When the opening has to change size

1. Pack out or trim back

Where the difference is small, typically under 60mm, the opening is adjusted with carpentry: packing out where the new frame is narrower, trimming back where it is wider.

No structural change, no engineer, and invisible once the reveal is made good. This handles most obsolete size situations.

2. Enlarge within the same rafter bay

Extending the opening up or down the slope, keeping the same width. No rafter is cut, so on a trussed roof nothing is compromised.

This is the underrated option. A taller window throws light further into the room than a wider one does, so it is frequently both the cheaper and the better answer.

3. Enlarge across rafters

Cutting a rafter and trimming around the opening. Routine on a traditional cut roof.

On a trussed roof it is a structural engineer’s job, because cutting a trussed rafter compromises the truss rather than the single member. That changes the cost materially and adds weeks.

Reframing entirely

Where the existing opening is awkwardly positioned, where several windows are being standardised on one size, or where the timber around the opening needs replacing anyway.

More work now and it leaves you with a standard size for the next thirty years.

The consequences

Any change to the opening is notifiable structural work. More tiles are cut and reinstated, which on an old roof with fragile or obsolete tiles is its own exercise. And the internal reveal is reformed, which means plaster and decoration.

What we recommend

Take the nearest current size that fits within the existing rafter bay, adapt the opening if needed, and go taller rather than wider if more light is wanted.

Why it comes up so often

A sealed unit lasts fifteen to twenty five years. Manufacturers revise their size ranges every decade or so. Put those two facts together and most replacements arrive well after the original size went out of production.

On anything fitted before about 2000, expect the size code to be obsolete. It is normal rather than unlucky, and it is entirely solvable.

Altering the opening is one of three routes, and on most jobs it is the right one.

What altering an opening actually involves

Less than people expect, when the difference is small.

Current sizes are frequently within 40mm or 60mm of an old one. Making up that difference means packing out the structural opening or trimming it back, refixing the battens around it, and making good the reveal.

Since the reveal is being replastered on a replacement regardless, the additional work is modest. When it is finished nothing is visible, and you are left with a standard current opening rather than an obsolete one, which is a real gift to whoever deals with it in twenty five years.

Making an opening smaller

The easier direction, and the more common one, because size ranges have tended to standardise rather than expand.

Timber is packed out from the existing trimmers and rafters to the new dimension, fixed securely so it carries load properly rather than sitting loose, and the battens are made good.

There is no structural implication: you are adding material rather than removing it. What matters is that the packing is fixed rather than wedged, because packing that settles takes the frame out of square with it.

Making an opening taller

The next easiest, and worth considering whenever more glass is wanted.

Lengthening up or down the slope does not cut rafters. It means removing battens, moving the trimmers at head and sill, and reworking the covering above and below the opening.

That is ordinary work on any roof, and it is why a tall narrow unit frequently delivers what a wide one was being asked for at a fraction of the cost.

The constraints are at the extremities: at least a course or two of covering above the head for the flashing to lap under, and enough below the sill for the apron. Ridge and eaves ventilation must remain clear.

Making an opening wider

The difficult direction, because roof openings are bounded by rafters or trusses at the sides.

In a traditional cut roof, cutting a rafter and installing trimmers is ordinary structural carpentry. The cut rafter bears onto trimmers at head and sill, and those transfer its load into the neighbouring rafters, doubled up where required.

It needs designing rather than guessing, and it is notifiable under Part A, and it is routine.

In a trussed rafter roof, a different matter entirely. Each truss is a single engineered component in which rafter, ceiling tie and webs work together. Cutting any member sends the load it carried somewhere it was not designed to go.

A structural trimmer has to be designed by an engineer, and that is the point at which the price steps up noticeably. It is not a cost worth avoiding.

A unit narrow enough to sit between two trusses at 600mm centres needs no structural work at all, which is why so many rooflights on modern estates are tall rather than wide.

The three routes when a size is obsolete

Adapt the opening to the nearest current size. Usually right. Invisible when finished, and it leaves a standard size for the future.

Find another manufacturer at the old dimension. Ranges overlap, so a size one maker has dropped may be current with another.

