How much does a flat roof rooflight cost?

Why a published figure would mislead you

Two houses in the same Oxford street can want the same size of rooflight and get quotations that are nowhere near each other, honestly quoted in both cases.

One has a modern warm roof with a sound single ply covering, a deck in good order and joists running the right way. The other has a 1980s felt roof over a cold deck, joists running the wrong way for the opening, no fall worth the name and a covering that will not take a new termination. The unit is identical. The job is not.

A headline price only works when the thing being sold is the same every time. Here it never is.

The unit is the smaller half

People assume the glass dominates the cost. On a straightforward installation it is usually somewhere near half, and on a difficult roof it can drop well below that.

The rest is labour, timber, insulation, covering materials, access equipment, waste removal and making good inside. None of that is optional and none of it is visible in a product listing. When a quotation looks unusually low, the question to ask is not what unit is being supplied but which of those items has been left out or handed to another trade.

What the glass specification does to the number

Within the same footprint, the glass build-up can change the unit cost substantially.

Solar control is the one people skip and later wish they had not. On a flat roof the glass faces straight up at the sky for the whole day, which is a harder thermal exposure than any vertical window.

Opening units, and everything that follows one

An opening flat rooflight costs more than a fixed one twice over: once for the hardware, and once for the trades it brings with it.

The actuator, hinges, weather seals and control gear sit inside a heavier frame. Then it needs a fused spur within reach, which means an electrician, and a switch position, which means chasing a wall or running a cable through a ceiling that may already be finished. A rain sensor adds a little more. If the room only needs air rather than access, the honest answer is often that a fixed unit plus a separate ventilator is cheaper and works better. We go through the options on whether a flat rooflight can be opened.

Whether a kerb already exists, and what state it is in

Building a kerb from nothing is a day of carpentry plus insulation plus the covering being dressed to it. Reusing a sound existing kerb removes almost all of that.

The catch is that an old kerb is often the wrong height, the wrong size for any current unit, or uninsulated. Reusing it to save money is a false economy if it means the new unit sits too low or cold-bridges the ceiling. On a survey we measure the existing kerb before anything else, because that single answer moves the price more than the choice of manufacturer does.

Access, and how material reaches the roof

A single storey rear extension roof reached from a small garden is the easy case: a tower or a short scaffold, material passed up by hand.

The cost climbs when the roof is over a rear return with no side access, when the only route is through the house, when a scaffold has to be designed around a conservatory or a boundary, or when the unit is large enough that hand-lifting is not sensible. A three metre pane of triple glazing is a mechanical lift, and that is a separate day with its own logistics. On Oxford terraces and college-owned properties, the access route is often the single most expensive line on the quotation.

The covering, and how much of it has to be renewed

New work around a rooflight always means new covering locally: up the kerb, over the top, and lapped or welded back into what is already there.

How far that extends depends on the material and its condition. Single ply welds to itself cleanly, so a local area is genuinely local. Torch-applied felt laps well while it is sound but poorly once it has gone brittle. GRP will not bond reliably to old GRP without preparation, so the area worked is wider. Where the covering is near the end of its life anyway, we will say so, and say that the sensible sequence is new covering first, unit second.

Structure: trimmers, beams and the joists you cannot see

Openings up to around the span of one or two joists are ordinary carpentry. Beyond that the work changes character.

A wide opening needs doubled trimmers, sometimes a steel or engineered timber beam, and occasionally a structural calculation. That is a cost, but it is also a programme item, because a calculation and a beam both take lead time. This is why we would rather find it at survey than on the morning the unit arrives. It is also why the number of units matters as much as their size, covered on how many rooflights a flat roof can take.

How we quote, and why it is two figures

Tell us the roof, the rough size and what the room is for, and we will send a range by email the same week. It is a range on purpose, because at that stage we are working from a description.

Then we survey: deck, structure, covering, insulation position, falls, outlets, access. After that you get one fixed price, with the kerb, the making good, the waste and the Building Control notification inside it rather than beside it. If the survey turns up something that changes the job, you hear it then, with the option to stop. We install and replace only, we have been doing it for fifteen years, and the workmanship is guaranteed for ten. Call 01865 704245 or email info@heritageskylightsoxford.co.uk.