How long does a skylight replacement take?

The lead time on the unit, which sets the date more than the diary does

This is where the weeks are. A standard manufactured roof window in a common size, in stock at a merchant, can be on site in days. A less common size in a specific glass build-up is a factory order. A made-to-size flush glazed flat roof rooflight is manufactured to your aperture and has its own production period. A non-standard powder coat finish adds another stage, because coating happens after fabrication and it is a separate process with its own queue.

Two things lengthen it further and both are common on replacements. Where the old size no longer exists in any current range, the answer is either a made-to-size unit or a standard unit with the opening adjusted, and either decision has to be taken before the order rather than after. That is set out at matching a replacement to an obsolete size. And where a Cotswold roof needs reclaimed stone matched for thickness and weathering colour, that sourcing runs to weeks on its own and it happens in parallel with the unit being made.

A like-for-like swap on a pitched slope, from first tile to last

On a concrete interlocking tile roof at Longlevens, Hucclecote, Quedgeley or Bishops Cleeve, with a sound structural opening and reasonable access, the outside work is one day and it runs roughly as follows.

The morning goes on stripping the covering back around the opening, stacking the tiles by course, taking the old flashings off and lifting the old unit out. Then the ten minutes that decides how the rest of the day goes: the trimmer checked with a moisture meter, the underlay looked at, the insulation around the frame perimeter assessed. Midday is the remedial work that finding throws up, plus renewing the underlay locally and dressing it properly around the new frame. The afternoon is setting and fixing the new unit, the new flashing kit, and re-laying the covering into it with cut pieces mechanically fixed. By late afternoon the roof is whole. Welsh slate and double lap clay plain tile both extend that, because the area disturbed is larger and old material breaks as it is lifted, so allow into a second day on either.

A flat roof unit, and whether the kerb is being rebuilt

A flush glazed rooflight or a dome coming off a single storey extension is often quicker than a pitched swap, because there is no covering to strip and re-lay. Where the existing kerb is treated, insulated on its outer face and properly weathered, the old unit comes off, the kerb top is prepared, the new unit is set and fixed and the perimeter is finished, and that is a comfortable day.

Where the kerb has to be rebuilt, add a day. The old kerb comes off, the deck around it is opened and checked, a new treated and insulated kerb is built, the waterproofing is dressed up the kerb and over the top so the upstand becomes part of the roof, and only then does the unit go on. On a single-ply or EPDM roof the detailing has to be made onto a dry substrate and adhesives have their own cure time, so this is not a sequence that compresses. Where the deck itself ponds and the falls have to be corrected first, that is a separate piece of roofing work programmed ahead of the rooflight.

When the opening changes size, and the days that adds

Growing the opening changes the shape of the programme rather than just its length. Taller within the same rafter bay is the cheap direction in time as well as money: the head trimmer comes out, the opening extends, a new head trimmer goes in, and it adds roughly half a day outside plus the internal work.

Wider means cutting a rafter and fixing doubled trimmers, which on a traditional cut roof is routine carpentry and adds a day, and on a trussed roof at Abbeydale, Ashchurch or Yate means a designed member carrying the affected truss, which adds an engineer’s calculation to the calendar before it adds anything to the site programme. Either way the internal reveal is rebuilt and re-plastered, so the internal side goes from nothing to two or three days including drying. The decision itself is covered at when the opening has to change size and the same-aperture question at can a bigger rooflight go in the same opening.

Diminishing courses, and the weeks before anyone arrives

A Cotswold stone slate roof is the case where the answer changes materially. The courses above the opening are stripped back up the slope, the frame is set, and the courses are re-laid downwards, re-establishing the diminishing gauge so the finished roof reads correctly. Slates are 20 to 30mm thick and the loading runs well over 100 kilograms per square metre, so the work is slower at every stage and the salvaged stone has to be stacked and sorted by course.

Two to three days outside is realistic on a Painswick, Bibury or Northleach roof, sometimes more where torching has to be dealt with and a breather membrane on counter battens is agreed as the replacement detail. In front of that sit the weeks of sourcing reclaimed stone and, on a listed building or in a conservation area, the pre-application conversation with the conservation officer, which is not a queue anyone can shorten. The upside is that it all runs in parallel with the unit being made.

Three or four units in one week

Where a roof has several units of the same age, doing them together takes considerably less time than doing them separately, and the saving is not linear. One platform goes up once. One set of protection goes in. One delivery of glass arrives. One notification covers the lot.

The sequence runs from the top of the slope downwards so the roof is closed every evening, one unit at a time. A set of four like-for-like roof window swaps on one elevation of an interwar semi is typically three days rather than four separate one day visits with four separate mobilisations. The reasoning, including why the units glazed on the same day tend to reach the end together, is at replacing several rooflights at once.

Winter, weather and the days that move

Between November and February the working day in Gloucester runs to around seven hours of usable light rather than fifteen, so the programme is built backwards from the hour it gets dark and the start times move earlier. Frost on a slope pushes a start back by an hour or two rather than losing a day. Wind is the one that actually loses days, because a large glazed panel is a sail and handling stops well before the point where standing on the roof would be uncomfortable.

None of that leaves a roof exposed, because nothing is opened that cannot be closed before dark. A date lost to weather is a date where the covering was never touched. The full winter picture is at can a rooflight be replaced in winter.

Plaster, drying, and the certificate that closes the job

The tail of the job is the part most often left out of an estimate. Where the reveal has been opened, insulation goes back tight and continuous, the vapour control layer is taped to the frame, then boarding and a skim. Fresh plaster wants several days to dry properly before it takes paint, longer in winter and longer again in a room that is not heated, and painting it too early traps moisture and shows within a season. So decoration is a separate visit rather than the same afternoon.

Alongside that runs the Building Control side. The notification goes in with the order, the inspection is booked around the work, and the completion certificate comes back to you in your name once it is closed out. That paperwork typically arrives a few weeks after the last day on site, and it is the document a buyer’s solicitor asks for. What it covers on a replacement is at do replacement rooflights need building regulations.

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. For a realistic date, email info@severnskylightsgloucester.co.uk with a photograph of the unit from outside, one from the room below, the size code off the old sash if you can reach it and the address. Pricing runs alongside it on the costs page, and the trade as a whole is on the skylight and rooflight installation and replacement page.