Can a bigger rooflight go in the same opening?

Where a slimmer frame gives you glass for nothing

Rooflight frames have got thinner in the way window frames generally have. A 1980s timber roof window carries a deep outer frame plus a substantial sash rail around the pane, and between the two of them a great deal of the aperture is timber rather than glass. A current unit in the same size code has a slimmer sash, a narrower sightline and, on some ranges, a pane taken further out towards the frame edge.

On a flat roof the difference is larger still. A twin-skin dome sits in a base frame with a wide flange, and the flange plus the frame can occupy a striking amount of the kerb aperture. A flush glazed unit on the same kerb has a slim thermally broken aluminium frame with the glass bonded across it, so the visible glass runs almost to the kerb line. The gain is real and it costs nothing beyond specifying it properly at survey.

Manufactured roof windows, where the next size up needs a bigger hole

This is where hopes usually meet arithmetic. VELUX, Fakro, Keylite and Roto all publish their ranges against structural opening sizes, and each size code has a specified opening it is designed to sit in, with a fixing tolerance of a couple of centimetres around the frame. That tolerance exists so the unit can be packed square in a roof that is not, and it is not spare capacity.

So a unit in the next code up will not go into your existing opening. The frame is larger than the hole. Going up a size means the opening is extended, a new trimmer goes in, and the internal reveal is rebuilt around it, all of which is entirely doable and none of which is a same-opening job. There is one useful exception worth checking: manufacturers changed their frame profiles over the decades and the codes have not always meant identical openings, so occasionally an old unit turns out to have been fitted into an opening slightly generous for its code. That gets checked at survey against the plate on the sash, described at how do I find out what rooflight I have.

A dome aperture converted to flush glazing

The clearest same-opening gain on any Gloucestershire roof is the 1970s or 1980s extension dome. Those units are commonly on imperial-derived apertures, and the base frame and dome flange between them eat a wide border around the hole. Swapping the whole assembly for a made-to-size flush glazed unit built to the aperture typically produces a noticeably larger visible glass area from exactly the same kerb.

Three dimensions have to agree before that works: the kerb’s internal aperture, the kerb’s external face where the new frame will sit down onto it, and the kerb height above the finished waterproofing, which wants to be 150mm measured from the highest point of the fall. Where the kerb is treated, insulated on its outer face and correctly weathered, it carries the new unit. Where it is untreated softwood with the felt turned up but not over, it is rebuilt, and at that point the aperture can be set to whatever suits. The criteria are at reusing an existing kerb and when not to and the conversion at replacing an old dome with flush glazing.

Flat roof units, and how far a frame can overhang its kerb

Made-to-size flat roof rooflights have one further piece of flexibility that manufactured roof windows do not. Because they are built to order rather than picked from a list, the frame can be specified to sit down over the kerb with a chosen overhang rather than inside it, and a modest amount of extra glass can be bought that way without touching the structural aperture.

There are limits and they are set by the weathering rather than by the frame. The waterproofing has to be dressed up the kerb and over the top with the frame bearing on it correctly, the upstand height has to remain right measured from the finished surface at the high point of the fall, and the frame’s own drainage has to discharge onto the covering rather than back against the kerb. Overhang beyond what the system is designed for turns a good detail into a shaded ledge that collects water. That is a survey conversation with real numbers in it, not a catalogue choice.

Growing up the slope inside the same rafter bay

Where a genuinely larger unit is wanted on a pitched roof, the cheap direction is upwards. A roof window’s width is set by the rafters and its length runs up the slope, so extending it towards the ridge passes between the same two timbers it already sits between. The head trimmer comes out, the opening extends, a new head trimmer goes in higher up, and no rafter is cut.

It is also the direction that gives the most daylight per pound spent, because the head of a window is what throws light to the far side of the room, and raising it lifts the sightline so somebody standing back from the window sees sky rather than reveal. This is not a same-opening job, the reveal is rebuilt and re-plastered, but on a cut roof it needs no structural design and adds roughly half a day outside.

Growing sideways, and the rafter or truss in the way

Width is the expensive direction, because it crosses a rafter. On a traditional cut roof that rafter is cut and doubled trimmers are fixed across the head and cill, transferring the load into the rafters either side. Over one bay that is routine carpentry. Over two or three bays it becomes a calculation, and the answer is sometimes a deeper timber trimmer, sometimes a flitch plate, sometimes a small steel, with the design submitted alongside the Building Control application.

On the trussed estates at Quedgeley, Abbeydale, Brockworth, Bishops Cleeve, Ashchurch and Yate the rule is simpler and harder. The webs and chords are part of a triangulated frame held with punched nail plates and they stay whole. Between two trusses at 600mm centres there is around 550 to 570mm of clear width, which is exactly why the manufacturers make narrow units at 550mm wide. Anything wider means removing part of a truss and carrying it on a beam spanning to the trusses either side, which is an engineer’s calculation and approval before the covering comes off. Often the better answer on those roofs is a second narrow unit in the next bay, which delivers more total glass than one wide one and lights the room more evenly.

Splaying the reveal, and measuring before anything is ordered

Where the opening genuinely cannot grow, there is one move left that adds daylight without adding glass, and it is often overlooked. Splaying the sides of the internal reveal and running a vertical lining at the head with a horizontal one at the cill widens the beam of light considerably and throws it deeper into the room. It costs another half day of carpentry and plastering, it uses the same aperture, and on a north facing room it is the difference between a bright room and a lit one. The reasoning is at rooflight reveals and why splaying them matters.

All of this is settled with a tape rather than a catalogue. At survey the structural opening is measured at both ends, since old openings are rarely square, the size code is read off the old sash, the rafter or joist centres and depth are recorded, the purlin position is noted, and on a flat roof the kerb aperture, external face and height above the finished waterproofing are all taken. From that, both options get quoted side by side: the best unit the existing opening will carry, and what a changed opening would give, with the difference in cost and days set out so the choice is yours.

Severn Skylights Gloucester fits new rooflights and replaces units that have reached the end of their service life across Gloucester and thirty miles by road, with a ten-year workmanship guarantee. Email info@severnskylightsgloucester.co.uk with a photograph of the unit from outside, one of the room from inside, the size code off the old sash and the address. Timings are at how long does a skylight replacement take and the process at replacing a failed or misted rooflight.