Replacing like for like, and when you cannot

What makes it possible

The size is still made, or close enough that the opening can absorb the difference. The opening and its timber are sound. On a flat roof, the kerb is square, level along the top and the right height.

Four things that rule it out

1. The size is obsolete. Common on windows from the 1980s and 1990s. Options are the nearest current size with minor carpentry, another manufacturer at the old dimension, or reframing.

2. The timber has gone. A unit that has been failing for years has usually been letting a little water past for some of them. Soft timber is cut out and replaced. There is no version of this job where a new unit is bolted to rotten timber.

3. The kerb is wrong. On a flat roof, a kerb that is out of square, too low for the covering to have terminated properly, or rotted at the corners has to be rebuilt. That means disturbing the covering around it, which is a different job with a different price.

4. The original was wrong. Occasionally we find the existing unit should never have been there: a roof window in a roof too shallow for it, a flat kit on a profiled tile, a rooflight sitting in a pond. Replacing it like for like reproduces the fault.

Why we survey first

Because all four are invisible from a photograph. A price quoted over the phone for a flat roof replacement is a guess about the kerb.

What you gain anyway

Even a straight swap brings twenty five years of glazing improvement: low emissivity coating, argon fill and a warm edge spacer, none of which was standard then.

The cheapest job in this trade

A genuine like for like replacement, a current size unit into a sound existing opening, is the least expensive rooflight work there is.

No structural alteration, no change to the opening, a straightforward strip out and refit, and the making good limited to reinstating what was there. On a single storey slope reachable from a tower it is comfortably a one day job.

The reason it is worth understanding what qualifies is that a lot of quotations are written as though a job is like for like when it is not, and the difference emerges on the day.

What has to be true

The size is still current. On anything fitted before about 2000, expect it not to be. Ranges are revised every decade or so and sizes get rationalised.

The opening is sound. No decayed timber in the trimmers, the rafters either side or the battens around it.

The opening is square. A frame that has moved, or an opening that was never square, will not take a new unit evenly.

The covering is workable. Slate and concrete interlocking tile lift cleanly. Old handmade clay is slow and a proportion breaks.

The requirements have not changed. Which is the one people forget, and it is covered below.

When you cannot: obsolete size

The most common reason, and it is entirely solvable rather than a problem.

Adapt the opening to the nearest current size, usually within 40mm or 60mm. Find another manufacturer still making the old dimension. Or reframe entirely.

Adapting is usually right, and it is a smaller step than it sounds because the reveal is being replastered on a replacement regardless. It also leaves a standard opening for the future.

What nobody should do is force a unit in and pack the difference, which survives about a year before the frame goes out of square and water gets past.

When you cannot: the escape requirement

The one that catches people out, and it applies in a habitable loft room.

Building Regulations frequently require the roof window in such a room to serve as a means of escape, with a minimum clear openable area, minimum opening dimensions and a maximum height of the bottom of the opening above the finished floor.

A centre pivot window sits across the middle of the aperture when open and frequently fails the clear opening requirement. A top hung unit that swings clear satisfies it.

So swapping a top hung escape window for a centre pivot unit because you prefer the cleaning feature can put a compliant room out of compliance. Like for like in appearance is not like for like in function.

Establish whether the window carries the duty before choosing, and check the specific model’s published clear opening figures in writing rather than assuming any top hung unit will do.

When you cannot: the property is designated

On a listed building, listed building consent is required for a replacement rooflight, on any slope, visible or not. Replacing something that is already there is still an alteration to a listed building.

That surprises people because it feels like maintenance. It is not, and there is no small works threshold.

In a conservation area with an Article 4 direction covering the elevation, a planning application may be needed. And flats and maisonettes have no permitted development rights at all, regardless of designation.

None of that prevents the work. It changes the sequence: consent first, order second.

When you cannot: what is found on strip out

Nothing about a replacement is certain until the old unit is out and the covering is back around the opening.

The four things we find, in order of likelihood: the old flashing kit was the wrong profile, there was never an insulation collar, there is no underlay at all on a period roof, and the timber around the opening has decayed.

Only the last of those costs money, and on a unit that has been misting or leaking for years it is a realistic possibility rather than a remote one.

That is why an honest quotation names a contingency with an indication of cost. A fixed price that assumes everything is sound is either carrying a hidden margin to cover the risk, in which case you pay for it whether or not anything is wrong, or it is going to change on the day.

What is never like for like, even on a straight swap

The flashing kit. A new one goes on as a matter of course. Reusing a flashing that has been on the roof for the same twenty five years as the failed window is a false economy and it is where the next leak comes from.

The glazing specification. A unit fitted twenty five years ago has no low emissivity coating, no argon, an aluminium spacer and no solar control. The replacement has all four as standard, so the room will be warmer, cooler in summer and quieter in rain whether or not you asked for it.

The appearance. Sightlines have generally slimmed, so a modern unit shows more glass and less frame. Cladding colours have changed. On a visible slope alongside older windows that is a mismatch, and it is one of the reasons doing the whole slope at once frequently makes sense.

The upgrade opportunity

Where the roof is open anyway, the marginal cost of a better specification is small because the labour and access are already being paid for.

Solar control glass on a south or west facing unit. Acoustic laminate over a bedroom. A capped electrical cable to the position for a future motor or blind. And the insulation collar, if the original never had one.

Coming back for any of those later costs nearly as much as the whole job, so this is the moment to decide.

What like for like actually requires

Four things have to line up, and it is common for one of them not to.

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

The timber is sound. The trimmers around an old opening have had a long time to be exposed to any slow leak, and slow leaks produce no symptom until the timber is gone.

The opening is square. Roofs move. An opening formed twenty five years ago in timber that has since dried and settled may not be the rectangle it started as.

Nothing else has changed. No loft conversion since, no change of use of the room below, no new escape requirement, no covering replaced with a different profile.

The three things that most often break it

An obsolete size. The usual and least serious. Adapt the opening to the nearest current size, which means packing out or trimming back and making good a reveal that is being replastered anyway. Invisible when finished, and it leaves a standard opening for the next thirty years.

Rot in the trimmers. Cut out, replace, splice to sound timber. Should have been carried as a provisional allowance in the quotation so that finding it is a priced variation rather than an argument on the day.

A changed requirement. A room that has become a bedroom may now need an escape window, and an escape window is about the clear unobstructed opening rather than the glass area. A centre pivot unit that was fine before may not satisfy it, because half the sash occupies the opening when it rotates.

When you should not do like for like even though you could

Where the original was badly specified. A twenty five year old basic double glazed unit with an aluminium spacer and no insulation collar is not a standard worth reproducing.

The access is being paid for once. Upgrading the glazing specification, adding the collar, insulating the reveal properly and sealing the vapour control layer costs a modest amount on top of a job that is happening anyway, and it is the only moment those things are reachable.

Where the position was wrong. If the window has always been badly placed, and the roof is open, moving it up or down the same slope between the same rafters is far cheaper than doing it as a separate exercise later.

The honest sequence

Photograph the data plate. Confirm with a supplier whether the size is current. Price a provisional allowance for timber. Decide the specification upgrades before ordering, because they cannot be added afterwards.

Then, once it is in, record the manufacturer, model, size code and glazing specification with the house paperwork so the next round starts from a document rather than a ladder.

The paperwork that makes it easy

Where the previous installation was documented, this whole question takes a phone call.

Manufacturer, model designation, size code, glazing specification and installation date, kept with the house papers, turns a survey into a supplier conversation.

Do it for the new one, because somebody will be standing under it with a torch in twenty five years.