How much does a pitched roof rooflight cost?

Your covering, from concrete interlocking tile to Cotswold stone slate

This is the single biggest variable in the county, and it is decided before anyone chooses a window.

Concrete interlocking tile on an interwar or post-war semi is the most economical roof to open. The tiles unclip, a standard flashing kit fits the profile, and the covering goes back in an afternoon. Clay pantile costs a little more because profiled soakers and a wider side flashing are needed. Double-lap clay plain tile costs more again, since the lap means tiles come off a long way up the slope and old nibs snap as they are lifted. Welsh slate sits alongside it: brittle at the nail holes, interleaved soakers course by course.

Cotswold stone slate is in a category of its own. Courses have to be taken back up the slope and re-laid to the original diminishing gauge, reclaimed stone is sourced to match thickness and weathering colour, and any mortar torching disturbed has to be replaced with an agreed detail. Sourcing matched reclaimed slate takes weeks rather than days. More on this at fitting a rooflight to a tiled roof.

Pitch, and what it does to a working day

A 30 degree concrete tile roof is walkable on staging and the work moves quickly. At 45 to 55 degrees, which is normal on Cotswold stone and on Forest of Dean slate roofs around Coleford and Cinderford, everything slows: roof ladders, more staging, materials passed up rather than carried, and longer head laps because driving rain climbs further on a steep exposed slope.

Below 15 degrees the manufactured in-plane roof window stops being the right product and a kerb-mounted unit takes its place, which brings carpentry, insulation and membrane work that a roof window does not need. Those thresholds are set out at the minimum pitch for a rooflight and how steep a roof can be.

Getting to the slope, which is priced from the address

Access is the item people underestimate most. A single storey rear addition with a driveway to stand on is one price. A three storey Cheltenham terrace with a parapet, a shared lane and a permit for the pavement is another. In the Stroud valleys, at Chalford, Rodborough and Nailsworth, scaffold is often hand carried up steps or along a footpath, and a lorry cannot get within fifty metres, which adds labour before a tile is touched.

Storey height, the number of lifts, how long the scaffold stands, whether a pavement licence is needed and whether a tower will genuinely do the job are all settled at survey. The full picture is on whether you need scaffolding to fit a rooflight.

Whether a rafter has to be cut, and who has to design it

An opening that fits between two existing rafters needs a head trimmer, a cill trimmer and nothing else. That is the cheapest structural outcome there is, and it is why the size you choose has such a large effect on the figure. Cut one rafter and doubled trimmers carry the load into the neighbours. Cut two or three, or work in a trussed roof at Quedgeley or Bishops Cleeve, and the answer is a designed member: a deeper timber, a flitch plate or a small steel, with an engineer’s calculation and a Building Control structural submission behind it.

The arithmetic is on what size rooflight fits between rafters, and the structural reasoning on does fitting a rooflight weaken the roof.

Unit specification: size, how it opens, and what is in the glass

Within one manufacturer’s range the spread is wide. A fixed unit is the least expensive; a manual centre pivot next; a top hung sash above that; a rod operated unit for a high opening above that again; and an electric unit with a rain sensor and a wall control at the top, since it brings wiring and an electrical sign off with it. See electric rooflight installation.

Glass moves the figure as much as the opening method. A standard double glazed pane, a solar control coating for a south or west facing loft, a laminated inner for rain noise and safety, triple glazing, obscure glazing for a bathroom and a self-cleaning outer coating are all separate decisions. The numbers behind them sit on energy efficient rooflights. A conservation unit with a cast frame and a central glazing bar is its own product, described at conservation rooflight installation.

How far the room below is taken

The reveal is real carpentry and plastering, and how far it goes is a genuine choice. A plain square-sided lining is quicker than a splayed one, but a splay throws light deeper into the room and keeps the corners drier. Insulation has to run continuous and tight to the frame with the vapour layer taped to it, and that is not optional on any specification.

Beyond that: whether the ceiling is plasterboard or lath and plaster, whether there is a fitted wardrobe under the opening, whether decoration is included, and whether the room is a bare loft or a finished bedroom that has to be sheeted and protected. A like-for-like unit swap in a finished room can often be done wholly from outside, which keeps the internal cost close to nothing.

Several units, one scaffold, one notification

Where a roof needs more than one unit, doing them together is materially better value than doing them one at a time. One scaffold, one set of visits, one Building Control notification, one delivery, and frames that match across the elevation rather than drifting through product generations. The same applies where an old unit has misted and its neighbours were glazed on the same day, which is covered at replacing a failed or misted rooflight.

It is also worth asking about the opening size while the roof is open. Going taller within the same rafter bay, at the point when the scaffold is already up and the covering is already off, costs a fraction of what the same change costs as a separate job two years later.

An indicative range by email, then a fixed price after the survey

There are two figures, in this order, and each does a different job.

The first is an indicative range by email. Send a photograph of the roof from the garden or the street, a photograph of the room from inside, what the covering is if you know it, roughly the size and position you have in mind, and the address. The address matters here more than anywhere: in this county the postcode usually tells you the covering, the likely pitch and the conservation position. The range comes back within a day or two and it is free.

The second is a fixed price after a survey, itemised so it is clear what is unit, what is labour, what is scaffold and what is Building Control. It holds unless you change the scope, or unless something structural is uncovered that was already written into the survey notes as a risk. The process is set out further on the costs page.