Roof lantern kerbs and who builds them

Sitting on top of the kerb, or dropping over it

Lantern systems fix to a kerb in one of two ways, and they need opposite dimensions.

A top-bearing base ring sits flat on the kerb top and is fixed down through it. The number the manufacturer wants is the outside face to outside face measurement of the kerb, because the eaves beam usually overhangs that face by a small amount and drips clear of it. Get the kerb 15mm wide and the beam still lands, just with a smaller overhang. Get it 15mm narrow and the beam overhangs too far and the internal lining has an awkward shadow behind it.

An over-kerb or skirted base ring drops down over the outside of the kerb like a lid, with an internal rebate that grips the timber. That system is far less forgiving. The kerb has to be inside a stated dimension on both axes and on both diagonals, because the skirt has to slide down all four faces at once. A kerb 6mm oversize on one corner stops the frame 40mm short of home and there is nothing to be done except plane the timber.

Ask which type is being supplied before the kerb is framed, and get the answer as a dimension with a tolerance rather than as a description. That is the single most useful sentence in a lantern order.

Timber section, treatment class and the fixings that hold a kerb down

A kerb is structural in a small way: it carries the whole weight of the lantern, and on a wide unit it also resists the outward thrust the sloping rafters put into the eaves beam.

The framing is treated softwood, commonly 150 by 50 or 100 by 50 sawn, doubled or built up in layers where the height demands it. Use timber treated to a use class suitable for external, above-ground service with a risk of wetting, because a kerb spends its life inside a waterproofing layer where nothing dries quickly. Cut ends get an end-grain preservative before they go together, since the treatment does not run right through a sawn section.

Corners are jointed rather than butted and skew-nailed: a halved or lapped corner with the two faces screwed through gives a joint that stays square while the membrane is being dressed round it. The kerb is then fixed down into the deck and into the trimmers below it, not just into the plywood or OSB. Screws into 18mm sheet material alone will hold a kerb square long enough for a plasterer to see it and no longer.

Why a kerb that measures 150mm on the deck is not 150mm when the roof is finished

This is the most common height error, and it is pure arithmetic that gets skipped.

The upstand figure that matters is measured from the finished surface of the waterproofing at the highest point of the fall. On a warm deck, the finished surface sits a long way above the structural deck: a vapour control layer, then 100mm to 150mm of rigid PIR, then an 18mm overlay board, then the membrane itself. Frame a kerb 150mm tall off the deck on that build-up and the finished upstand is around 15mm, which is not an upstand at all.

The sum on a typical warm deck extension runs: 150mm upstand, plus 120mm of PIR, plus 18mm of overlay board, giving 288mm of kerb to frame off the deck. That is two 150 by 50 sections stacked, or a 300 by 50, and it changes the whole appearance of the kerb from below. On a cold deck with the insulation between the joists, the same 150mm upstand needs only about 170mm of framed kerb. On an inverted or ballasted roof, add the depth of the ballast or the paving on top of the membrane as well, because water can stand within that layer.

The practical answer is to work the height out at survey from the specified roof build-up, write it on the survey sheet as a framing dimension rather than as an upstand, and mark the high point of the fall on the kerb timber in pencil before the insulation goes down. Once the roof is finished nobody can see where that high point was.

Setting a square kerb on a rear extension that is not square

Gloucester’s Victorian terraces produce back extensions that are out of square by 20mm or more over three metres, because the original brickwork was built to a boundary rather than to a set square, and the 1980s flat roof was built off the brickwork. The deck opening is usually out of square by the same amount.

The kerb does not follow the deck. It is set out square in its own right, from a baseline struck on the deck, with the diagonals checked before a single fixing goes in. The out-of-square is then absorbed in the surrounding flat roof, where nobody will ever measure it, rather than in the aluminium, where it shows as a tapering gasket line down one rafter for the next thirty years.

The place the discrepancy usually surfaces is the internal reveal. A square kerb inside an out-of-square room gives a plaster lining that is 15mm wider at one end than the other, and against a straight ceiling line that reads badly. So the setting-out gets agreed against whichever line the eye actually follows: on an open-plan kitchen that is normally the run of units or the line of the bifold head, not the party wall. Where the room is genuinely skewed, splaying the internal lining unevenly is the honest fix, and it is invisible once painted.

Building a kerb off blockwork or stone rather than timber

On new work, a garden room or an orangery in the wolds, the kerb is often masonry. A blockwork upstand rendered and waterproofed, or a stone parapet with the lantern set behind it, both change the detailing.

A masonry upstand needs a timber head plate bolted to the top of it to take the base ring fixings, bedded level in mortar and packed where the blockwork wanders. It also needs insulation on the outside face exactly as a timber kerb does, because a block is a better conductor than softwood, not a worse one. Where the lantern sits behind a coped Cotswold stone parapet at Painswick, Bibury or Northleach, the timber kerb is built independently inside the parapet and the single-ply or lead is taken up behind the coping into a chase. The lantern does not bear on the coping and the coping is not asked to weather anything. Stone that stays saturated through a Cotswold winter spalls at the arris, and it is the top of a wall that suffers first.

The parapet gutter behind the coping is then the only drainage the roof has, so the kerb position has to leave that gutter its full designed width and its fall to the outlet or the chute.

Factory-made insulated kerbs, and where a site-built one still wins

Several lantern makers now supply a matching upstand: a timber or composite frame with rigid insulation already bonded to the outer face, delivered square, in a standard height of around 150mm or 300mm above the deck.

