Replacing In-Plane Rooflights on Profiled Metal Roofs

Replacing an in-plane rooflight on a profiled metal roof is a sheeting operation, not a glazing one. The rooflight sheet takes the place of a metal sheet within the roof plane, so getting it right comes down to matching the profile exactly, handling the liner and cavity on twin-skin roofs, sealing the side and end laps for the actual roof pitch, and fixing to the correct grade and torque. Get any of those wrong and the new rooflight leaks or sits proud, which is why the profile is taken physically off the roof at survey rather than guessed from a catalogue.

This page sets out how in-plane replacement is done on the profiled metal roofs that make up most of the industrial stock around Ipswich. It sits under our commercial skylight installation service, and it is for facilities managers and building owners who want to know what a proper replacement involves before they commission one.

Profile matching, and why it decides the job

The rooflight sits between metal sheets, so it has to match their profile. Profile is not one number. It is the pitch between crowns, the crown width, the rib height and the exact geometry of the side lap, all together. There are dozens of profiles in service across UK industrial stock, and several of the manufacturers that made the older ones have discontinued them or closed.

Most Ipswich industrial buildings went up between the 1970s and the early 2000s, and a large share of their original profiles are now out of production. That leaves three routes.

• Moulded to match. A rooflight sheet made to a profile physically taken off your roof. This is the cleanest answer for a discontinued profile and it carries a cost uplift for tooling and a minimum order quantity.
• Close match with a purpose-made side lap. A current profile that is close enough, fitted with a designed side lap detail to bridge the small difference. This works on some profiles and not others, and the survey establishes which.
• A change of approach. Where a large part of the roof is being addressed anyway, sometimes the right answer is kerbed units or a barrel vault rather than chasing a dead profile. Our barrel vault and continuous rooflight installation page covers that route.

The failure to avoid is a rooflight that sits a few millimetres proud on every side lap. That roof leaks on the first driven rain, and no amount of sealant closes a lap that does not sit down. We take a physical profile from the roof at survey, not a photograph and not a guess from a catalogue, so the sheet that arrives fits the sheets it goes between.

Single skin or twin skin

The roof build-up decides half the specification. A single-skin roof is one weather sheet with no insulation below, common on unheated storage and older sheds. Replacement there is a single sheet swap, matched and lapped.

A twin-skin roof is an outer weather sheet, insulation, and an inner liner panel below. Most heated industrial roofs are twin skin, and they bring two extra requirements that a single-skin swap does not.

The liner rooflight

If there is a rooflight in the outer sheet, there must be a matching rooflight liner directly below it, or the daylight simply arrives in the insulation and goes no further. The liner rooflight is also the safety-critical component in a twin-skin build-up, because the liner is what a person falls through if they come through the outer sheet. A non-fragile outer over a fragile or missing liner is a mixed message on the roof plan, and it gets resolved at specification, not discovered later. Our non-fragile rooflight specification page covers how the assembly classification is set.

The cavity and condensation

The gap between the outer rooflight and the liner has to be handled. Warm, moist air from a heated building rising into an unventilated cavity condenses on the cold underside of the outer sheet, and that interstitial condensation shows up as staining on the liner, drips onto stock, and corrosion at the fixings. It reads as a leak and it is not one. Correctly sealed liner details, a proper spacer system and vapour control at the liner are what prevent it. Getting this wrong turns a dry roof into a wet one from the inside.

Side and end laps

Laps are where profiled roofs leak, and rooflight laps are the weakest because they join two materials that move at different rates. Getting them right is most of what stops the new rooflight leaking.

• Side laps need continuous lap sealant and stitching fixings at the specified centres, not fixed at the crowns and hoped for. A lap that is not stitched closed breathes open and shut with temperature and lets driven rain track through.
• End laps need the correct minimum length for the roof pitch. Shallow roofs, and industrial roofs are frequently down to five or six degrees, need longer end laps because wind drives rain back up the slope. An end lap sized for a steeper roof leaks on a shallow one every time.
• Fixings need the right size of sealing washer and a controlled torque. Over-tighten and the sheet dishes and the surface crazes, which starts local degradation. Under-tighten and the washer never seals. On coastal and port-side roofs the grade matters too, and 316L stainless is specified because standard fixings pit in chloride air. Our coastal industrial roof corrosion answer explains why.
• Thermal movement is planned for, not ignored. GRP and steel expand at different rates, and a long continuous run detailed as if it were rigid tears its own fixing holes over a few summers. Fixing patterns allow the sheet to move.

Access and safety on a working roof

Almost every in-plane replacement is on a roof that is fragile in parts and over a building that is still working below. The access and safety plan is as much of the specification as the sheet.

1. Treat the existing rooflights as fragile. The sheets being removed are, by the fact of their age, fragile surfaces. Work is planned so nobody relies on them, using staging, crawl boards or a mobile platform that spans the openings. Nobody walks on or near a suspect rooflight.
2. Collective protection first. Safety netting is installed beneath the work area before any sheet comes off, giving fall arrest for the operatives and catching tools and debris above a floor that may still be in use. Where the internal environment rules netting out, the alternative is a covered platform or out-of-hours working.
3. Weather-tight by nightfall. Only as much roof is opened as can be closed the same day. If the forecast turns, the day’s opening is reduced rather than gambled on. No roof is left open overnight over a working building.
4. Cordon below. The floor beneath the open bay is cleared and cordoned in advance, agreed with the operations team, so stock and people are out from under the work.

The full method for keeping a building running through the work is on our replacing rooflights in an occupied building page.

How the replacement runs

1. Survey. Physical profile taken from the sheeting, purlin spans measured, build-up confirmed, existing fixings and cut edges assessed, every position recorded.
2. Specification. Sheet grade and surfacing film, liner treatment, lap details for the actual pitch, fixing grade, and the non-fragility classification against your real spans and fixing pattern.
3. Order. Typically four to eight weeks for standard profiles, longer for moulded-to-match sheets on discontinued profiles. Nothing on the roof is opened before material is on site.
4. Install. Netting set, first bay handed over, old sheets out, purlins and fixing points checked, liner fitted where twin skin, new sheet set with correct laps, sealant and fixings, each bay closed weather-tight the same day.
5. Test and hand over. Hose test on completed areas, internal walk beneath every replaced position, and the documentation set issued including the as-installed rooflight schedule and non-fragility classification.

Get your profiled roof surveyed

In-plane replacement lives and dies on the survey: the profile taken physically, the spans measured, the build-up confirmed. We do that, then come back with a scope priced by bay and by sheet count that fits your building and your operating pattern.

Request a survey. Read across to barrel vault and continuous rooflight installation and specifying non-fragile rooflights, and see budget rates on the costs page. Kerbed commercial units on flat roofs are covered under flat roof skylight installation.