Planning a loft room around VELUX windows

A loft room is shaped by the roof, not by the walls. Under a pitched roof, only a strip down the middle has the height to stand in, and that strip is what you are really designing. Stairs usually need about 2m of headroom, so the line of the staircase often fixes where the door and the landing go before anything else.

Take a tape and mark three zones on a plan of the loft floor. The first is full height, where you can walk without ducking. The second is the sloping edge, where the ceiling comes down to knee or waist height and furniture goes. The third is the dead space under the eaves, good for storage and not much else.

A trussed roof, the kind built from the 1960s onwards with a web of thin timbers, is a poor starting point because those timbers cannot simply be cut out. Older cut roofs with purlins and common rafters give far more freedom. We check which you have before any layout is promised, since it changes how many windows fit and where.

In a typical semi or terrace, the rear slope is the favoured place for roof windows, because it keeps the street elevation unchanged and tends to be the simpler planning route. That is a sensible default, not a rule. A bedroom at the front of a Victorian villa in Torquay may want its window on the front slope, and the planning position is different there.

In planning terms a house rooflight normally needs no application, so long as it sticks out no more than 150mm from the roof plane and stays below the highest point of the roof. A window facing sideways must be obscure glass and fixed shut unless the part that opens is over 1.7m from the floor. A loft conversion that adds a dormer is a different matter, with its own volume limits and a bar on dormers on a front slope facing the highway, so a dormer is a separate planning conversation. Owners of flats and maisonettes cannot rely on householder permitted development, a listed building needs consent before its exterior is altered, and a conservation area may have an Article 4 direction that withdraws the right. If any of that describes your house, ask the planning authority. In Exeter that is Exeter City Council, and in a Dartmoor edge village it may be the national park authority.

How many windows? Think of the room rather than the roof. A single wide window centred in a small bedroom reads as calm and deliberate. Two smaller windows side by side with a rafter between them can look fussy, unless the room is long enough to want both. Whatever the number, the windows look best when they line up with something in the room, such as the bed, the desk or the centre of a gable wall, instead of sitting wherever the rafters happened to allow.

Two things decide how a roof window behaves in use: how steeply the roof slopes, and how high the sill sits above the floor. Together they decide whether you can reach the handle, lean out to the glass, or clear a desk beneath it.

Start with the pitch. A shallow roof means the window lies closer to flat, so it catches daylight from higher in the sky but is also more exposed to summer heat. A steeper roof puts the window closer to a wall, which feels more like an ordinary window. VELUX publish the pitch range each model is approved for, and it is worth checking against your own roof before you fall for a model.

Then the opening type. VELUX roof windows come in two main families, and you will see the codes on every specification sheet.

Centre-pivot (GGL, GGU)

The sash turns about a bar across its middle, with the handle at the top. GGL is white-painted pine and GGU is white polyurethane, which suits damp rooms such as a loft shower room. Pivoting lets you clean the outer pane from inside, and the window opens within its own footprint, so it clears furniture and sloping ceiling lines. It works best where the sill sits low, because the handle is then easy to reach above.

Top-hung (GPL, GPU)

The sash is hinged at the top and swings out from the bottom. GPL is painted pine and GPU is polyurethane. When open it gives a larger clear gap at the base, which suits a window set higher up the slope, over a bed head or a stair, where you stand beside it and use it as an opening rather than a skylight overhead. The handle sits at the bottom, so you can reach it from below.

The sensible sill height depends on the room. In a living space, a low sill suits someone sitting down. In a bedroom, a sill that sits near knee height may look pleasing and suit a bed against the wall below. Wherever it sits, think about children and falls, and about whether a window that opens wide near the floor is right for the household. Specifying laminated glass on the inner pane is good practice in rooms like these, and our guide to VELUX glazing options goes through the choices.

If the loft room is a bedroom, or the new storey sits above the first floor, fire safety shapes the plan. Approved Document B sets out how a loft conversion should provide escape, and for a two-storey house that usually means a protected route down the stairs, with fire-resisting doors at the new landing and a means of getting out of any bedroom if the stairs are blocked.

One common route is an escape window: a roof window that is big enough to climb through and low enough to reach. The guidance sets a minimum clear opening size and a maximum height for the bottom edge of the opening above the floor. As we understand it, a roof window can serve if it meets those numbers when open, but the details depend on the type of window, its opening angle and the exact edition of the guidance. We do not quote them here, because your Building Control officer will tell you which apply to your house, and a window that does not meet them cannot be relied on.