They are genuinely good on a new build or a straightforward new extension where the deck is flat, square and dry. The frame and the kerb are guaranteed to agree, the insulation continuity is designed rather than improvised, and half a day of carpentry comes off the programme.

They are less useful on an existing roof. A factory kerb comes in fixed heights, and an existing 1970s felt roof at Longford or Tuffley with a 1:100 fall and 130mm of new insulation going over it needs a height that is not one of them. It also has to land on a deck opening trimmed to a tighter tolerance than an existing joisted roof will usually give without work. Where the deck has to be re-fallen anyway, a site-built kerb framed to the measured build-up is quicker and lands the upstand where it belongs.

Surveying a kerb built by another trade before a frame is ordered

Where a builder has already framed the kerb, the sensible sequence is to measure it rather than to trust the drawing it came from. That visit takes twenty minutes and it prevents the expensive version of this job.

What gets recorded: both axes measured across the outside faces at three points, both diagonals, level along each side and corner to corner, the height above the finished waterproofing at the high point of the fall, the plumb of each outer face, the width of the kerb top including insulation, whether the membrane has been taken over the top or only up the face, and how the timber is fixed down. Photographs of each corner go with it.

If the kerb is inside tolerance, the order goes off the measured dimension with the tolerance stated. If it is not, the correction is small at that point and enormous later. Planing a kerb top before the membrane is dressed over it is an hour. Doing the same thing after the roof is finished means the waterproofing comes off and goes back on.

How the frame is fixed to the kerb top, and what it is bedded on

The eaves beam is bedded on the finished kerb top on a continuous bead of low-modulus silicone, or on the butyl tape the manufacturer supplies, and mechanically fixed down at the centres the system specifies, commonly every 400mm to 500mm and always within 150mm of each corner.

Three details in that operation decide whether it stays dry. The bedding compound is continuous, with no breaks at corners, so there is no route under the beam. The fixings go through the beam’s own pre-drilled or marked positions, into solid timber, not into the joint between two layers of a built-up kerb. And the membrane termination underneath is complete before the beam goes on: the beam sits on waterproofing, not on bare timber with the membrane stopped at the outer arris.

Silicone is not the weathering. It is a bed that excludes air and stops capillary movement under an aluminium section, and if it is being asked to keep water out of a kerb then something upstream of it has already failed.

What the kerb has to clear: outlets, abutments and a soil vent pipe

A kerb position is decided in plan against everything else the flat roof is doing, and that list is longer than people expect on a rear extension that has been altered twice.

The outlet needs its catchment. Putting a 2000 by 1200 kerb across the middle of a roof that drains to one corner splits the catchment in two and leaves the upslope half draining nowhere. Either the falls are re-formed around the kerb with a tapered insulation scheme, or a second outlet goes in. Both are decided before the kerb is set out, not after.

The abutment with the house wall wants at least 150mm of flat roof between the kerb and the brickwork, and realistically 300mm, so a flashing can be chased in and dressed down without the two upstands fighting each other. Then the existing services: soil vent pipes, extract terminals, boiler flues and, on a Gloucester terrace, sometimes an old cast iron vent still serving a first floor bathroom. Each one gets located on the survey drawing with its own clearance, because moving a soil vent pipe is a different trade and a different week.

Stroud valley flat roofs, and a kerb built from material carried up steps

Around Stroud, Chalford, Rodborough and Nailsworth the houses are built into the valley side, so a rear extension is frequently at first floor level relative to the garden, reached by a flight of steps from a shared lane with a retaining wall tight against the elevation.

That changes the kerb job in a practical way. Timber comes in by hand, insulation boards come up the steps one at a time in whatever wind is running down the valley, and there is nowhere to set out a kerb flat before lifting it. So the kerb is normally built in place rather than assembled on the ground and lifted, which makes the squareness check on the diagonals more important rather than less. It also means the sequence gets planned so that the whole kerb is built, insulated and weathered in one visit: leaving a half-built upstand on a valley roof overnight in October is asking for the thing that must not happen.

On the modern estates at Quedgeley, Bishops Cleeve, Ashchurch and Yate the opposite is true. A driveway, a level garden and a garage roof or a rear extension deck built in the last twenty years to a proper fall means the kerb is a half-day of straightforward carpentry, and the whole job runs to the shorter end of the programme.

Getting the kerb agreed before anybody orders a frame

The order of decisions on a lantern job runs kerb, then frame, then glass, and reversing any two of those costs money. Aluminium lanterns are made to order with a lead time measured in weeks, and a frame in a factory queue cannot be adjusted once the section has been cut and the corners cleated.

So the useful sequence looks like this. Agree the lantern footprint and the pitch at survey. Work the framing height from the specified roof build-up. Establish from the supplier whether the base ring is top-bearing or over-kerb and get the required dimension with its tolerance in writing. Build the kerb, insulate it, weather it, then measure it and photograph it. Place the order off that measurement. Where a builder is doing the flat roof, the same numbers get issued to them before their carpentry starts and the finished kerb gets measured on the same visit.

That is also the point at which the structural side of the opening is settled, because a kerb is only as good as the trimmed joists under it, and the sizing of those is set out on the structural openings and steel page. Where the deck and its falls turn out to want re-forming first, that is a better thing to discover with a tape than with a frame standing on the scaffold. Severn covers Gloucester and thirty miles by road around it, listed town by town on the areas pages, and what moves a figure on a lantern job is set out under costs. For a first answer, email info@severnskylightsgloucester.co.uk with a photograph of the flat roof taken from an upstairs window, which shows the falls, the outlet and anything already coming through the deck better than any description does.