The short answer
They can, particularly on south and west slopes in summer. Glass in a roof catches the high sun almost square-on, so more heat gets in than through a wall window of the same size. Most overheated loft rooms combine that with thin roof insulation and no way for hot air to escape. An external blind, solar-control glazing and ventilation at both low and high level make a large difference, and are better chosen before the window goes in.
Why roof glass gains so much heat
The sun in summer is high in the sky. A vertical wall window sees it at a glancing angle, and the overhanging eaves shade part of it. A roof window sits on a slope pointing up towards the sun, so the light strikes the pane almost head-on, with nothing above it to cast shade. More energy enters as heat, and the room under a sloping ceiling has little thermal mass to absorb it. The air warms quickly and, because hot air rises, collects at the top of the room under the ridge.
The effect is strongest on a slope facing south or west, in the afternoon, and weakest on a north-facing slope. A big window on a south pitch above a bedroom is the classic overheating combination. Size matters as well: a larger area of glass lets in more.
The roof around the window
The window is often blamed for heat that the roof is also letting in. A loft room with thin insulation, or one where the insulation was squeezed or left gappy, heats up under the sun on the tiles and radiates it down. Dark roof coverings absorb heat. If the rafters are only filled with a thin layer and the plasterboard is bare, the whole ceiling acts as a radiator. Improving the insulation is a part of cooling a loft room, and it works in winter as well.
Poor ventilation is the other half. A room with a single small window and a closed door has no way to flush its warm air out. Roof windows are good for this when opened, since they vent the highest part of the room, but only if a lower opening supplies fresh air.
What cools a room
Shading comes first, and it is best on the outside. An external blind or awning stops the sun before it reaches the glass, so the heat never enters the room. An internal blind helps with glare but the heat is already inside the glass. Many makers offer external roller shutters or blinds for roof windows, operated by hand or a motor. The page on electric roof windows covers the remote-controlled options, which are useful because blinds only work if they are used.
Glazing comes second. Solar-control coatings on the glass reflect part of the sun's heat while letting light through, and they are a choice at the order stage. They can slightly tint the daylight, and they cost more than plain double glazing. Third is ventilation. Opening the window at the top while a lower window or door is open draws air through the room by stack effect, the same chimney action that makes warm air leave through the highest opening. At night, cooler air can flush out the day's heat.
Choosing for a hot room
If you are planning a loft room or a new window, think about orientation first. Where possible, put larger windows on the north and east slopes and keep the south and west ones smaller, with external shading. Consider a mix of openable and fixed windows, and avoid putting all the glass at the same height. A smaller window in the right place often works better than a large one in the wrong one. Our overview of roof window types shows the styles and their ventilation options.
Fitting a roof window to a loft that already overheats is a good moment to improve the insulation, because the roof is open. Ask about both when you request a survey.
Night cooling and daily habits
How you use the room changes the result. Close the blinds on a sunny morning before the heat builds, rather than after. Open the window at night, once the outside air is cooler than the room, and shut it again early in the day when the air outside warms. A small fan placed to push air out of the top window draws cooler air in through a lower one. In a bedroom that is too warm to sleep in, a combination of an external blind and night ventilation usually does more than any single change.
If the room is already too hot
You can add a blind, a film or a reflective screen without changing the window. External solutions are better than internal ones. An extractor fan or an openable window at the highest point helps ventilation. If condensation or damp is a concern in the room as well, see the guide to condensation on roof windows, since the fixes interact with ventilation. Sometimes a better glass specification can be fitted in the existing sash, which is what the glass-only replacement answer explains.
On the survey we note the orientation, the window size, the insulation in the rafters and the ventilation, and advise on what to specify. The fixed quote lists the glazing and any shading. Everything else we do with roof glass starts from roof windows and skylights.
Every roof is different. A free survey with photos settles it for yours.
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