What that means in practice is a decision made early. If one window must be the escape, its position is fixed by sill height and by the stairs, and everything else is placed around it. A different stair arrangement is also a Building Control conversation.

Roof windows are made to standard sizes, and the standard sizes are built around the usual spacing of rafters. VELUX size codes are a letter for width and two digits for height, for example MK04 or CK02. The letter gives the width of the frame, and the digits the height. Our page on finding your VELUX size code covers reading one off an existing window.

If the opening is narrower than the gap between two rafters, no structural work is needed beyond fitting trimmers, and the job is quick. Wider than that, one or more rafters must be cut and the load carried around the opening by new, doubled timbers. That is structural work, and it is why cutting a new window into an existing roof counts as building work that needs Building Control approval. In an older roof we also check the condition of the timbers, since rot at the foot of a rafter near a failed old window is a surprise nobody wants to find while the roof is open.

Think in terms of the finished ceiling, not just the frame. A window set partly over a rafter will have one splayed side and one narrow side unless the rafters are trimmed to suit. On a roof of tiles or slate, the flashing kit also has to match the covering and the depth of the battens, and for profiled tiles it is a different kit from the one for slate. Our VELUX flashing kits explained sets out the codes and what each is for.

A room directly under the slates is the first to warm up in July. The reasons are simple: the ceiling is the roof, there is no cold loft between, and glass in the roof slope sees more sun than a wall does. The fix is spread across the design rather than found in a single product.

Insulation is the first line, and it has to continue behind the lining around the window, or the lining becomes a cold or hot surface. Next comes the glass. Solar control coatings reduce the heat that gets through, at some cost to the clearest daylight, and acoustic or laminated panes are an option where wanted. Then blinds. An external awning-style blind stops heat at the glass, while an internal blind catches it after it has entered the room. External blinds do more for overheating and cost more.

Ventilation counts too. An opening window can purge hot air quickly, and one at the low end of the slope with another higher up lets warm air leave while cooler air comes in. We cover that more in our answer on whether VELUX blinds are worth it, and the glass options are compared in the glazing guide above.

This is the part most plans skip. Once the roof window is in, the gap between the frame and the sloping ceiling has to be closed with a lining, and in a loft room the lining is short, because the ceiling follows the rafters. A short lining still has choices. The sill can be horizontal or follow the line of the slope, the head can be level so it meets the ceiling at a clean angle, and the sides can splay out slightly.

The usual guidance is to set the head horizontal and the sill vertical, so the lining runs level at the top and plumb at the bottom. That gives the window a settled, architectural look and lets daylight move into the room instead of stopping at the plaster. Our full guide to how a rooflight is finished inside covers boarding, skimming, beads and the paint line in more detail.

In a loft room there are a few details worth deciding at the planning stage. The first is the material. A modern plasterboard ceiling takes a board and skim lining. An older ceiling in lime, as in a Georgian house where the loft is being brought into use, takes lime. The second is where the window meets a knee wall or a dormer cheek, because the corner there is awkward, with three surfaces meeting at odd angles. The third is paint, agreed before work starts and written on the finish schedule.

A loft room is a change of use of the roof space, and the whole conversion is notifiable, not just the windows. Building Control looks at the structure that carries the new floor, the stairs, fire protection and escape, the insulation, and the windows, which must meet Part L. If your loft conversion uses a builder for the structure and a specialist for the roof windows, agree who notifies Building Control and who holds the certificate, so nothing is missed at the end.

For our part of the job, we install the roof windows to current Building Regulations and handle the Building Control notification where that work is notifiable. That covers the new openings, the trimming of rafters and the windows themselves. It does not cover the rest of the conversion if someone else is building it, and we say so at the quote stage, so the responsibilities are written down.

A tidy brief makes a survey faster and the quote more precise. Before you contact anyone, have these decided or at least roughly sketched.

• The use of the room, since a bedroom, a study and a shower room want different glass, blinds and ventilation.
• The slope or slopes you would like windows on, and whether the house is in a conservation area or listed.
• Whether one window must serve as an escape, and the layout of the stairs that will fix where it can go.
• The structure of the roof, if you know it: cut rafters and purlins, or trusses.
• The ceiling material, so we can plan the lining and the making good, including whether any of it is lath and plaster.
• How you want the lining left: ready to paint, painted, or trimmed in timber.

Photographs of the loft and the roof from outside help a great deal. At survey we measure the rafters, check the pitch, note the roof covering and look at the ceiling the new opening will meet, and then write all of it into a quote with a finish schedule. If you would like us to plan the windows in your loft room, ask for a quote. More on the product itself, including replacement and the range, is on our VELUX window installation page.