Roof Lantern Upstands: Timber, Kerb or Insulated

It is a structural platform first: the lantern's weight, along with wind pressure and any snow load, bears down onto the kerb walls, so they have to be built to carry it whatever material they are made from. It is a weatherproofing junction second: the roof's felt, EPDM or GRP covering has to turn up the outside face of the kerb and lap under the lantern's frame, and guidance for flat roofs looks for that upturn to reach at least 150mm above the finished roof surface at its lowest point, so standing water and wind-driven rain cannot get behind it. It is an insulation zone third, since the kerb has to close the gap in the roof's insulation left by the opening, and the standard the whole lantern is checked against, a limiting whole-unit U-value of 2.2 W/m²K under Approved Document L, is measured across the kerb and frame together, not the glass on its own.

Put together, the kerb does three jobs at once:

• Carries the lantern's structural load down into the roof.
• Gives the roof covering somewhere to lap and seal against.
• Sets the height the glass stands at, and with it, the depth of lining between the ceiling and the sky.

That last point is the one people miss. The kerb's height, added to the roof's own build-up, is the well the sightline has to cross before it reaches open glass. Our guide to sizing a roof lantern for your extension works through how that depth interacts with room width and ceiling height; here, the question is which kerb gets you there, and what each route costs in warmth and in view.

Its strengths follow from being built in place. A carpenter can true it to a roof that is slightly out of square, step the height to follow the fall rather than build a separate upstand for each corner, and adjust the size on the day if the trimmed opening comes out a few millimetres different from the drawing. On an older cold roof, or an extension where the joists and the opening were never going to be perfectly regular, that flexibility earns its keep. It is also the option most roofing crews already know how to build well, without waiting on a delivery.

Its weakness is thermal. Timber studs conduct heat faster than the insulation either side of them, so a kerb insulated only between the studs carries a run of small cold bridges up its full height, each one a stripe where the inner lining sits a little cooler than the board either side. Detailed properly, with a continuous layer of insulation wrapped around the outside of the frame as well as the fill between the studs, a timber kerb closes that gap and meets the same U-value as any other build-up. Detailed poorly, it becomes the reason a lantern feels cold to the touch along its lining even when the glass itself is a modern, well-specified unit. Timber also moves a little as it dries out and as temperatures swing through the seasons, which is one of the reasons a lantern can be heard rather than seen; our answer on why a roof lantern makes cracking noises covers what that movement sounds like and when it is worth having looked at.

Because the insulation runs continuously through the whole kerb, the U-value across it is even and predictable, which matters most where the rest of the extension has been designed close to the Part L limit and there is little margin left to lose at the roof. The unit arrives square and level, which shortens the time the roof stands open on site, a real advantage over a working kitchen in winter, and it removes one variable from the finished job: the kerb cannot be built slightly out of true or insulated unevenly by hand, because it was made to a fixed specification before it reached the roof.

The trade-off is that a factory unit is ordered to a size, so it suits a lantern chosen from standard dimensions or a made-to-measure order placed with enough lead time, rather than a kerb trimmed to fit an opening that turns out slightly different on the day. It also typically costs more than an equivalent timber kerb for the same footprint, which is worth weighing against the roof it will sit on; our guide to what affects the cost of a rooflight sets out where the upstand sits among the other variables on a quote.

The advantage is height. A low-profile kerb can shave real centimetres off the total depth between the roof deck and the underside of the glass, which matters most on a shallow loft conversion or a room with a low ceiling, where every millimetre of well depth is sky lost from the sightline. It also arrives as one factory-sealed component with the frame, so the junction between kerb and lantern is made under controlled conditions rather than on the roof. The limitation is that it is fixed to that particular lantern system, so it is not a like-for-like swap onto an existing timber upstand without checking the frame it was designed to sit on. Where an existing lantern's kerb has been assessed and found sound, keeping it and changing only the glass and frame above is often the simpler route; our guide to replacing an old polycarbonate roof lantern covers how that kerb is checked before anything is ordered.

A tall kerb, built to clear a deep warm-roof insulation layer or to follow a steep fall on one side of the roof, pushes the glass further from the ceiling line. Seen from a chair a couple of metres away, that extra height can be enough to bring the near edge of the lining into the eyeline before the sightline reaches open glass, turning a clear patch of sky into a lit box of plaster. A shorter kerb, whether it is a factory low-profile unit or a timber frame built to the minimum the roof allows, keeps the well shallow and lets more of the room's sightlines clear the lining and reach the glass.

