Tile roofs and brick rows in Grimethorpe, lit from above by skylights and rooflights

Height up the slope

The glass should reach the point where a person stands, or at least where the handle can be turned without stretching. Too low and it sits behind a bed or a wardrobe. Too high and the bottom edge is above head height, which wastes the daylight on the ceiling. A position partway up the slope suits a typical layout, and we mark it from inside the room with the furniture in place.

One window or two

A long bedroom may take two windows side by side. Each has its own trimmers, and the rafter between them is either kept or cut and carried round. Some makes offer a combined flashing for a pair, which closes the gap between the frames in one piece.

Width from the rafters

The width of the opening follows the gaps between rafters. Makers publish their sizes as codes, so the loft measurements point to one before any unit is ordered, and the pair of figures is explained in reading VELUX size codes.

Paperwork for a loft room

Barnsley Council receives any planning application for the address. The limits that apply to a roof window on a house are listed in the skylight planning page, and a loft turned into a bedroom brings its own rules about the stair and the way out, which the planning page also touches on.

A second source of light

A loft room with a gable window gets light from one end and one slope. A roof window on the other side brings daylight from a second direction, which evens out the room through the day and stops one corner from going dim as the sun moves round.

Reaching the handle

A roof window set low enough to reach opens by hand. One set higher, over a stairwell or a tall landing, is better as an electric model or one worked with a pole, and the choice is made at the same time as the position so the two do not clash.

The bed under the glass

If the bed will sit directly under the window, a centre-pivot sash suits because it tilts about its middle and leaves the lower edge clear of the pillow. Where there is walking room, a top-hung sash opens wider at the bottom and lets more air through.

What changes in a swap

The old frame comes out with its flashing, and the tiles beside it are lifted. A new frame is set in the same gap, flashed to the tile profile and glazed to a better standard. Part L sets 1.4 W/m²K for a roof window (band B on the rating scale also meets it) and 2.2 W/m²K for a rooflight. (We notify Building Control about a swap as we do for a new opening.)

Matching the size

A unit of the same code fits the old opening with no carpentry. A slightly smaller one needs the gap filled with new timber. A slightly larger one means trimming further, which turns it into a bigger job, as replacing a roof window with the same size sets out.

Choosing which units to swap

Where a house has two or three roof windows of the same age, they tend to fail in the same way. It makes sense to look at all of them in one visit, so that the scaffold, the delivery and the plaster work are shared across the job.

Reading the signs

Damp marks under a window can come from the flashing, the frame or condensation, and the cause has to be traced before a replacement is chosen. Moisture inside the glass is the clearest sign of a failed seal. When a skylight should be replaced lists the other signs to look for.

The day of the swap

Tiles beside the old frame come up, the frame is lifted out, and the new one is set, flashed and glazed before the tiles are relaid. Inside, the reveal is checked and the sheets come off. The roof is weathertight again when the crew leaves.

Better glazing in the same hole

In the newer unit, the spacer at the glass edge conducts less heat and the panes carry a coating. The opening is the same, but less heat escapes through it, which matters most over a bedroom.

A lantern on a kerb

A lantern sits on a kerb and its glass pitches up to a ridge. Aluminium gives thinner bars than uPVC for the same strength, which lets more glass show. Quoted sizes are taken inside the upstand by some makers and outside it by others, so we work from the opening itself and build the kerb to match, and a size is chosen as roof lantern sizes describes.

Permission for something that stands up

A lantern stands far higher than a rooflight set in a slope. So the question for Barnsley Council is a real one, unless the extension's own planning consent already covers the roof. We read any consent that exists before suggesting a lantern.

What the existing flat roof means

If the kitchen roof is old, the deck and its covering are looked at while the opening is planned. A roof close to the end of its life can be re-covered at the same time, so the new kerb is weathered into new material instead of old.

Light and heat in a kitchen

A south-facing lantern can warm a kitchen on a sunny afternoon. A coating that cuts solar heat limits that, and the guide to whether rooflights make rooms too hot explains how the coatings work.

What the kitchen looks like beneath

Inside, the lantern opens a lined shaft in the ceiling with the glass at the top. White plaster on the shaft sides bounces the light into the room, and a pale ceiling helps it reach the far wall. A dark ceiling soaks up the benefit, which is worth thinking about when the kitchen is being redecorated.

Flat rooflight as the alternative

A lantern can stand awkwardly high against the back of the house, and a flat rooflight keeps a lower line.