Pane replacement against whole unit replacement

When the glass alone can be changed

Some models have a sealed unit held in a removable glazing bead, so the glass can be replaced with the frame in place. On those, if the frame is sound, replacing the glass is meaningfully cheaper.

On other models the glass is bonded into the sash as one component, and the sash is replaced rather than the glass. That is still cheaper than a whole window, but by less.

The code on the data plate tells us which you have.

Why it is often a false economy

Consider what a twenty five year old window is: an old frame with an old finish, worn hinges, hardened seals, an obsolete ventilation flap, and a thermal specification two generations behind current.

Putting a new sealed unit into that frame gives you modern glass in a worn window. The seals will still let draughts past, the hinges will still be stiff, and if the frame timber has taken moisture the new unit may not sit properly.

The labour and access are almost the same either way. Most of the cost of this job is getting to the window, not the component.

The rule of thumb

Frame under fifteen years old and sound: replace the glass.

Frame over twenty years old: replace the window. You get current thermal performance, new seals, new hinges and a warranty on all of it.

Frame showing any timber decay: replace the window, always. New glass into a decaying frame is money thrown away.

Doing several

Where a loft has several windows of the same age, they will fail at about the same time. Replacing them together shares the access cost, which is the largest fixed element.

The arithmetic that decides it

Most of the cost of a rooflight job is getting to the window rather than the component you are changing.

The access, the setting up, the protection inside, the strip out and the making good are nearly identical whether you change a sealed unit or the whole window. Once you accept that, the decision becomes simple: you are choosing what to leave behind, not what to spend.

When pane replacement is possible at all

It depends on the model, and the data plate tells us in one step.

Bead held glazing. Some pitched roof windows hold the sealed unit with a removable glazing bead, so the glass can be changed with the frame and sash in place. This is the situation where reglazing is straightforward.

Bonded glazing. On many modern units the sealed unit is structurally bonded into the sash, so the two are one component. The sash is replaced rather than the glass, which is a bigger item and closer in cost to a whole window.

Bonding is what allows very slim sightlines, so it is a consequence of a design choice rather than a defect. It does mean the reglazing option is off the table.

Flat roof rooflights. Almost always a single bonded assembly. Pane replacement is generally not available and the answer is a whole unit.

The age test

Where reglazing is possible, three straightforward rules cover almost every case.

Frame under fifteen years and sound: replace the glass. You get modern glazing, with a low emissivity coating, argon fill and a warm edge spacer, in a frame that has plenty of life left. That is good value.

Frame over twenty years: replace the window. A unit of that age has hardened gaskets, worn hinges, a finish approaching the end of its life and thermal performance two generations behind current. New glass in that frame buys you the shorter of the two lifespans, and you will be paying for the access again within a few years.

Any timber decay: replace the window, always. Not a judgement call. Decay is caused by water getting in and it does not stop until the cause is corrected, and putting new glazing into a frame surrounded by decayed structure is money spent on the wrong part of the problem.

Press the frame at the sill and the lower corners. Timber that gives under a thumb has gone.

What the frame is doing that the glass is not

Worth spelling out, because it is why an old frame is not a neutral thing to keep.

The gaskets seal the sash against the frame. Hardened or split gaskets mean the sash does not seat, which lets water past and is reported as a leak.

The hinges carry the sash and hold it square. Worn hinges mean a sash that catches at one corner.

The frame itself has to stay square. A frame that has moved, or been packed to fit an opening it never suited, loads a new sealed unit unevenly from day one.

And the external cladding is what keeps weather off the timber core. Where that has been damaged or has failed at a joint, the timber behind it is already taking water.

The upgrade that comes with reglazing

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

Solar control, on a south or west facing unit. The most valuable upgrade available and the one people most often want retrofitted, because overheating is what they are living with.

Acoustic laminate, where the rooflight is over a bedroom and rain noise is the complaint.

A warm edge spacer, which removes the band of edge condensation an aluminium spacer produces on cold mornings and which most people have been mistaking for a failing unit for years.

