The short answer
Only partly. A new roof window needs the roof covering cut back and a flashing kit fitted around the frame, and that has to be done from the outside. Some stages, such as cutting through the lining, fixing the sash or finishing the reveals, are done from inside. A like-for-like replacement can sometimes be handled largely from inside and a ladder or roof ladder, but a new opening or a larger one cannot.
What happens where
A roof window is a hole through the roof, sealed with a frame, a flashing and the covering around it. Most of what keeps water out lies on the outside: the flashing kit that overlaps the tiles, the underlay collar that directs water around the frame, and the tiles themselves cut and laid to suit. All of that is fitted from the roof. Without it the window may look right from inside but will leak.
The inside work is real, though. The rafters are exposed and measured from the loft, the opening is marked from the room, the frame may be fixed from within, insulation is packed round it, and the lining and plasterboard finishing happens indoors. In practice a fitter works from both sides, climbing in and out through the opening.
The like-for-like replacement case
If an existing window is being replaced with one of the same size from the same maker, the opening already exists. The sash can often be removed from inside, the frame unscrewed, and the new frame set in the same position. The flashing kit may still need replacing, and a worn one is worth changing, since the old one has aged with the window. Some makers supply replacement units designed to fit the old openings with the minimum disturbance to the roof around them.
Even then, access to the outside is generally required, if only briefly, to cut the old flashing free, lift tiles at the sides and fit the new flashing. A roof ladder, a scaffold tower or a short scaffold are the usual ways. The exception is a small repair, such as replacing the sash or the glass only, which can sometimes be done from inside, as the page on replacing just the glass in a roof window explains.
New openings cannot be done from inside
A new window needs a new hole. The tiles or slates round it are lifted from outside, the rafters are cut and trimmed and a new frame is fixed, then the covering is reinstated around it. Doing this from inside would risk dropping debris and, worse, leaving the covering and flashing unfinished. Anyone offering to fit a new roof window entirely from indoors is skipping the part that keeps the weather out.
If the new window is in a roof with an awkward covering such as slate or large concrete tiles, the work outside takes more care still. The page on roof window flashing kits explains what is fitted and why each piece is needed.
Safety and access
Working from the inside is not automatically safer. A fitter standing in an open roof hole is exposed, and falling debris is a risk to people below. A scaffold or a platform at the eaves gives safe access on a taller house. On a low roof or an extension, a roof ladder may suffice. The method is chosen for the roof and the window, and a reputable fitter explains it at the survey. If the cost of scaffold seems high against the cost of the window, it is worth asking whether it can be shared with other work such as gutter or roofline repair.
What to ask a fitter
Ask how the flashing will be fitted and from where. Ask whether the old flashing is being replaced or reused, and whether the underlay collar will be renewed. Ask what protects the room below from dust and falling debris, and what cover is on site if rain arrives while the roof is open. A fitter who answers without hesitation has done it before. One who says the whole job can be done from the loft in an hour should be asked to explain how the outside gets sealed.
Ask also about the finish. The lining round the window, the plaster returns and the insulation behind them are part of the job, and a gap or a cold bridge here shows as condensation in winter. The best installers pack the space between frame and rafters with insulation and fit a vapour barrier so the window sits warm.
How long and how messy
The job itself is quicker than people expect for a like-for-like swap, and the room is disrupted for a shorter time. The exposure of the roof opening is limited to the time it takes to fit the frame and flashing, and the weather cover is kept ready. Our page on how long it takes to fit a roof window gives the typical stages.
When we visit, we check the roof from outside and the structure from the loft, and tell you what can be done from where. The quote is fixed and in writing, with the access method named. The full service is under roof windows and skylights.
Every roof is different. A free survey with photos settles it for yours.
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