The short answer
A like-for-like replacement is often done in a day or less, and a new window in a roof where none existed takes longer, typically a day or two for the structural and roof work. Plastering, lining and decorating inside come after and are usually separate. Larger openings, awkward coverings such as slate, several windows at once and bad weather all add time. Ask for a stage plan with the quote.
The two kinds of job
The first question is whether you are replacing a window or forming a new opening. Replacing means the hole is there already, the structure is sound and the new unit is the same size or compatible. That job is mostly about removing the old window cleanly, setting the new frame, fitting the flashing and making good the covering. The second kind is a new opening: the covering has to be cut back, rafters cut and trimmed, and the roof reinstated round the frame, which is a bigger, slower and more skilled job.
A third case is several windows in one go, for instance in a loft conversion. The work is repeated, and a team can do two or three in a short time, but the scaffold and set-up costs are shared. The wider job of making a loft room adds time for floor, walls and stairs.
Stages and rough times
For a replacement, the sequence is: protect the room, remove the sash, release the old frame and flashing, lift the covering at the sides, fit the new frame, fix the flashing kit, relay the tiles and finish the lining. A competent pair can do it in a few hours to a day, depending on the covering and the access. Slate and handmade tiles take longer to cut and relay than interlocking tiles, and our page on fitting a roof window in a slate roof explains why.
For a new opening, add the structure. The position is marked from the loft, the covering stripped to suit, the rafters cut and the trimmers fixed, all before the window can go in. That is typically a day or two, longer for a wide opening or one that needs engineering advice. The room then needs lining and finishing. The guide on moving or enlarging a roof window goes through the structural side.
What slows it down
Weather is first. An open roof must not be left to the rain, so work waits for a dry spell or the opening is made small enough to close in the same day. Access is next: a roof that needs scaffold takes time to set up. The next delay is the unexpected: rot in a rafter, a hidden pipe or cable, a chimney too close to the position, or tiles that crack when lifted. Delivery of a made-to-order window can take weeks, which is not on-site time but affects when you can book.
A building control inspection may be needed, since new roof windows are notifiable. Allow for the inspector's visit, and the paperwork afterwards.
Finishing the inside
The part people overlook is what happens in the room. After fitting, the reveals around the window are lined with plasterboard, filled and sanded, then painted. Plaster needs time to dry. A tidy fitter leaves the opening ready for decoration, and you or a decorator finish it. If you want it all done in one go, ask for the lining and plastering to be included, and for a timescale. Dust sheets and a sealed doorway protect the rest of the house.
If the old window leaked, the surrounding ceiling and insulation may be wet and need drying out or replacing before the new unit is lined. That can add days. The guide on replacing an old roof window covers what to expect with an older unit.
Can it be done faster from inside?
Some of it can, but the outside work remains. Our answer on fitting a roof window from inside explains which stages need roof access. The risk of rushing is a window that leaks in the first winter.
Several windows, or windows with a conversion
Fitting two or three windows on one roof is not a multiple of the single-window time. The scaffold or access is set up once, the crew moves from one opening to the next, and the roof is closed in section by section. A pair of matching replacements can often be done in the same day. In a loft conversion, windows are timed with the other trades, since the roof must be weathertight before plastering and the floor go in, and the window openings are usually among the first jobs after the shell is stripped.
Planning your time
Plan to be at home or have someone available for the main stage, clear the room beneath, cover furniture and move what is on the floor. Ask for a start window, since the weather decides the actual day. Ask who will be on site and how long they expect to be there. Ask what happens if the job spans two days and the roof is open overnight, which it should not be without secure weather protection.
When we survey, we measure, check the structure and tell you how long we expect on site, with a note on the finishing. The fixed quote then lists the stages. The rest of the service is under roof windows and skylights.
Every roof is different. A free survey with photos settles it for yours.
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