Partly, and the honest answer is that no roof window is fitted entirely from inside. The opening is formed and the frame is set from within the roof space, but the flashing kit and the covering around it are external work. What can change is how much external access is needed, and on some roofs that matters a great deal.
What is done from inside
More than people assume.
Marking out, cutting the underlay, trimming the rafters, fixing the brackets and setting the frame are all done from within the roof space, standing on a boarded floor rather than on the roof.
What has to be done from outside
The parts that keep water out.
The flashing kit is dressed over the frame and under the covering above it, and tiles or slates are cut and reinstated around it. Neither is reachable from below.
Which is why the question is really about access rather than about the window.
Why installers say from inside at all
Because on an easily reached roof the external work is short.
Where a fitter can reach the opening from a tower for the flashing and the courses, and everything else happens inside, the job needs very little in the way of scaffold.
That is what the phrase usually means in practice.
Where it genuinely matters
A roof over a conservatory, an extension roof, or a boundary.
Where a scaffold cannot be stood because there is a glass roof, a neighbouring garden or a narrow passage below, minimising the external work changes whether the job is possible at all.
That is a survey conversation before it is a product one.
The sash comes out
The detail that makes the external work quick.
A roof window sash lifts out of its frame, so the frame is fitted as an empty box and the glass goes in afterwards. That reduces the weight handled on the roof and lets the frame be passed up through the opening.
Reaching out through the opening
How the flashing is often dressed on a small unit.
With the sash removed, a fitter can work through the frame to fix the side and bottom flashings, which covers a good part of the external detail without standing on the covering.
The head of the flashing, above the window, is the part that generally cannot be done that way.
Working off the roof itself
Sometimes possible, sometimes not.
A sound covering on a moderate pitch can take a roof ladder hooked over the ridge. A brittle covering, a steep pitch or a fragile sheet cannot, and that is a decision made on the roof rather than from the ground.
Access equipment generally
Costed as part of the job rather than assumed away.
A tower, an edge protection scaffold or a full lift each suit different heights and different ground conditions, and which one a house needs is settled at survey.
It is one of the larger variables between two otherwise identical jobs.
Where the ground is the problem
Common on a terrace or a town centre house.
A scaffold needs somewhere to stand, and a narrow side passage, a light well or a neighbouring garden all complicate it. Agreement with a neighbour may be part of the arrangement.
What that looks like in practice is on the East Worthing page.
Replacing rather than fitting new
Where internal work covers most of it.
Swapping a unit for the same size in the same range reuses the opening and often much of the flashing, so the external work reduces to lifting a few courses rather than cutting new ones.
Which is why a like for like replacement is a shorter job.
The underlay collar
Formed from inside, and it is the detail that leaks when it is rushed.
The underlay is cut, dressed around the opening and lapped so any water reaching it is carried out over the flashing rather than in behind it.
The direction of that lap decides whether the detail works.
Insulation and the reveal
All internal, and all after the window is weathered.
Insulation is carried up to the frame and the reveal is boarded and finished, which is the trade after the roof work rather than part of it.
It is also the part that decides how the window performs.
Weather during the day
The reason the sequence matters.
The roof is open for a period, so the work is ordered so the opening is formed and weathered within a working day rather than left relying on a sheet overnight.
On a marginal forecast a job is moved rather than started.
What a homeowner sees
Access and dust inside, and a tidy exterior.
Most of the disruption is in the room below, which is protected and cleared before anything is cut, and the loft is walked first so nothing is trodden through a ceiling.
Whether the room has to be empty
Cleared rather than emptied.
Furniture is moved away from beneath the opening and covered, and the floor protected, which is generally enough for a single window.
Whether a fitter needs the loft empty
Cleared around the opening rather than emptied entirely.
Boxes and insulation directly beneath the position are moved so there is a working platform and a clean edge to cut to, and anything stored under the slope is shifted a metre or so clear.
A loft that has not been cleared is the commonest cause of a late start.
What we settle first on a pitched roof
We settle the access question at survey rather than on the day, because a roof over a conservatory or a boundary changes the method, and we sequence the opening so it is weathered before we leave.
The rest of the process is on the roof window installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.