New glass or a whole new unit?

Strip a rooflight down and you find two panes of glass held apart by a spacer bar, bonded round the edge with sealant. That sandwich is the sealed unit. It sits in a rebate in the frame, held by beads or a capping strip, with gaskets or sealant between the unit and the frame to keep rain out.

A glass-only job lifts that sandwich out and sets a new one into the same frame. The gaskets and beads are renewed with it, because they are cheap to change and awkward to reuse. The frame, the kerb and the flashing stay where they are, and the job can often be done without disturbing the roof covering at all.

A whole-unit job takes the frame out too. On a pitched roof that means opening the tiles or slates around the window, fitting a new window and dressing new flashings into the covering. On a flat roof it means lifting the rooflight off its upstand, and sometimes renewing the upstand.

The new unit in either case is made to measure. Glass cannot be trimmed on site, so every dimension is taken at survey and checked again before the order is placed. See why rooflights mist up for how the seal gives way in the first place.

Glass is only as good as the seat it is bedded in. A brand new sealed unit placed in a frame that has twisted, softened or corroded is likely to fail again. So the frame gets judged before anything is ordered.

On a timber frame we press a probe into the bottom rail and the corners, where water collects first. Sound timber resists. Rot gives way under the point, and so does old paint that has been hiding it. Staining that runs down from the glazing bead is a clue that water has been standing in the rebate.

Aluminium and steel frames show their age differently. We look for pitting under the paint, white powdery corrosion around fixings and joints that have opened at the corners. A frame that no longer closes evenly, or a hinge that has dropped, tells us the frame has moved, and a new unit would sit under strain.

A uPVC frame can look fine from the room and still have warped. Hold a straight edge along the long side and you can see it bowing. The welded corners are the next place to look for hairline splits.

The two outcomes

A frame that passes these checks is a good home for new glass. A frame that fails any of them means the whole unit should be replaced, and we name the test it failed. The conversation is usually short once you have looked at a probe sinking into a rail.

Old frames are sometimes sound enough but too narrow for the glass we would like to fit. A deeper, better-performing unit needs a rebate that can take it. Where it cannot, the frame is the limit, and a whole new window is the way to get the better glass.

Branded roof windows carry a data plate, usually on the frame edge inside the room or on the sash when it is open. It gives a size code and a type. In a code such as MK04, the letters give the width and the digits the height, so the plate tells us what the original opening was before we pick up a tape. The plate also helps us check whether a replacement glass unit is available for that window.

Rooflights on a flat roof rarely carry a code. They are measured on the roof: the clear size of the upstand opening, the outer size of the frame and the thickness of the glass, since the unit has to match the old one closely enough to fit the rebate and seat on the gaskets.

Older makes are the awkward ones. A window installed a long time ago may belong to a range that has been altered or withdrawn, and a sealed unit made for it may not be on offer in the same profile. When the unit cannot be matched, the whole window is the only route to a good seal, and we can see that from the plate and the glazing before you commit to anything.

A made-to-measure rooflight in aluminium or timber, such as one fitted to an orangery or a conservatory roof, is different again. The new unit is made to the size of the old glass, and we take templates where the shape is not square.

Mist and leaks are different faults that sometimes turn up together. Mist is moisture trapped inside the unit, and cleaning the glass does not reach it. A leak is rain getting past the frame, the kerb or the flashing and coming through into the room. New glass cures the first and has no effect on the second.

On a flat roof, the kerb can be the culprit. A timber upstand that has been wet for years loses strength, and its roofing felt or membrane can crack where it turns up the side. The rooflight sits on it, so the unit is carried by a base that is already failing. A glass swap would hide the problem for a season. A whole-unit job, with the upstand rebuilt and the covering dressed up against it, ends it.

On a pitched roof the same pattern shows in the flashing. If the soakers that overlap the tiles have lifted or the lead has split at a corner, water is running under the window. Replacing only the sealed unit leaves that route open. We look at the flashing from the roof, which is why the survey includes a climb and not only a look from the room.

Staining on the ceiling lining that starts at the window edge, a damp patch on a splay or a tide-line on the reveal all point the same way. Our guide on whether a misted skylight leaks covers how to tell the two faults apart from the room.

Glass-only work is the quicker of the two, and it is done from the room, the roof side or a scaffold tower, depending on how the unit is held, often without disturbing the covering. The room below is protected with sheets, and the opening is not left uncovered while the new unit is on its way.

A whole-unit job opens more of the roof, so it is planned around the weather. The old window comes out, the opening is checked for rot in the rafters or the trimmers, and the new frame goes in with new flashings dressed into the covering. On a flat roof the old rooflight is lifted, the upstand is inspected or rebuilt, and the new unit is bedded and sealed to it.

The finish inside depends on how much of the lining was disturbed. A swap leaves the ceiling splay alone. A whole-unit job may mean the lining around the opening is made good and painted.

A new unit is an opportunity to bring a rooflight up to current standards. Under Part L, a new or replacement roof window on an existing home takes the window limit of 1.4 W/m²K, or a minimum Window Energy Rating of band B, while a rooflight or roof lantern takes 2.2 W/m²K. A whole-unit job therefore has a target to meet. For a glass-only swap, what the rules require for the work as described is something we confirm at survey, as it depends on the roof and the unit.

Building Control wording matters here too. Where the work is notifiable, we handle the Building Control notification, and the council or a registered building control approver signs the job off. You are not asked to chase it.

Planning seldom comes into a straight replacement in the existing opening, because the opening itself neither moves nor grows. If you are changing the size, or adding a new window elsewhere, the permitted development rules in our skylight planning permission guide are the place to start, and the energy numbers are set out in Part L and rooflights.

New glass also brings new coatings. A sealed unit built to today's specification can hold more heat in the room than a failed older unit, and the room feels it near the glass on a winter evening.

You can make a fair first guess before anyone visits. If the window opens and closes smoothly, the frame is straight, the paint is sound and no water has ever marked the lining, a glass swap is a reasonable expectation. If any of those are untrue, plan on the whole unit.

Then the survey settles it with the probe, the plate and a look at the roof side. It is free, and the written quote that follows is fixed, with the 10-year workmanship guarantee in writing. Photos of the window from inside and outside, taken in daylight, help us come prepared: send them with the form or by WhatsApp on 07880 284133.

If cloudy glass is only one of your concerns, how long rooflights last explains what age does to each part of the unit.