Replacing just the glass in a rooflight

A rooflight pane is not a single sheet. It is a sealed unit: two or three layers of glass held apart by a spacer bar around the edge, with the gap between them filled with air or a gas such as argon. The edge is sealed so the gap stays dry. Because the unit is sealed, moisture inside it has nowhere to go, and a misted rooflight is usually one where that edge seal has failed and humid air has got between the panes.

The unit is made as one piece, to the millimetre, and bedded into the frame on gaskets or glazing tape and held by beads, cappings or a pressure plate. Taking it out and putting a new one in is the same operation whatever the reason, so what matters is never whether the glass can be changed. It is whether the thing around the glass deserves a new unit.

On a roof window with a factory frame, the unit is often a specific product that fits only that range. On a flat rooflight or a roof lantern, the glazing is more likely to be a standard-size sealed unit sitting in a kerb or a set of glazing bars, which gives more room to specify the replacement. If you are not sure what you have, the label on the frame edge and a few clear photos are usually enough to start.

Replacing the unit alone suits a rooflight that has one problem and that problem is the glass. A good test is to look at everything except the pane and ask whether you would be happy to keep it for another decade or more.

• The frame is straight, solid and free of soft timber, corrosion or warping.
• The opening mechanism, if there is one, works smoothly and closes tight.
• The flashing or kerb weathering is sound and the roof round it is dry.
• The inside lining and ceiling are clean and undamaged, with no stain climbing out of the sill.
• The size and position suit the room, so nothing about the opening needs to change.

A typical case is a younger roof window or lantern, a few years old, where the seal on a single pane has failed and everything else is fine. Another is an aluminium lantern on a kitchen extension where one glazed panel has misted but the frame, the gaskets and the ridge are in good order. In both, a new unit gets the room back at modest cost and with very little disturbance to the ceiling.

It is also a sensible route in a room you have just decorated, or in an old house with a lath and plaster ceiling you would rather not disturb. The work happens at the glass. The ceiling and its edges are left as they were, which is one reason the route appeals in period homes in Southernhay, St Leonards and Topsham.

Often the sealed unit is simply the first part to show its age, and the frame has been ageing alongside it. Putting a new pane into a frame that has reached the end of its life gives you a clear pane and the same draught, the same cold edge and the same stained sill. The frame also has to take the weight and the movement of a new unit without distorting.

Warning signs in the frame are plain enough to spot with a torch and a screwdriver handle. Timber that gives under a gentle press at the bottom rail is rotting. Paint that has flaked off the outside edge and left the pine grey is past protecting. Aluminium with white powdery corrosion at the joints, or a lantern whose glazing bars have moved so the gaps are wider at one end, is telling you the structure has shifted. Seals that have hardened and cracked will not hold a new unit watertight however good the unit is.

A common middle case is a frame that is fine today but already near the end of its life. If the roof is going to be re-covered soon, or the ceiling is going to be redecorated anyway, replacing the whole rooflight at the same time costs less than doing the glass now and the whole unit in a few years. Our guide to replacing an old skylight goes through that side of the decision.

A replacement unit is made to order, so the numbers have to be right before anything is cut. Glass that is a few millimetres out will not seat on its gaskets, and glass that is too thin or too thick for the rebate will either rattle or not close. A survey takes these measurements on site rather than from a drawing.

Overall size

Width and height of the unit, measured across the glass edge to edge and checked in two places because old openings are rarely square.

Thickness

The total build of the unit, from the outer face of the outer pane to the inner face of the inner pane. The rebate in the frame is cut for a particular depth, and a unit deeper than the rebate cannot be held by the beads.

Make-up

Which panes, how thick, what coatings and what gas, as set out below. This is where an old unit is usually out of date, and where a new one can be better than the one you are replacing.

Edge and spacer

The seal and spacer that sit around the perimeter. A warm-edge spacer, made of a material that conducts less heat than aluminium, reduces cold at the edge and the tendency for condensation to form there.

On older frames, the existing unit may be unusual in thickness or shape. Triangular, curved or tapered glass can be made, but it takes longer and costs more, and a survey will tell you early whether glass-only is realistic for your frame.

Replacing a unit is a chance to improve it. The frame stays the same, but the glass can be better than the original in several ways.