Worth it where the opening is genuinely awkward to alter: tight against a purlin, a steel, a party wall, or a historic timber that cannot be cut. The trade is a mixed set of windows on one roof, which on a rear slope is irrelevant and on a front elevation may not be.

Reframe entirely to a current size. More work now, and right where several windows on a slope are being standardised, or where the timber needs replacing anyway. If decayed trimmers are coming out regardless, reframing costs very little more.

What never to do

Force a modern unit into an old opening and pack the difference loosely.

It survives about a year. The packing settles, the frame goes out of square, the sash stops closing evenly against the compression gasket, and water gets past. By then the covering is back on and nobody can see what was done.

Where a quotation for a replacement with an obsolete size is noticeably cheaper than the others, it is worth asking directly how the size difference is being dealt with.

What changes when the opening changes

The flashing kit. Size specific, so a new one is needed regardless. Never reuse one.

The covering. A larger opening means more tiles or slates reworked, and on a fragile or handmade clay roof a proportion will break.

Building Control. Altering an opening engages Part A as well as Part L. Adapting by 40mm is normally straightforward; widening significantly is a structural alteration and needs designing.

Permission, possibly. A significantly larger rooflight in a visible slope may need it where the original did not, and on a listed building consent is required regardless.

The escape provision. In a habitable loft room, changing the size changes the clear openable area. Where the window carries a means of escape duty, the new unit has to satisfy it, and that is checked in writing before ordering.

The alternative worth pricing

A second unit alongside rather than one bigger one.

Two rooflights spaced across a slope give more even daylight than one large one, each can frequently sit clear of the structure without trimming, and they fail independently.

On a trussed roof that is often the difference between a straightforward job and an engineered one, and the result in the room is usually better rather than merely cheaper.

How we establish it

From a photograph of the data plate, before anybody visits. The size code describes the structural opening, which is not a dimension you can take from inside the room.

From that we can tell you whether the size is current, what the nearest alternatives are, and which of the three routes suits your situation.

The four reasons it happens

The original size is obsolete. The most common. Ranges are revised every decade or so and sealed units last fifteen to twenty five years, so most replacements arrive after the size went out of production.

The room has changed use and now needs an escape window, which is about the clear unobstructed opening rather than the glass area.

More light is wanted, which is a choice rather than a necessity.

The timber around the opening has to be replaced anyway, which makes resizing nearly free while it is open.

Up the slope against across it

The distinction that governs the cost, and it is worth understanding before asking for a price.

Extending up or down the slope, between the same two rafters, means moving the trimmers at the head and sill. No rafter is cut, no load is redistributed. It is ordinary carpentry.

Widening means cutting at least one rafter and carrying its load into trimmers each side, which are usually doubled. On a traditional cut roof that is designed work. On a trussed rafter roof it needs an engineer, because a truss is an engineered component and cutting one without a designed repair compromises the roof.

The practical consequence: 500mm more height may cost very little; 500mm more width can cost several times as much.

Making it smaller

Easier than making it larger, and it is what most obsolete size adaptations require.

Timber packing pieces or a new liner fixed to the existing trimmers, properly fixed and square rather than wedged. Then the reveal made good, which is happening anyway because the plaster comes off at the frame regardless.

When it is finished nobody can tell, and the opening is now a current standard size.

What changes with the opening

A new flashing kit, because they are size specific. That is true of any replacement, so it is not an additional cost of resizing.

The reveal rebuilt on all four sides rather than patched, which means more plastering and a longer drying period.

More covering off and back on, with breakages to allow for.

And a check on whether an enlarged window now falls under escape window requirements, or whether it has moved too close to the ridge for the flashing kit clearance.

The opportunity

While the opening is open, the timber is visible for the only time in its life, and the insulation and vapour control layer are reachable. Use it.

Telling the neighbours

Where the enlarged opening is close to a boundary, or the work involves a party wall, the Party Wall Act may apply and notice is required.

It rarely stops the work and it does take weeks, so it belongs in the programme rather than being discovered when a neighbour objects on the day the scaffold arrives.