Is a rooflight cheaper than a dormer?

What a dormer actually involves

A dormer is a projection built out from the roof slope with its own walls, its own roof and a window in a vertical face. Building one means opening the slope, cutting and trimming the rafters, and putting in new structure sized to carry what the cut members carried. Then a small building is constructed on top of that: cheeks, a flat or pitched roof, insulation to current standards, a vapour control layer, an external finish, and a window and cill.

Every junction that structure makes with the existing roof has to be weathered. The sides, the head where the dormer roof meets the slope above, the cheeks against the covering, and the abutment at the base are all separate details in lead or in the covering material.

It is a genuine piece of construction rather than a fitting job, usually involving more than one trade. That is where the cost sits.

What a rooflight involves by comparison

A rooflight is set into the plane of the roof that already exists. Where the opening drops between existing rafters, no structural alteration is needed at all. Where it does not, the trimming is a contained piece of carpentry rather than a new structure with a roof on it.

The covering is cut back and re-dressed around the opening, with soakers worked into the courses at the sides, lead dressed at the head, and the laps above and below maintained so water leaves the roof the way it always did. The unit is fixed, the internal reveal is formed and insulated, and the roof is closed. The perimeter detailing is the part that decides whether the installation stays dry.

None of that makes a rooflight trivial. A conservation-pattern unit set flush into the plane, on stone slate with scaffold to arrange, is real work with real cost. It is still a much smaller undertaking than building out from the roof, and on a like-for-like comparison it lands well below.

The planning position is not the same either

This is the part people underestimate. A rooflight generally does not alter the shape of the roof, which is why rooflights on many houses fall within permitted development, subject to the conditions that apply. A dormer changes the roof profile by definition, so it is far more likely to need planning permission, and on a front slope it very often will.

Designation changes it again. Where the building is listed, sits in a conservation area or is covered by an Article 4 direction, both propositions are assessed on appearance, and a dormer is asking a great deal more of the elevation than a flush dark rooflight is. Whether either is acceptable at a particular address is a matter for the local planning authority for that district.

The cost of a consent route is time as much as money. An application that has to be prepared, submitted and determined sits in front of the programme, and that is a real difference between the two options.

Headroom is the real reason people build one

All of which would settle the question if the two things did the same job, and they do not. A dormer gives usable head height where the roof slope has taken it away. That is the thing it does, and a rooflight cannot deliver it.

In a loft with a shallow pitch, or under a low ridge, the floor area is there but the standing space is not, and the useful part of the room is a strip down the middle. A dormer squares off part of the slope and turns floor area at the edge into space somebody can stand in, and where a staircase lands or a bathroom has to fit, that can be the difference between a scheme working and not working.

A rooflight adds no head height at all, and the geometry of the room stays exactly what it was. Where the honest problem is headroom, comparing prices is beside the point, and we will say so rather than sell you glass that does not answer the question.

Two rooflights often answer the problem that was posed

Where the problem is genuinely daylight, though, the picture changes. Somebody has been told they need a dormer because a room is dark, and the daylight is what they actually want. In that situation two well-placed rooflights frequently do more for the room than the dormer would have, for far less work.

The reason is that a roof slope sees more sky than a wall does. A vertical window in a dormer cheek looks out horizontally and takes light from part of the sky, while glass in the slope faces upward into the whole of it. Two modest units set apart also light a room more evenly than one opening at one end.

The room stays the shape it was, which is the trade. But where the ceiling already follows the pitch and the height is adequate, the dormer was solving a problem the room did not have. Establishing which problem you actually have is the useful first step.

Compare what each achieves, not only what each costs

So the honest comparison is not a pair of figures. It is two different pieces of work answering two different questions, and the sensible order is to decide what the room needs before anybody prices anything.

If the answer is standing height, usable floor area at the edge of the room, or a vertical window with a view out at eye level, that is a dormer, and a rooflight is not a cheaper version of it. If the answer is daylight, ventilation, or light reaching a landing or a stairwell that has none, a rooflight is very likely the better piece of work as well as the smaller one. Plenty of schemes end up with both, a dormer where the height is needed and rooflights where only light is.

We install rooflights and nothing else, so we are not the people to build you a dormer, and we would rather tell you a rooflight is the wrong answer than fit one that disappoints. On cost we publish no figures, because a number given before anybody has seen the roof is a guess. We survey the roof, write a specification against what is there, and put one written figure against it. That figure includes the Building Control notification, and our workmanship carries a ten-year guarantee.