The short answer
A sun tunnel suits a small room that needs daylight and has little roof space to give up, such as a landing, hallway or bathroom. A roof window suits a room that also needs fresh air and a view. The tunnel brings light only, through a reflective tube. The window opens, but it needs a larger opening and more structural work.
Both bring daylight into a dark room, but they do it in different ways and ask different things of your roof. The room and the loft space above it decide.
What each one actually does
A sun tunnel is a rooftop dome connected to a ceiling diffuser by a reflective tube. Daylight bounces down the tube and spreads into the room below. It looks like a ceiling light that happens to switch itself on at sunrise. There is no glass to see out of, no sash to open and nothing to clean from inside.
Does a sun tunnel give as much light as a window?
It depends on the room and the run of the tube. A short, straight tube delivers a good wash of daylight to a small space. A long tube, or one with bends, loses light along the way. A roof window admits light directly through a much larger pane, and it adds a view of the sky, which a tunnel cannot do.
Ventilation is the other split. A roof window opens, so damp air from a shower or a cooking session can leave the room. A sun tunnel can only light it. Some tunnel kits include a fan or a vent, but that is an extra component to specify. Check the kit contents before you compare prices.
Roof space, rafters and the loft
This is where the two diverge most. A roof window needs an opening framed into the roof. Where the opening is wider than the gap between rafters, the inclined timbers under the tiles, one or more rafters are cut and the load is carried round the hole by new timbers. That is structural alteration, and it changes the price and the paperwork.
Will a sun tunnel fit between my rafters?
Often it will, because the tube is narrow and the hole in the roof is small. That can mean the rafters stay uncut. We confirm that when we survey, going up into the loft, because pipework, wiring, insulation and a water tank can all sit in the path of the tube. The tube also has to run from roof to ceiling, so a room with a large loft void above it gives more choice.
Sun tunnels still need the tiles or slates lifted and a proper flashing around the dome. A neat small opening is no less of a waterproofing job than a big one. Our page on fitting a new roof window shows what the framing involves.
Choosing for a landing, bathroom or box room
Think about what the room does all day. A windowless landing or hallway mostly needs light, and a tunnel does that well with very little disturbance. A bathroom is a closer call, because steam needs to leave. A box room used as a bedroom or study usually wants both light and air, which points to a roof window.
Can I have both in one house?
Plenty of homes do. A tunnel lights the stairwell while a roof window serves the loft room above it. The two are different jobs and the quotes are separate. Where the loft is being turned into a room, the window may also have to serve as an escape route, and our answer on roof window fire escape sets out the sizes that apply.
Do not choose by the picture on a brochure. Choose by the room, the loft above it and what you want to do in the space. We survey, send you pictures taken on the roof and in the loft, and write one price for the option that fits.
What the work involves and what it costs
A sun tunnel is usually the smaller job. The roofer lifts a few tiles or slates, cuts a round hole, fits the dome base and flashing, runs the tube through the loft and fits the diffuser from below. A roof window involves more steps, including framing, a full flashing kit, lining out the opening and often some decorating.
Which costs less?
The tunnel is generally the smaller outlay, but the covering, the access, the length of the tube and what is in the way all move the price, and only a visit settles them. For a roof window, MyJobQuote lists £1,100 as the standard mid-range figure and about £100 less for an easy swap (Sep 2026). Read that as a typical UK range, not a quote for your house.
Either way, the covering and the waterproofing must be right where they meet the new fitting, and a small hole leaks as readily as a large one if that detail is wrong. Read the main page on roof windows and skylights page, then send us a photo of the room and the landing below the roof, and we will advise which suits.
More on sun tunnels and roof windows
Can a sun tunnel be fitted on a slate roof?
Yes, in principle. The dome sits on a base with a flashing, and the slates around it are cut and dressed to suit. Slate needs more care than concrete tile because each slate is thin and brittle. We look at the condition of the slates and battens on the survey before agreeing to the opening.
Does a sun tunnel make a room colder?
It can, if the tube and diffuser are poorly insulated, because a dome is a thin spot in the roof. Ask for a product with an insulated tube or an insulated diffuser, and check the product data. Tell us how the room is used and we will look at the loft insulation around the tube.
The full picture: Velux roof window and skylight installation in Taunton · More short answers