The short answer
A like-for-like replacement takes less time than a new opening, because the hole, the framing and the lining already exist. A new opening involves cutting rafters, framing the gap, weathering it in and finishing the inside, and the finishing can run beyond the roof work. The roof must be left weathertight each day. The exact timing depends on the roof, the access and the room, and we give it after the survey.
How the time splits between roof and room for a replacement and for a new opening, and what can lengthen the job.
A like-for-like replacement
When a new window goes into the opening an old one has just left, most of the work is already done. The roofer lifts the tiles or slates around the frame, removes the old sash and frame and its flashing, fits the new window to the same rafters and weathers it in. Because the size matches, the timber framing and the inside lining usually stay as they are.
That is why this is the quicker job, and why a single window is often a short visit. The time that is hard to predict is the surprise behind the old unit: rotten timber around the frame, a lining that crumbles when the old window comes out, or flashing that is welded to the covering. A good roofer lets you know about those before you agree to start.
The covering matters. Plain tiles and slates are individually lifted and re-laid around the frame, which takes longer than an interlocking tile that can be cut to suit. Our answer on Velux in a slate roof covers that case. Hours are best settled by the survey, which tells us far more than a rule of thumb, and the written price comes with a clear plan of the day.
A new opening in the roof
Cutting a new opening is a longer job. The first step is to open the roof and cut the rafters that cross the gap, then to fix a trimmer above and below the new window and a header either side, so the load goes round the opening. The window frame is set into that box, and the covering is cut and laid back against it with flashings made for your tile or slate.
The roof must be weathertight whenever the roofers leave. A new opening is the moment when the house is most exposed, so the work is planned to get the frame and flashings in and made weathertight in one session. Weather can still move the date. Our answer on what happens if it rains during a roof replacement explains how a roof is kept covered.
Paperwork can come before the work. Crawley Building Control treats new window openings in roofs as work needing approval, so ask your own building control body whether notice is due and allow time for it. Planning is usually straightforward under Class C, though a listed building or a flat needs separate consent.
Finishing inside the room
The inside often takes longer than the roof. A new opening needs a lining of boards or plasterboard fitted into the splayed reveal, then filler, plaster or tape, then paint. Plaster must dry before it can be painted, which makes the finish a stage that cannot be rushed. A replacement may need only a small repair to the existing lining.
Insulation goes in around the frame before the lining closes, and a vapour check helps keep room moisture from reaching the timber. A deeper reveal, an angled lining on a steep roof or a window over a stairwell all add steps. Ask whether the price includes the finish inside the room, as quotes differ on this. Our answer on a skylight light well shows how a deeper shaft is built.
If you are fitting a blind, a blackout or a special finish, say so at the start. Some come in kits that fit the frame, and a few need extra fixings put in while the lining is open. A clear list of what happens each day lets you plan around the dust, the noise and who needs to be at home.
What can lengthen the job
Access is the first thing. A window low on a roof may be reached by ladder or a small tower, whereas one at the back of a tall house may need scaffold, and putting it up adds time before the roofing starts. The roofer must also carry the window and the materials up, and a cramped loft slows work on the inside.
Next comes what the opening reveals. Old rafters may be wet or damaged, the flashing of an earlier window may have to be cut out, and a roof that is out of square can mean the frame needs packing. Several windows fitted in one visit save time per window, because setting up and clearing away happen once. Our cost to fit a Velux window answer shows how those factors affect price.
The best protection against delays is a survey. Our survey takes in the roof, rafters and room, with photos sent over, and the plan is agreed in writing alongside one total price. If the roof opens up a problem, we tell you at that point and explain the options before going further. Our roof windows and skylights page covers the wider service.
More on this in Velux Roof Windows & Skylights in Crawley
Two more timing questions
Will my room be open to the weather while the window is fitted?
Only briefly. The roofer works in stages so the opening is closed and weathered in before the roofers leave, and sheeting protects it if the weather turns. Clear the area beneath the window, move furniture and cover what remains, since dust and debris fall into the room during the work.
Can I stay at home while a Velux window is fitted?
Yes, in most cases. Expect noise, dust and people on the roof and in the loft, and keep little ones and animals out of the room below. If a new opening is being cut, you may prefer a different room for the day. Ask for a plan of each day before it starts.