Roof windows for loft conversions in Devon homes

A converted loft is a room under a slope. There is no wall to put a window in apart from the gable ends, which many houses in terraces and semi-detached pairs do not have as free walls. The roof itself is the wall, so the windows go in the roof.

That makes the planning of the openings part of the design of the room, and it affects the work in the roof in several ways. The rafters have to be trimmed and doubled around each opening. The insulation has to be continued around the frame so there is no cold spot. The ceiling is fixed to the underside of the rafters, so each window needs its own lining returning from the frame to the sloping plasterboard. And the glass has to be big enough to bring in daylight, because the floor area of a loft room is often long and narrow, running back from the stairwell to the gable.

The right place to start is the floor plan and the roof slope together: where the bed goes, where the stairs land, where a person will stand upright and where the roof slopes down to knee height.

Two measurements steer the position of a window. One is the height of the bottom edge above the finished floor, and the other is how far up the slope the top edge reaches.

Seated, you look out at about the height of the sill when it is placed around a metre up from the floor, measured vertically. That suits a bedroom or a study, and it leaves wall below for a bed head or a desk. Put the window lower and you can see out from a bed or from standing, but you give up the space beneath. Put it higher, and the window becomes a source of daylight and ventilation that you look up into rather than out of.

In a room with a steep roof slope, the same height gives a long window, and a long window can be too much of a good thing in a small room; a shorter window set higher up the slope can serve better. On a shallower roof the window takes up less of the surface and several together may be needed.

Most loft conversions put the windows in the rear slope, away from the street, which also avoids the front elevation questions that arise in conservation areas. The front slope can take rooflights too, but its appearance matters more to neighbours and to the planning authority. We go through that in the section on planning below.

The number follows the rafters and the room. Rafters in Devon houses are commonly spaced somewhere between 400mm and 600mm apart, and a window width is chosen to fit between a pair of them, or to straddle one or two with the rafter trimmed. A standard size of roof window needs no more than one rafter cut, but a wider frame can mean doubling timbers at either side, which adds structural work. Our answer on how wide a roof window can be between rafters explains the limits.

Designers tend to aim for a healthy glass area relative to the floor area, and loft skylights in Exeter homes are sized the same way. The practical answer is that one window in a bedroom of ordinary size is the minimum, two windows spread along the slope feel brighter, and a pair side by side with a short gap, sometimes called a combination, reads as a single large opening.

Size codes follow a letter for width and two digits for height, for example MK04 or CK02. The letter pair sets the frame dimensions, and a surveyor chooses the code once the rafter positions are known. If a quote gives only a code, ask for the size in millimetres as well.

Before windows, a loft conversion has to work as a room. The Building Regulations expect a safe staircase to reach it, with enough headroom over the flight, and a floor strong enough to carry a bedroom. Those are outside what a window installer does, but they affect where windows can go.

The stair usually arrives at the high point of the roof, under the ridge, and the room is tallest there. A window close to the stairwell has the most headroom around it and is the easiest to open and close. Windows further down the slope sit over the low part of the room, where you are likely to be sitting or lying, so their handles and the swing of their sash need thinking through. We cover the two ways a roof window opens in our guide to top-hung and centre-pivot roof windows.

Where the roof is too shallow to give standing room, or the room needs more floor area than the slope allows, a dormer may be the better answer. That decision has been argued over in many houses, and we set it out in roof window or dormer for a loft room.

A bedroom or other habitable room in a loft conversion needs a way out in a fire. Approved Document B sets out the usual arrangement: a protected route down the stairs, and, in many houses, a window that a person can climb out of, or that firefighters can reach with a ladder. A roof window can serve as that escape window when it meets the size and position rules in the Approved Document, and the Building Control officer decides whether a particular window, in a particular position, is acceptable.

In broad terms, an escape window needs an openable area of a minimum size, a minimum clear width and height, and a bottom edge not too high above the floor, so that someone can get through it without climbing. Those figures are in the document and not in this page, because they have been revised and you should read the current edition. The point to carry forward is that the escape window is chosen first, and the other windows are chosen around it. Our page on whether a roof window can be a fire escape sets out the details to ask about.

A loft room is the warmest room in the house in summer and, without care, among the coldest in winter. Roof windows help with both.

For fresh air, an opening roof window provides purge ventilation, so a stuffy bedroom can be aired quickly. Most also have a ventilation flap at the head that admits a little air with the sash shut. Hot air rises, so a window opened at the high point of the slope lets out the warmest air, and a second window lower down lets cooler air in, which gives a draw through the room.

