In-plane rooflights are rooflight sheets laid flush within the profiled roof covering, taking the place of a metal sheet in the roof plane rather than sitting on an upstand above it. They match the profile of the metal sheets around them and lap into them, so the daylight comes through the roof surface at the same level as the rest of the roof. They are the cheapest way to bring daylight into a large profiled roof, and the most common type on industrial buildings.
This answer sits under our commercial skylight installation service and explains what they are and how they differ from kerb-mounted units.
How they work
A profiled metal roof is made of sheets that lap side to side and end to end. An in-plane rooflight is a translucent sheet, usually GRP or polycarbonate, moulded to the same profile as those metal sheets, that takes the place of one of them. It laps into its neighbours exactly as a metal sheet would, so the roof surface stays continuous and flat, with the rooflight simply being the see-through part of it. Because it sits in the plane of the roof, there is no upstand and no separate frame, which is what makes it cheap.
Profile matching
The sheet has to match the profile of the metal around it, and profile is not one number. It is the pitch between crowns, the crown width, the rib height and the side lap geometry, all together. A rooflight that does not match sits proud at the laps and leaks. On older buildings the original profile is frequently discontinued, so the answer is a sheet moulded to match a profile taken physically off the roof. Our in-plane rooflight replacement on profiled metal roofs page covers the detail.
Liner panels on twin-skin roofs
On a twin-skin roof, an outer weather sheet over insulation over an inner liner, an in-plane rooflight in the outer sheet needs a matching rooflight liner directly below it, or the daylight arrives in the insulation and goes no further. The liner rooflight is also the safety-critical layer, because it is what a person falls through if they come through the outer sheet. Both the outer and the liner form part of the tested non-fragility assembly. Our non-fragile rooflight specification page covers how that is classified.
How they differ from kerb-mounted units
| Feature | In-plane | Kerb-mounted |
|---|---|---|
| Position | Flush in the roof plane | Raised on an upstand above the roof |
| Roof type | Profiled metal | Flat, membrane, built-up, or profiled with a kerb |
| Cost | Lowest | Higher, includes the kerb and weathering |
| Weathering | By profile match and laps | By upstand height and flashing |
| Ponding risk | Low, sheds with the roof | Managed by upstand height on flat roofs |
| Daylight per opening | Modest, flat sheet | Higher, especially curved barrel vaults |
The choice between them usually comes down to the roof type. On a profiled metal roof, in-plane is the natural, cheap answer. On a flat, membrane or built-up roof there is no profile to match, so a kerb-mounted unit, a rooflight or a barrel vault on an upstand, is the way daylight comes through. Our in-plane or kerb-mounted commercial rooflights answer compares them in full.
In-plane is a sheeting job, not a glazing one. Because the rooflight is part of the roof covering rather than a framed unit sitting on top, it is installed like a roof sheet: matched, lapped, sealed and fixed. This is why in-plane replacement is quoted from a survey that takes the profile physically off the roof, and why the laps and fixings, not the sheet, decide whether it keeps the water out.
Where they are around Ipswich
Almost all of the profiled metal industrial stock across Ipswich, the port estates and the Suffolk and north Essex estates uses in-plane GRP rooflighting, most of it installed between the 1970s and early 2000s. That puts the great majority of it at or past the end of its service life, which is why in-plane replacement is the most common commercial rooflight job in the region. On the coastal belt the fixings and cut edges corrode faster, so replacement there also renews the fixings to a marine grade. Our coastal industrial roof corrosion answer covers that.
Are in-plane rooflights the same as skylights?
They are a type of rooflight, and the industrial equivalent of what a house might call a skylight. In-plane rooflights sit flush in a profiled metal roof and replace a metal sheet, rather than being a framed unit set on an upstand. On a commercial roof they are the most common and cheapest way to bring daylight in.
Can an in-plane rooflight go on a flat roof?
No. In-plane rooflights need a profiled roof to lap into. A flat, membrane or built-up roof has no profile to match, so daylight comes through a kerb-mounted unit or a barrel vault raised on an upstand above the roof surface. The roof type decides which is right.
Get your in-plane rooflights surveyed
In-plane replacement lives on the survey: the profile taken physically, the spans measured, the build-up confirmed. We do that and price the work by bay.
Request a survey. Read across to in-plane or kerb-mounted, why GRP rooflights degrade, and the in-plane replacement guide.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.