Answered · Skylight & Rooflight Installation & Replacement Chelmsford

Can a bigger rooflight go in the same opening?

The short answer

A little bigger, usually yes. Substantially bigger, only if the structure allows it, and that is a question about rafters, trusses and joists rather than about glass. The dividing line falls at the point where the opening stops fitting between existing structural members and starts cutting through them.

The moment an old unit comes out is the cheapest opportunity there will ever be to change the size, because the opening is formed, the access is set up and the labour is already committed. It is worth asking the question properly at survey rather than a year later.

How much bigger is being asked for

Three quite different requests arrive under the same words, and they have three different answers.

Easing an opening by twenty to fifty millimetres to reach the next standard unit size is usually a lining job rather than a structural one, and it is the change most often available. Going up one nominal size, perhaps two hundred millimetres in a direction, generally means moving a trimmer and is achievable in a cut roof. Doubling the area, or turning two small units into one large one, is a structural alteration with a calculation, a Building Control conversation and a different programme. Knowing which of the three you are asking for saves a great deal of circular discussion.

The lining, and the free millimetres inside it

The first thing checked is whether the visible opening is the structural one, and surprisingly often it is not.

Old installations frequently used a thick timber lining inside the trimmed hole, sometimes twenty five millimetres of board on each face, plus plasterboard and a skim. On a nineteen seventies dome the internal lining can be forty or fifty millimetres in from the structure on every side. Strip that back and the true structural opening is a hundred millimetres larger in both directions than the hole you were looking at, which is often exactly enough to reach the next unit up without touching a single rafter.

Pitched slopes, and the rafter either side

In a traditional cut roof, rafters run at 400 or 600 millimetre centres and a roof window sits between two of them, with the rafters either side doubled and trimmers framed above and below.

Widening within that bay is limited to the clear space between the doubled rafters. Going wider means cutting a rafter, which is ordinary carpentry provided the load is picked up: the neighbouring rafters are doubled again and the trimmers are sized for the new span. That is achievable in most cut roofs on Victorian terraces in Chelmsford and Colchester and on interwar semis across the commuter towns. Going deeper up the slope is generally easier than going wider, because it works with the structure rather than across it.

Trussed roofs, and the width that will not move

Anything built from the nineteen sixties onwards is likely to have a trussed rafter roof, and the estates at 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 bottom chord are carrying load in tension and compression, held together by punched metal plates. Cutting one is not the same as cutting a rafter. In practice the opening is limited to the clear space between trusses, which usually caps the width at around 550 millimetres unless a structural engineer designs a trimming solution transferring load to adjacent trusses. Going deeper up the slope stays available. Going wider does not, without design.

Flat roofs, and the joists under the deck

On a flat roof the same logic applies to the joists, with one useful difference: the deck hides everything, so nobody knows the spacing until it is opened or a small inspection is made from below.

Joists at 400 or 600 centres allow an opening between two of them without any structural work. Beyond that, trimmers are introduced and the cut joists bear onto them. Below about a metre and a half of clear span, timber trimming is usually sufficient. Beyond that, and particularly where the opening runs across the span rather than with it, the load has to be picked up and a steel becomes the usual destination, with a calculation and a longer lead time attached.

The kerb, and whether it grows or is rebuilt

On a flat roof the upstand is the part that decides whether a size change is cheap or expensive.

A kerb cannot simply be extended outwards, because the covering is dressed up its existing faces and terminated under the old unit. Enlarging the opening means the kerb comes off and a new one is built to the new dimensions, with the covering cut back and reinstated up the new faces. That is half a day of carpentry and a covering operation, and it is the same work whether you are going up fifty millimetres or five hundred. Which is why, on a flat roof, going considerably bigger often costs proportionally less than going slightly bigger.

The ceiling and the reveal underneath

Every millimetre added at roof level appears at ceiling level, and the room below is decorated.

A larger opening means the plasterboard comes back further, the reveal is reformed and the surrounding ceiling is made good, so the decorating scope grows faster than the glass area does. It also means checking what is in the way before committing: a light fitting, a bathroom extract duct running through the void, a cupboard, or a downstand nobody thought about. The compensation is that a splayed reveal, opening outwards towards the room, spreads light across the ceiling and makes an opening read considerably larger than it is.

Two units rather than one wider one

Where the structure will not give you the width, two narrower units side by side is very often the honest answer, and it is not the consolation prize it sounds like.

Two roof windows between trusses, separated by the truss itself, deliver a similar total glazed area, spread the light more evenly across a room than a single opening does, and need no structural design at all. On a flat roof, two flush units in a run read as a deliberate composition. The alternative is a bespoke unit made to the largest size the existing structure permits, which adds four to eight weeks of lead time and leaves the roof entirely alone. Both routes are priced at survey alongside the full replacement option, and a widened opening brings its own Building Regulations position with it.

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.

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