What size roof window should I get?

1. Rafter spacing

A window that fits between existing rafters needs no structural work. One that does not means cutting and trimming, and on a trussed roof that means an engineer.

Establish the spacing before choosing anything. The difference in light between the widest window that fits and the next size up is usually small; the difference in cost is not.

2. Cill height

Where the bottom of the window sits decides whether it is a window or a light source. Around 900mm to 1100mm above the finished floor gives a view standing and from most seated positions.

Higher and it becomes a source of daylight you cannot see out of, which is fine in a bathroom and disappointing in a bedroom.

This is worth deciding in the room with a tape measure rather than from a plan.

3. Glass area

A common rule of thumb is glazing of about 20 per cent of floor area for a habitable room, more in a north facing space.

Remember the code is the frame size and the glass is meaningfully smaller, typically 120mm to 180mm less each way. Two small windows lose more to frames than one larger one.

Two rather than one

In a long loft room, two windows spaced along the slope light it far more evenly than one large one, and each may fit between rafters where a single wider one would not.

Escape

If the room is a habitable loft room, an escape window is generally required, and it has minimum clear opening dimensions that constrain the size. That has to be in the specification from the start.

Start from the structure, not the catalogue

The most useful constraint is usually what the roof allows without structural work, and it produces two or three real options rather than a free choice.

On a trussed rafter roof, a unit narrow enough to sit between two trusses at 600mm centres needs no trimmer, no engineer and no structural design. Anything wider is a different job at a different price.

On a traditional cut roof, trimming is ordinary carpentry, so the constraint is looser, but rafter spacings on older buildings are irregular and the clear span at the position you want has to be measured.

Then the glazed area

Around 15 to 20 percent of floor area gives a bright room without turning it into a greenhouse. Below about 10 percent a room reads as adequately lit rather than bright. Above 25 percent you are into overheating territory on any sunward facing slope.

Doubling the area doubles the solar gain, so the largest unit that fits is rarely the right answer.

Two smaller rather than one large

Better in almost every room. One large unit gives a bright pool with the corners darker by contrast; two spaced across the slope read as ambient daylight.

They also fail independently, and on a cut roof with a purlin two narrower units are frequently easier to fit because each can sit clear of the timber.

Taller rather than wider

Where more light is wanted and the structure limits the width, lengthening the opening up the slope does not cut rafters. A tall narrow window is frequently the answer and it costs a fraction of a widened opening.

The escape requirement

In a habitable loft room the roof window frequently carries a means of escape duty, with a minimum clear openable area, minimum opening dimensions and a maximum sill height above the finished floor.

That is a Building Regulations matter rather than a preference, and it is settled before size is chosen on any other grounds.

The upgrade that changes the answer

Splay the reveal. Cutting it back at an angle at head and sill rather than boxing it square opens the beam out and can make a modest window feel considerably larger.

It costs a little more in plastering and it frequently means a smaller unit does the job.

What to send us

A photograph looking up inside the loft, which settles cut roof against trussed rafters and tells us what the structure allows without a trimmer. A photograph of the room, and its floor area. And which way the slope faces.

From those we can suggest a size and a number of units rather than a single option, and we will tell you where a taller unit would do what a wider one is being asked to do at a fraction of the cost.

Where the room is a habitable loft, tell us, because the means of escape requirement is settled before size is chosen on any other grounds and it rules out some designs.

Start from the rafters, not the catalogue

The structure decides the realistic options before anything else does, and it usually narrows a free choice down to two or three.

A unit narrow enough to sit between two rafters, touching neither, needs no structural work at all. On a modern trussed rafter roof at 600mm centres, that sets the maximum width without an engineer.

Anything wider means cutting a rafter, trimming, and on a trussed roof a designed repair. The cost step is substantial.

Go taller rather than wider

This is the move that gets people what they actually wanted.

Extending the opening up the slope cuts no rafters, so a tall narrow unit delivers a large glass area and a real gain in headroom at between the rafters cost.

Two tall units in adjacent bays with a combination flashing frequently look better than one wide one and cost a fraction as much.

The other constraints

Distance from the ridge, because the flashing kit needs a minimum number of courses above the head.

The purlin, on a traditional cut roof, which is not a member to cut and frequently decides the position outright.

Sill height above the floor, which affects reach, cleaning, and whether the unit can serve as an escape window in a loft room.

Overheating, on a south or west facing slope. More glass does not give proportionally more perceived brightness, because the eye adapts, but it does give proportionally more solar gain.

The stock size argument

Staying within the standard range gets days rather than weeks of lead time, a lower price, and blinds and flashing kits that fit exactly. Adapting an opening by 50mm to hit a stock size is almost always worth it.