Obscure glazing, where privacy has become an issue.

Coming back for any of those later costs nearly as much as the whole job.

What a whole unit replacement gets you that reglazing does not

A new flashing kit. On a full replacement this is included as a matter of course. Reusing a flashing that has been on the roof for the same twenty five years as the failed glazing is a false economy, and on an exposed roof it is where the next leak comes from.

The underlay collar checked and renewed. Frequently the actual cause of a leak, and it is only accessible with the covering lifted.

An insulation collar. Older installations often have none, which is why the reveal runs with condensation every winter. It goes in as part of a full replacement.

An inspection of the timber. With the unit out and the covering lifted, decay is visible. Reglazing from inside tells you nothing about what is behind the frame.

Modern mechanism and gaskets. A sash that closes properly for another twenty five years.

The sash only option

Worth knowing because it sits between the two.

On some models the sash lifts out and a new one goes in while the frame stays. Where the frame is genuinely sound and reasonably recent but the sash has failed, that is a legitimate repair and it is quick.

It carries the same caveats as reglazing: the flashing, the underlay collar and the timber around the frame are untouched and uninspected.

What we will not do

Drill and dry a misted unit. Some companies drill the sealed unit, dry the cavity and fit small vents. The visible misting clears.

The seal is not restored. The argon has gone and is not coming back. The unit now deliberately breathes outside air with a desiccant that is already spent, and the low emissivity coating continues to degrade.

On an overhead unit at height, the labour to do that is most of the labour to replace it properly, so you are paying nearly full price for a temporary cosmetic fix. We do not offer it.

The decision in practice

Photograph the data plate. That tells us the model, whether the glazing is bead held or bonded, and how old the unit is likely to be.

Then press the timber at the sill and the lower corners, and tell us what you find.

Between those two pieces of information we can usually tell you which option applies and what each costs, without anybody visiting, and the answer is frequently clearer than people expect.

The question behind the question

Almost always: the sealed unit has misted and you want to know whether the glass alone can be changed.

The answer turns on how the glass is held, and there are two constructions.

Gasketed and beaded

Most mainstream pitched roof windows. The sealed unit sits in the sash held by gaskets and removable beads, so it can be taken out and a replacement fitted by somebody who knows the system.

Manufacturers supply replacement units against the size code from the data plate, and the work can frequently be done without disturbing the covering at all.

It is a genuine and considerably cheaper option.

Structurally bonded

Most flat rooflights, and all structural glass walk on units.

The glass is bonded to the frame with structural silicone under factory conditions, with controlled surface preparation and cure. That bond carries load and forms the seal simultaneously, and it cannot be remade on a roof.

So on these, the whole assembly is replaced. There is no pane to change.

Worth knowing at purchase, because it changes the long term arithmetic between a flat rooflight and a pitched roof window.

When reglazing is the wrong answer even though it is possible

Where the frame is the actual problem. Rot in the timber, a failed hinge, a distorted sash, a gasket that has hardened and no longer recovers. New glass in that frame is money on the wrong component.

Where the model is old enough that replacement units are no longer made to that size.

And where the original specification was poor. A twenty five year old basic unit with an aluminium spacer is not a standard worth reproducing, and if you are opening the sash anyway the upgrade is available.

Deciding by age and access

Age, and access.

Under about fifteen years with a sound frame, reglaze. The frame has plenty of life and you are buying the improvement without the surrounding work.

Over about twenty, replace. The hinges, gaskets and mechanism are the same age as the glass and heading the same way, and new glass buys a few years before everything else needs doing.

The access cost is identical either way, and on a small job that is frequently the largest line. Which is why the age test usually beats the price comparison.

What to check before ordering either

The data plate, photographed rather than transcribed, so the size code and model designation are right first time.

Ordering against the code

Replacement sealed units are supplied against the size code from the data plate.

Photograph the plate rather than transcribing it. A mistyped code buys a unit that does not fit, and it is discovered with the sash apart.