Council's planning team, or whichever council covers your address, before settling on a position, especially on the slope that faces the street. If you live in a listed building, a separate consent is needed whatever Class C says.

If the window serves a loft bedroom or other habitable room, it may need to work as a way out. Approved Document B, paragraph 2.2, contains the 4.5 m rule. A habitable room on an upper storey that is no higher than 4.5 m above the ground, and that is reached by a single stair, needs a window or external door that can serve as an emergency exit, or else a route straight onto a protected stair. Paragraph 2.10 describes the escape window: a clear opening of 0.33 m² or more, no narrower or shorter than 450 mm, and the lowest edge of that openable area not more than 1100 mm above the floor.

That last dimension affects where you put the window, because a sill set high on the slope can fail it. Storeys higher than 4.5 m above ground follow separate provisions, and building control will confirm what applies. Send us photographs and the room dimensions and we will tell you which positions suit your roof. On the visit we photograph the slope as it stands, and a written price follows. See our skylights and roof windows service for more.