Can you replace a roof window with a bigger one?
Yes, and how much bigger is decided by the rafters rather than by the product range. Going taller within the same rafter bay is usually straightforward carpentry. Going wider means cutting a rafter and rebuilding the trimming around it, which is still routine on a traditional cut roof and a structural question on a trussed one. The covering has to come off further than the old opening in either direction, and the ceiling below has to be made good.
It is a common request. An older unit fitted for daylight alone often turns out to be too small once the loft becomes a bedroom, or too small to satisfy an escape requirement. The exchange is a sensible piece of work with a clear method, and it is set out below alongside VELUX window installation.
Going taller within the same rafter bay
Height is the cheap direction, and it is the one to exhaust first. The window runs along the rafters rather than across them, so extending upwards or downwards means moving a trimmer rather than cutting a structural member.
The trimmer at the head or the sill is taken out and reset at the new position, and the rafters either side are unaffected. What limits it is what is above and below: a purlin crossing the slope, the ridge, the wall plate at the eaves, or the finished ceiling line inside. On a steep Victorian rear slope there is often three or four hundred millimetres of usable height available that nobody thought to take, and it buys a great deal of daylight for very little extra work.
Widening across a rafter, and what that involves
Width is the expensive direction because it interrupts the load path. To widen past a single bay, a rafter is cut back, the rafters on either side of the new opening are doubled up to carry the load that rafter was taking, and new trimmers are fitted at head and sill to close the box and transfer the load into the doubled members.
On a cut roof with rafters at six hundred centres that typically takes the opening from around five hundred and fifty millimetres to around eleven hundred. Cutting two rafters gets you wider again, and past that the trimming stops being a standard detail and starts being a designed one. The carpentry is not exotic; it just has to be done properly rather than by cutting the rafter and hoping.
Where the truss sets the width for you
On the estates built from the nineteen sixties onwards, the roof is trussed rather than cut. Great Notley, South Woodham Ferrers, Beaulieu Park and most of Basildon and Harlow are full of them.
A truss is an engineered component whose webs and chords carry load in tension and compression, and cutting one is not the same as cutting a rafter. In practice the opening stays between two trusses, which caps the width at around five hundred and fifty millimetres, unless a structural engineer designs a trimming solution with steel or a proprietary hanger arrangement. Where somebody wants a wider opening in a trussed roof, two windows in adjacent bays weathered as a pair is very often the better answer.
The covering, and how far it has to come off
The old flashing kit comes out and a new one goes in, and the new kit needs the covering stripped back further than the old opening on all four sides. Expect two or three courses beyond the new frame line at the head and sides, and enough below the sill to dress the apron cleanly.
How that goes depends on what is up there. Concrete interlocking tiles lift and re-lay easily and spares are readily matched. Natural slate breaks when disturbed, so allowance goes in for cut and re-hung slates around the whole perimeter. Handmade clay pegs on the older stock around Coggeshall, Thaxted and Saffron Walden are slower again, because each one is bedded individually and matching a colour range is a matter of sorting rather than ordering.
The ceiling below, and the plaster that follows
A bigger opening in the roof means a bigger opening in the ceiling, and the internal lining is rebuilt rather than extended. The old reveal comes out, the plasterboard is cut back to the new frame line, and the head and sides are re-boarded, taped and skimmed.
This is the moment to splay the reveal if the old one was square. A splayed lining, head opened out towards the room and sill kept close to vertical, spreads light across the ceiling and roughly doubles the apparent size of the opening. It costs a little more plastering and nothing in glass. Since the plasterer is coming anyway, it is the cheapest improvement on the whole exchange.
Building Regulations on a larger opening
Replacing a roof window is notifiable work in England, and enlarging one certainly is. Part L applies to the thermal performance of the new unit, which any modern double glazed window meets comfortably. Part B applies where the window serves as means of escape from a loft room, and enlarging is frequently how an existing window is brought up to that standard.
Where the opening grows enough to affect the roof structure, the structural work is inspected as well. Sunspire handles the notification as part of the job. An unnotified alteration surfaces years later during a sale, when a buyer’s solicitor asks for a completion certificate that nobody applied for.
Doing it as one visit rather than two
The whole exchange runs in a single sequence and the roof is not left open. Protect the room below and set up access. Strip the covering back to the new line. Remove the old unit and its flashing. Alter the trimming to the new dimensions, doubling rafters where the width is changing. Set and fix the new frame, square and out of wind.
Then the underlay collar goes on, the new flashing kit is dressed, the covering is re-laid and made good, and the sash is hung and adjusted. Internal boarding and plastering follow. On a straightforward height increase in a cut roof that is a day. On a width increase with a rafter cut, two. The covering is closed and watertight at the end of every working day whatever the forecast has decided to do.
What gets measured before anything is ordered
Five things, all from the loft side. The clear span between the rafters either side of the existing opening. The rafter section depth, since a doubled rafter has to be made up from matching timber. The distance from the existing head trimmer to the purlin or ridge, and from the sill trimmer to the wall plate. The covering type and its condition. And whether the roof is cut or trussed, which is visible in ten seconds and changes everything.
With those, a range comes back by email and a fixed figure follows the survey. Where the existing unit is simply worn rather than undersized, a like for like exchange into the same opening is a shorter job, covered under skylight replacement.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements