Planning a rooflight for the grey half of the year

On a clear day, sunshine comes in at a steep angle through a wall window and lands on the floor. On an overcast day there is no single direction. The light comes evenly from the whole sky, and the brightest part of the sky is straight above.

A wall window sees a slice of that sky, usually cut off by the opposite house, a fence or a tree. A window in the roof looks up at the whole dome. In a room that is deep from front to back, or has a wall of extension on one side, that difference is the one you notice. The far corner is dim even at noon, and a rooflight is what reaches it.

This is the reasoning behind everything on this page. If an opening will work on a dull November afternoon, it will not disappoint in June.

People often ask for the rooflight in the middle of the ceiling. The better place is over the part of the room that is furthest from the existing window. Light from a window falls away quickly with distance, so the rooflight should fill the gap.

On a survey we stand in the room at the time of day it feels darkest and mark where the shadow sits. Then we check what is above that spot. A rafter, a chimney breast, a water tank or a run of wiring may mean moving the opening slightly. A small shift in position rarely costs much light, because under cloud the whole sky is bright.

For a long narrow room, two smaller openings can spread the light more evenly than one large one. For a large open-plan room, a single wide rooflight or a roof lantern often gives the best result.

In the dull months the weather is also cold and damp, and that is when glass gets into trouble. A cold pane next to warm, humid indoor air collects condensation. In a kitchen or a bathroom, with cooking and showers adding moisture, this shows first.

Better glazing keeps the inner pane warmer, which reduces condensation. Modern double or triple glazing with low-emissivity coatings and warm edge spacers does this. The energy-efficient rooflights page explains the options. For existing homes, Part L of the Building Regulations allows roof windows up to 1.4 W/m²K (or a Window Energy Rating of B) and rooflights up to 2.2.

Ventilation helps too. Cold bridging at the frame is the other cause. A well-insulated frame and a lining that is sealed against the warm room keep the edge of the glass from becoming the coldest surface in the house. A rooflight that opens, including an electric opening rooflight, lets damp air out at the highest point of the room. Where the glazing has misted between the panes, the seal has failed, and we cover that on the page about replacing misted rooflights.

A dry July tests very little. A rooflight is tested by weeks of rain, driven by wind, with leaves, moss and standing water on a flat roof. The detail around the opening matters far more than the glass.

On a flat roof, the rooflight sits on an upstand, a raised kerb round the opening, so water has to rise before it can get in. A taller, well-formed upstand with a continuous waterproof layer carried up and over it copes with a wet spell better than a minimal one. On a pitched roof, the flashing channels water round the frame and back onto the roof covering. It has to match the covering, which is different for tiles, slates and profiled sheets.

Moss and leaves collect at the low side of a flat rooflight and in the valley of a pitched roof, and long damp spells make them worse. We leave the detail clear and easy to wipe down, so a quick clean from time to time keeps water moving the way it should.

These are the details our workmanship guarantee stands behind, and you can read the terms on the guarantee page.

Under cloud, brightness comes from how much sky the opening can see, not from direct sun. So the size of the glass and the slimness of the frame count for more than the tint.

• A wide, flat flat roof rooflight has a large clear area and a low frame, which is why it suits an extension.
• A roof lantern admits light from the top and the sides, and gives a bright ceiling to the room below.
• A roof window in a pitched roof is the natural choice for a loft room.
• A sun tunnel carries sky light down a tube. It is a good answer for a landing, a hall or a bathroom with no outside wall.

Glass with a heavy solar-control tint cuts the light you need in winter, so for a room that is dark mostly in the gloomy months, a clearer glass tends to be the better call. We talk about that on the survey, because it depends on the room and its use. Frame colour matters in the same way. A white or pale internal frame bounces light into the room, while a dark one frames the view and takes a little away. Outside, a darker frame is often chosen to match windows and roof.

A small room needs less light in total but suffers more from the lack of it. A landing with no window can be dim at any hour. A small opening matters more here, and a sun tunnel suits the job well when the room sits below a loft or a pitched roof, because the tube can pass through the roof space.

Where there is a flat roof above, a small flat rooflight can be set into the ceiling directly. Lining the sides of the opening with a pale, splayed finish helps it spread the light around the room.

For deeper advice on picking between these, read our guide to choosing a skylight for a dark room.

A survey in the grey months has an advantage: the room shows its worst. We look at how dim it is, where the damp or condensation collects, and what the roof looks like after weeks of rain, including the state of the gutters, any staining and the age of the covering.

We also look at the lining inside, because a rooflight that opens into a deep, dark shaft wastes much of what it catches. A splayed, light-coloured lining is part of the plan, not an afterthought. We also check access in poor weather, and what planning rules apply. Most rooflights on houses fall under permitted development, but flats, listed buildings and homes with an Article 4 direction are different, and the planning permission guide explains the rules. If you are ready, ask for a free survey.