What Size Conservation Rooflight Do I Need?

Three things settle the size

There is no standard answer, because the best size comes from three things meeting at the same spot: what the room needs, what the roof structure allows, and what the covering will look like around the finished unit. Start with all three and the choice narrows quickly.

What the room needs

Think about what happens in the room. A landing or a bathroom needs a modest amount of daylight, and one small unit can do it. A bedroom or study that you want to read in benefits from more glass. The slope of the roof matters too. The steeper the slope, the more light a given piece of glass lets in from the sky, so a small unit in a steep roof can outperform a larger one in a shallow roof.

Position changes the effect more than size does. A rooflight set high on the slope throws light deep into the room, and one set low lights the floor near the eaves. We stand in the room at the survey and agree where the light is wanted before talking about dimensions.

What the roof structure allows

In most older roofs, the opening is easiest and strongest when it sits between two rafters. Cutting a rafter means fitting a trimmer to carry the load, which is entirely possible and a normal part of the work, but it is more involved. The spacing between your rafters, which varies from one house to another, therefore shapes which sizes are straightforward.

Old roofs are seldom regular. Rafters in a Victorian terrace may not be evenly spaced, and a roof that has been altered may have several sizes of timber. We open up enough of the ceiling or measure from the loft side to see what is really there, instead of guessing from the outside. The new opening is a structural change, so Building Regulations approval covers it.

What the covering will look like

This is where conservation rooflights differ from ordinary ones. On a slate roof, the edges of the unit should follow the slate courses, so the rooflight looks planned. Cutting across the courses leaves awkward partial slates around the frame. On a pantile roof, the unit is easiest to dress when its width follows the pan width. We pick a size that respects the covering, then check that it still works for the room.

A smaller unit also keeps the roof's own pattern in charge. An old roof carries a large plain area of slates that people notice, and one narrow window in it reads as a light touch. Several smaller units spaced evenly can look better than one large one, and they spread the daylight across a longer room.

Why bigger is not automatically better

It is tempting to go for the largest unit on the price list. In an old roof, that often backfires. A big pane means a heavy sash if it opens, more glass to lose heat overhead, and a window that dominates the slope from the street. It can also tip the council against an application where one is needed. See rooflights in conservation areas for how councils weigh these things.

On the other side, a unit that is too small gives disappointing light and can look mean in the roof. The right size is one where nobody can say afterwards that it should have been different.

How to check before you order

• Measure the clear space between two adjacent rafters from the loft side, if you can reach it.
• Note the roof pitch and which way the slope faces.
• Count the courses of slate or the pantile rows near the planned position.
• Stand in the room and decide where you want the light to fall.

None of this has to be exact. We measure again at the survey, and the written quote shows the size, the position and each cost on its own line.

Pairs and rows

A long attic often gives better light from two matching units than from one large one. Set the same distance from the ridge and spaced evenly along the slope, they look deliberate from the street and light the room from two directions. They also let you keep each unit between a single pair of rafters. The cost of two is not simply double, since some of the work is shared, and the written quote will show both options so you can compare them.

What if the roof has a chimney or a valley nearby

Chimney stacks, valleys and the junction with a neighbouring roof all limit where an opening can go, because the flashings need clear space to be dressed properly. A unit placed too close to a stack can leave no room to form the lead or to carry water around the frame. We mark the spot, step back and check the surrounding roof before settling on a width, and where the best position is tight, a slightly narrower unit often solves it.

After the size is settled

With a size chosen, the next questions are the frame, the glass and the cost. Our page on conservation rooflight cost shows how size feeds into a quote. If you are unsure whether to have it open, read can a conservation rooflight open, and for the whole job visit the conservation rooflight installation page.