How much does it cost to replace a skylight?

A replacement is priced from the roof you already have

New work is priced from a specification: a size, a covering, a slope, a room. Replacement is priced from an existing installation whose condition is only partly visible until the old unit is out. That is the single structural difference between the two.

The survey exists to shrink that unknown as far as it can be shrunk. The size code off the old sash, the depth and centres of the rafters or joists, the state of the underlay at the eaves and at the opening, a moisture meter on the trimmer where it can be reached, the height and construction of the kerb on a flat roof, the condition of the waterproofing it is tied into, and the internal reveal from below. Everything on that list can move the figure, which is exactly why the fixed price comes after it rather than before.

The unit itself, and what a current specification carries

The glazed unit is the smaller half of most replacement figures, and its own price moves on four decisions. Size, which on a made-to-size flat roof unit is continuous rather than stepped. Whether it is fixed, manual or electrically operated, since an electric unit brings an actuator, a control and a wiring run with it. The glass, where a solar control coating on the outer pane, a laminated inner pane and a triple build-up each add something. And the frame finish, where a non-standard powder coat is a factory operation with its own lead time.

Where the old size no longer exists in any current range, which is common on 1980s and 1990s units, the answer is either a made-to-size unit built to the aperture or a standard unit with the opening adjusted to suit. That decision is set out at matching a replacement to an obsolete size and it can move the figure in either direction.

Whether the existing opening and kerb carry the new unit

The cheapest replacement is the one where the structural opening is sound, square and the right size, the kerb is treated, insulated and correctly weathered, and a new unit drops into what is already there. Nothing structural, no plaster, one day outside.

From there the ladder goes up in clear steps. A kerb that needs its outer face insulating and the covering re-dressed over the top. A kerb that has to be rebuilt because it was untreated softwood with the felt turned up but not over. An opening that has to grow up the slope inside the same rafter bay, which needs a new head trimmer and a rebuilt reveal. An opening that has to grow sideways, which means cutting a rafter and doubled trimmers on a cut roof, or a designed member on a trussed one. Each of those is a real step rather than a percentage. Which one applies is decided at survey and written into the quotation. The detail is at when the opening has to change size.

What the ten minutes after the old unit comes out can add

This is the part of a replacement figure that a new installation simply does not have. What is commonly waiting underneath a thirty year old unit: original underlay brittle and slit straight rather than dressed; a head trimmer with historical staining and the ghost of an old water run down one side; insulation compressed flat or missing entirely for 100mm around the frame; battens split where the original fixings went in; lead face nailed down both sides and cracked along the nail line.

Occasionally there is no trimmer at all, just a cut rafter and thirty years of hoping. That is a structural finding and it goes in writing with its own figure before anything new goes near the opening. A good survey flags the likelihood of these in advance and prices the risk openly rather than discovering it on the day, which is the whole point of what is found when the old unit comes out. Where soft timber is the finding, the work involved is set out at what if the timber around the opening has gone soft.

Access, and the addresses in this county where it is the largest line

Access is itemised separately, and on some jobs it is the biggest single item. What drives it is the height of the working line above whatever is directly beneath it, how long the platform has to stand, whether the ground will take a tower, and how far material has to be carried.

A flat roofed rear extension with a paved yard beside it sits at the easy end. The main slope of a two storey house needs scaffold everywhere in the county. A three storey Cheltenham terrace with a parapet needs a boarded lift at the right level and possibly a highway licence with its own lead time. Chalford, Rodborough and Nailsworth sit at the other end entirely, where components are hand carried up a flight of steps and glass may need a small mechanical lift because a lorry cannot get within fifty metres. That is honestly more expensive and it is priced from a survey where the lane has been walked. The full reasoning is at can you replace a rooflight without scaffolding.

Taking the old unit away, and what that carries with it

Removal and disposal are in the quotation as a line rather than assumed. The old glazed unit, the old frame, perished gaskets, old lead, brittle underlay, cut timber and any kerb material being renewed all leave site, separated into streams and moved under a waste transfer note. Glass is the awkward one, particularly where a unit is cracked or a dome has gone brittle, because it is bagged or crated on the platform and lowered rather than carried down.

Two things can change this line materially. Salvaged tiles or slates are stacked and re-laid rather than skipped, which is a labour cost rather than a disposal cost but it is real. And where cement sheeting of an age that might contain asbestos surrounds the opening, it is identified at survey and its removal arranged with a specialist contractor before the rooflight work is programmed, which changes the sequence and the figure. The whole process is at do you remove the old rooflight and dispose of it.

Making good inside, which runs from nothing to a rebuilt reveal

How far the work goes into the room is a genuine choice and it has a wide range. At one end, a like-for-like swap in a finished loft room done entirely from outside leaves the plaster and the paint untouched, and there is no internal line at all.

In the middle sits the case where the reveal is opened at the head because insulation is found missing behind the plasterboard, then reinstated tight and continuous with the vapour control layer taped to the frame, boarded, skimmed and left to dry. At the far end sits a changed opening, where the whole reveal box is rebuilt, the sides splayed to widen the beam of light, the ceiling line altered, and decoration follows after the plaster has dried. Fitted furniture, a wardrobe or a bath under the opening, comes out and goes back. What happens to the finish is covered at what happens to the plaster around the opening.

Several units, one visit, and the overheads that are only paid once

Where a roof has three or four units of the same age, doing them together is materially better value than doing them one at a time, and the arithmetic is not subtle. One platform instead of three. One Building Control notification instead of three. One set of travel, setting up and protection. One delivery of glass. The frames match across the elevation instead of drifting through three product generations.

The units glazed on the same day as the one that has already misted are usually within a couple of years of the same point, so a second platform later costs more than the extra units do now. The reasoning is at replacing several rooflights at once.

Why the figure arrives in two stages

First an indicative range by email. It is based on photographs of the rooflight from outside, the room from inside, the covering, the size code off the old unit if you can reach it, and the address. It is free, it takes a day or two, and it exists so that nobody spends an afternoon on a survey for a project that was never going to work financially.

Second, after the survey, a fixed price, itemised so it is clear what is unit, what is labour, what is access, what is disposal and what is Building Control. That figure holds unless the scope changes at your request, or unless something structural is uncovered that was written down as a risk in the survey notes beforehand. What that document should contain, line by line, is at what a replacement quotation should contain, and the general process is on the costs page.

Severn Skylights Gloucester fits new rooflights and replaces units that have reached the end of their service life, across Gloucester and thirty miles by road, with a ten-year workmanship guarantee. Email info@severnskylightsgloucester.co.uk with those photographs and an address for the first figure. The wider replacement job is described at replacing a failed or misted rooflight.