Use in-plane rooflights on profiled metal roofs and kerb-mounted units on flat, membrane or built-up roofs. In-plane sheets sit flush in the roof plane, cost the least and shed water with the roof, but only suit a profiled covering. Kerb-mounted units sit raised on an upstand, cost more, and are the way daylight comes through a flat roof, with the upstand height managing the ponding risk. The roof covering and pitch decide which is right.
This answer sits under our commercial skylight installation service. It is the practical comparison behind a common specification question. Our what in-plane rooflights are answer covers the in-plane type in detail.
How they compare
| Factor | In-plane | Kerb-mounted |
|---|---|---|
| Roof covering | Profiled metal | Flat, membrane, built-up, or profiled with a kerb |
| Position | Flush in the roof plane | Raised on an upstand above the roof |
| Weathering | By profile match and laps | By upstand height and flashing into the covering |
| Cost | Lowest | Higher, includes the kerb and weathering |
| Ponding risk | Low, sheds with the roof pitch | Managed by upstand height on flat roofs |
| Thermal performance | Modest single skin, better twin skin | Good, the kerb carries the insulation line |
| Daylight per opening | Modest, flat sheet | Higher, especially curved vaults |
| Best pitch | Pitched profiled roofs | Flat and low-pitch roofs |
Weathering
In-plane rooflights weather the same way the metal roof does: by matching the profile and lapping into the sheets around them, so water runs off the roof surface across the rooflight without meeting an obstruction. The weakness is the laps, which have to be the right length for the pitch. Kerb-mounted units weather differently, by rising above the roof surface on an upstand and flashing into the covering, so water runs around the base of the kerb rather than over the unit. The weakness there is the kerb-to-covering joint and the upstand height.
Ponding risk
This is where the roof pitch matters most. On a pitched profiled roof, water runs off, and an in-plane rooflight sheds with it. On a flat roof, water can sit, and a rooflight sitting flush in a flat roof would sit in the water. A kerb-mounted unit lifts the glazing above the water line, and the upstand height is set against how much the roof can pond. A flat roof that drains slowly needs a taller upstand. Putting an in-plane-style flush rooflight in a flat roof is asking for a leak, which is why the covering, not preference, drives the choice. Our flat roof skylight installation page covers upstand detailing.
The rule by roof covering and pitch. Profiled metal, pitched: in-plane, the cheap and natural answer. Flat, membrane or built-up: kerb-mounted, because there is no profile to match and the upstand beats the ponding. Low-pitch profiled roof: in-plane with careful lap lengths for the shallow pitch. Where a lot of even daylight is needed on any roof: a kerbed barrel vault. The covering rules out the wrong choice before cost or preference enters.
Thermal performance
Kerb-mounted units generally handle heat better, because the kerb carries the roof’s insulation line up and over the opening and the unit itself can be a well-insulated glazed or multiwall assembly. In-plane single-skin GRP is thermally modest, though twin-skin in-plane with a liner closes much of the gap. On a heated building the thermal build-up matters, and it is set so the lighting saving is not lost to heat. Our double or triple skin answer covers that within the sheet.
Cost
In-plane is the cheaper option wherever it suits the roof, because it is a sheet swap with no kerb, no upstand and no separate weathering. Kerb-mounted units cost more because the kerb has to be formed, insulated, structurally supported and weathered into the covering. That extra cost is not waste, it is what makes a rooflight work on a flat roof, but it does mean you do not choose kerb-mounted on a profiled roof to save money, only where the roof needs it or the daylight demand calls for a vault. Our barrel vault rooflight cost answer covers kerbed vault pricing.
What it means around Ipswich
Most of the older industrial stock across Ipswich and the Suffolk and north Essex estates is profiled metal, so in-plane is the common answer on those roofs. Some newer distribution and production buildings around the port and the A14 corridor have flat or membrane roofs, where kerb-mounted units and barrel vaults are the route to daylight. On the coastal belt near Felixstowe, both types take the marine fixing and coating upgrade. The survey confirms the covering and pitch, which is what decides the type.
Can I use in-plane rooflights on a flat roof?
No. In-plane rooflights need a profiled roof to lap into and shed water across. A flat roof can pond, and a flush rooflight would sit in the water, so daylight comes through a kerb-mounted unit or a barrel vault raised on an upstand above the water line. The upstand height is set against how much the roof can pond.
Which is cheaper, in-plane or kerb-mounted?
In-plane, wherever the roof suits it, because it is a profile-matched sheet swap with no kerb or separate weathering. Kerb-mounted units cost more because the kerb has to be formed, insulated, supported and weathered in. You choose kerb-mounted where the roof is flat or membrane, or where you need the extra daylight of a vault, not to save money on a profiled roof.
Get the right type for your roof
The roof covering and pitch decide between in-plane and kerb-mounted, and the survey confirms them. We recommend the type that suits your roof and price it accordingly.
Request a survey. Read across to what in-plane rooflights are, barrel vault rooflight cost, 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.