Better thermal performance

A unit with a low-emissivity coating on one pane, a warm-edge spacer and an argon-filled gap loses less heat than an older uncoated unit. The frame still limits the overall result, which is why a whole-rooflight replacement usually reaches a lower figure.

A laminated inner pane

Laminated glass holds together if it breaks, because a plastic interlayer bonds the fragments. Fitted as the inner pane, it is good practice in overhead glazing and is recommended by industry guidance, though building regulations do not require it for rooflights.

Solar control

A tinted or coated outer pane reduces the heat that comes through on a south-facing roof, which helps in a kitchen extension or a loft bedroom that overheats in summer. The choice of coating changes how much daylight passes, so it is worth looking at a sample against the room.

Self-cleaning glass

A coating on the outer face breaks down dirt in daylight and sheds it in rain. It suits roofs that are hard to reach.

Acoustic glass

A thicker laminated pane, or panes of different thicknesses, reduces rain and traffic noise. It is an option, not a cure, and the frame and its seals still matter.

Not every frame can take every option. A thicker unit needs a deeper rebate, and a heavy unit puts load on hinges and opening gear. That check belongs in the survey, not on the day.

Fitting a new sealed glass unit into the frame you already have needs no planning permission, since the outside of the roof is unchanged, and the Building Regulations do not require notification either. Anyone in a listed building should ask before altering any glazing, because listed building consent reaches further than most people expect, and a conservation area with an Article 4 direction can change the answer again. A short call to the council planning team will settle it. Our conservation rooflight page covers the products that suit those homes.

Replacing the whole rooflight, frame and glass together, is a different matter. A window is a controlled fitting, so a complete replacement must meet Part L of the Building Regulations and be notified to Building Control, or self-certified by an installer on a competent person scheme. Where the job is notifiable we handle that notification, and it goes on the written quote so there is no surprise later.

It is worth knowing which side of that line you are on. Some quotes for a whole replacement are described as a glass change to make them sound smaller. If the frame is coming out, it is a replacement, and the paperwork follows.

A glass-only job is quick compared with a full replacement, and it is mostly outside the house. For a roof window or flat rooflight the work is done from the roof or from a platform at the edge, and for a roof lantern it may need access equipment at the ridge.

1. Protect the room below with dust sheets, because glazing putty, old sealant and grit can drop through.
2. Remove the beads or capping and lift out the old unit, keeping the frame and the weathering round it undisturbed.
3. Clean the rebate, check the frame for rot or movement that was not visible before, and renew the glazing tape or gaskets.
4. Set the new unit on packers so its weight sits correctly, and refit the beads or capping.
5. Seal and check that the gaskets sit evenly, then clear the roof and the room.

If the frame shows a problem that could not be seen from outside, we tell you at that point, with a photograph, and the choice is yours. The written quote agrees in advance what happens if the frame turns out to be unsuitable.

New glass does not touch the lining, which is both its advantage and its limit. If the lining was in good order, the room looks as it did, with a clear pane at the top. If the old unit was misted for a long time, the sill may carry a stain from condensation and the paint may have bubbled or blackened. A new unit stops the cause, but it does not remove the mark.

Stained plaster needs treating before it is painted, or the mark comes back through the new paint. In a lime plastered ceiling, a breathable finish is needed rather than a sealed modern emulsion. If the lining was never insulated well, glass alone will not cure a cold sill, and the better route may be to form the lining again as part of a whole replacement. The way a lining is built is covered in how a rooflight is finished inside.

When we survey for a glass-only job, the finish schedule records what the lining and ceiling look like today and whether the stain is to be treated and repainted. That keeps the job honest: you know whether the room will look finished when the glass goes in, or whether the sill still needs attention.

If the frame is sound and one unit has failed, replacing the glass is a sensible, proportionate job. If the frame is tired, the product is old or the room is about to be redecorated, a whole replacement tends to be better value over the years. A short survey settles it, and we tell you which one we would choose and why.

Start with our misted and failed rooflight replacement service, which covers both routes. To understand why units fail in the first place, read why rooflights mist between the panes, and to check whether yours has reached the point of a full change, the signs a rooflight needs replacing are listed in one place. If you want to understand the effect on the ceiling, replacing a rooflight without damaging the ceiling goes through it.

When you are ready, ask for a quote and send a few photos of the rooflight from inside and out. We will survey, measure and write down what is being changed in the quote.