For heat in summer, glass in a south-facing slope gains a lot of warmth. Reflective or solar-control coatings on the glass, external awnings or blinds, and internal blinds all help, and they are best decided when the windows are ordered, since external blinds can be fixed to the frame. In winter, double or triple glazing with warm-edge spacers and good insulation around the frame keeps the room comfortable, and Approved Document L sets limiting U-values that the new window must achieve.

Condensation in a loft room usually comes from warm, damp air meeting a cold surface. A properly insulated lining, a vapour control layer lapped onto the frame and a little regular ventilation keep the window reveals dry. A lining that is cold, and so wet, is the usual reason that paint blisters at the sill.

In a loft room the ceiling follows the rafters, so the lining of a roof window is short. It runs from the frame down through the thickness of the rafter, the insulation and the plasterboard, and it often measures well under 300mm. That is less than the deep shafts of a flat-ceilinged house, so the lining is quick to form, but the way it is formed shows in every part of the room.

The top and bottom of the lining are usually set so that the head runs level and the sill stands vertical, and the sides splay slightly to ease the line into the sloping ceiling. A splay on a short reveal is a subtle gain; it softens the edge of the opening and spreads the glow of daylight along the slope. We explain the idea in what a splayed rooflight lining is.

The sloping ceilings in a conversion are normally plasterboard, either taped and jointed or skimmed, and the lining is finished to match. In an older house where the converted roof still has lime plaster or lath beneath the new board in places, the junction needs care. The lining must meet the ceiling with a sharp, bead-protected edge, and the paint line against the frame has to be dead straight, because in a room with only roof windows for light every edge is lit from above and shows.

That is why the finish is part of the job, not an afterthought. Each quote includes a finish schedule that records how each window lining will be formed and finished, so the room is agreed before work begins. Our guide to how a rooflight is finished inside shows the layers in order.

For a house, roof windows normally come under Class C, Part 1, Schedule 2 of the General Permitted Development Order. Three conditions govern it: the window stands no more than 150mm off the roof plane, it stays below the roof's highest point, and a window in a side slope is obscure-glazed and fixed unless its opening section is over 1.7m above floor level. Householder rights are not available to flats and maisonettes. A conservation area may carry an Article 4 direction that removes the right, a listed property cannot have external work done without listed building consent, and Dartmoor and Exmoor have some rights restricted. Where it varies, check with the local planning authority, which in Exeter is the city council and elsewhere is the relevant district council or park authority.

A loft conversion as a whole has its own rules, and a dormer or a rear extension to the roof is judged against separate permitted development limits that this page does not cover.

The Building Regulations treat a new roof window as building work, since rafters are trimmed and the window must satisfy Part L. The conversion as a whole needs Building Control approval, covering structure, stairs, fire safety and insulation. For the window work itself, we make the notification. In older roofs, bats and their roosts are legally protected, and if signs are found during the work it stops until advice has been taken, so it is sensible to mention any known bat activity at the survey.

Lofts differ with the age and shape of the house. A Georgian or Regency terrace in Southernhay or St Leonards often has a shallow-pitched or mansard-style roof, with slate and old timbers, and the rafters may be irregular or hand-cut, so every window position has to be checked against the real timbers. A Victorian villa in Torquay or Exeter usually has a steep slate roof, tall enough for a good loft room, with a centre-pivot window working well on the slope.

A 1930s semi in Heavitree and many postwar houses have concrete or clay tile roofs at a moderate pitch, with factory-made trusses in later houses that limit where rafters can be cut. Truss roofs are a structural matter, and an engineer's design is usually needed before any conversion is planned around them. Cob cottages in the Culm and Exe valleys can have thick walls, low eaves and irregular rafters, where the windows may need a custom approach.

New builds in Cranbrook and similar estates are mostly roof trusses, and the conversion is a bigger project. In every case, the best start is a survey that looks at the real roof, not a plan from a catalogue.

A good quote for loft roof windows names the model and its opening type, gives the size in millimetres, states the flashing for your roof covering, and says who handles the Building Control notification. The flashing kit has to suit the roof: codes begin with ED, using EDW for profiled roofing such as interlocking tiles, EDZ for flat roofing such as plain tiles or thicker slate, and EDL for thin slate. Our guide to roof window flashing for slate and tile roofs explains the choice.

It should also describe the finish inside, which is what the schedule is for. Read the written quote before you agree it, and ask about anything that does not match how you picture the room.

The full picture of the work, from choosing the model to the installation day, is on our roof window installation page. When you are ready, ask for a quote and tell us about the room, and we will arrange a survey.