The lining's shape does some of the same work as the kerb's height. A reveal that runs square down from the glass, straight sided, holds the view to almost directly underneath the lantern, while a splayed reveal, angled outwards as it drops, opens the same kerb height back up to more of the room. On a tall kerb, splaying the lining is often the cheaper fix than trying to shave height off the upstand itself, because it changes the angle the eye travels through rather than the distance.

Cold bridging is the main mechanism: any point where a material that conducts heat quickly, timber studs, an unbroken metal fixing, or a gap left in the insulation at a corner, runs from the warm inside face to the cold outside face, heat crosses it faster than it crosses the insulated field around it. The visible sign is often condensation, not draught: a cold stripe on the lining collects moisture from the room's air before the rest of the surface does, and in a kitchen, where cooking adds a lot of moisture to the air, that stripe can show up as a damp line along the kerb well before the glass itself streams. Our answer on why condensation forms inside a roof lantern goes through the glass and ventilation side of that picture; the kerb is the other half of it.

Heat loss through the kerb also adds to the figure on the quote for the lantern as a whole. The 2.2 W/m²K limit in Approved Document L is checked against the whole unit, kerb included, so a kerb with poor continuity of insulation can pull the combined figure the wrong side of the standard even when the glass alone would easily pass. Our answer on whether roof lanterns lose heat sets out where a lantern loses warmth across frame, kerb and glass together, and our guide to roof lantern Building Regulations covers the standard in full; we install to current Building Regulations and handle the Building Control notification wherever the work is notifiable.

On the newer flat-roofed extensions common across the townships of Bretton, Orton, Werrington and Hampton, a warm-roof build-up with insulation above the deck is typical, which already asks for a taller kerb to clear that layer before the waterproof covering even begins. A factory insulated upstand suits that situation well, because it holds a consistent U-value through a kerb that has to stand taller than a cold-roof equivalent, and it arrives ready to sit on a warm deck without extra site detailing. On an older Fletton brick terrace with a shallower, cold flat-roof extension to the rear, the existing upstand is often lower to begin with, and a timber kerb built to match that shallower depth, with continuous insulation wrapped around the frame, keeps the reveal shallow and the sightline open without forcing extra height the room does not need. Around Stamford and the Nene valley villages, where Collyweston stone slate and limestone roofs sit over older structures, the existing roof build-up decides more than either preference: a bespoke timber kerb, cut to fit a structure that was never built to a standard modern module, is usually the more workable route.

None of that rules either option in or out on its own. It is a reason the kerb gets specified at survey, against the roof that is actually there, rather than chosen from a brochure before anyone has been up a ladder.

Where the opening is being cut fresh and the roof's structure is being assessed for the load in any case, it is worth deciding the kerb type at the same time as the lantern's size and shape; our answer on whether a flat roof can take the weight of a roof lantern covers how the joists and trimmers around the opening are assessed, which is the same visit that settles what kind of kerb the roof can carry. Every option, timber, factory insulated or a low-profile kerb built into the frame, is drawn onto the same sightline sketch that shows the rest of the lantern, so you can see the well depth and the view it gives before anything is ordered, not after the roof is opened.

Can I change the kerb type later without replacing the whole lantern?

Not usually. The kerb is built into the roof structure and dressed into the waterproof covering, so changing it means opening the roof again in the same way as fitting a new lantern. If the glass and frame have failed but the kerb is sound, it is normally kept and reused rather than rebuilt.

Does a taller kerb always mean a worse view?

Not on its own. Height matters together with how far you sit from the opening and how the lining is shaped. A splayed reveal can recover much of the sky a taller kerb would otherwise hide, which is why the lining finish is decided alongside the kerb height rather than afterwards.

Is a factory insulated kerb worth the extra over timber on a small lantern?

It depends on the roof. On a small lantern over an already generous glazing allowance, a well-detailed timber kerb can meet the same standard for less outlay. Where the extension's overall Part L figure is tight, the consistent performance of a factory unit can be the difference that gets the whole scheme through.

How do you decide which kerb to specify for my job?

We measure the existing roof build-up, or the one your builder is planning, check what depth and fall it needs to clear, and draw the result on your sightline sketch alongside the lantern itself. To start, book a home visit or message us on WhatsApp, and see our full roof lantern installation service for what the whole job involves.