How a Roof Lantern Upstand Is Built

What a correctly built kerb looks like

A roof lantern upstand is a continuous timber box, insulated, faced, and dressed with the roof covering, sitting a minimum height above the finished roof surface with the lantern’s eaves beam fixed to the top of it. Every stage has a number attached: a minimum timber size, a fixing centre, an insulation thickness, a height, and a levelling tolerance. Skip or guess at any one of them and the kerb becomes the weak point in an otherwise sound installation. This page sets out the build-up we work to, stage by stage, on both inland and coastal jobs.

If you are trying to work out whether an existing kerb was built correctly rather than planning a new one, the symptoms of a kerb that misses these numbers are covered separately on when a roof lantern upstand is the problem, not the lantern.

Kerb specification at a glance

Stage one: the timber frame

The kerb frame is built from structural softwood, not sawn general-purpose timber, because it is carrying wind and dead load from the lantern and it needs to hold its shape for the life of the roof. Studs run vertically at 400 to 600mm centres around the full perimeter of the opening, tied into the trimmers or ring beam that forms the structural opening below. Corners are doubled and fixed with noggins added at mid-height, which stops the frame racking sideways under wind load before the covering and insulation add their own stiffness.

How that opening was formed in the first place, trimmers for smaller spans, a ring beam beyond around 1.5m, is structural work that happens before the kerb goes on, and it sits outside the scope of this page. On any opening approaching or beyond that size the frame usually involves a structural engineer’s design, covered in more depth on large-span and multi-section roof lanterns.

Stage two: insulation, fitted with no gaps

Insulation goes to the outer face of the kerb frame, typically 100 to 120mm of PIR board, and it has to be continuous with the insulation in the surrounding roof build-up. A gap at a corner or where the kerb meets the roof deck is a cold bridge waiting to happen, and it is the single most common reason a correctly specified kerb still condenses on the inside reveal. Boards are cut tight and taped at every joint, corners included, rather than left with a visible seam.

The insulation is then faced with 18mm WBP plywood or OSB3, screwed back to the timber frame beneath it. This facing does two jobs. It protects the insulation from mechanical damage during the rest of the build, and it gives the roof covering a flat, continuous substrate to bond or mechanically fix to. Dressing a membrane directly over unsupported insulation board is how membranes tear at the arris in the first hard frost.

On the inside face of the kerb, below the insulation line, a vapour control layer is carried up from the ceiling and lapped into the kerb build-up rather than stopped at the wallhead. Warm, moist room air will find any gap in that layer and condense against the first cold surface it reaches, which in an insulated kerb is usually the timber just behind the plasterboard. Continuity here matters as much as continuity in the insulation itself, and it is one of the details most likely to be missed on a retrofit where the ceiling below is being worked around rather than opened up fully.

Stage three: dressing the membrane up and over

This is the detail that actually keeps water out, more than the lantern’s own gaskets. The roof covering, whether EPDM, GRP, single-ply or felt, is dressed up the full height of the faced kerb and taken over the top edge, so that the highest point of the waterproofing sits above the level the lantern’s own base will sit at. The sequence runs in this order:

1. The field membrane is run up to the base of the kerb and turned up the vertical face, fully bonded or mechanically fixed depending on the system.
2. At the top edge, the membrane wraps over the arris rather than stopping short of it. This is the point most commonly skipped on a poor installation, because it takes longer and the covering is harder to dress neatly around a corner.
3. Corners are formed as a welded detail on single-ply and EPDM systems, or with a purpose-made corner piece, not built up from cut strips and sealant.
4. The termination at the top is finished under a proprietary trim, or turned into the frame rebate, so the membrane’s edge is mechanically protected rather than left exposed to UV and movement.

Sealant plays no structural role in this sequence. It is a gasket at a termination edge, not a waterproofing layer, and any detail that depends on a bead of sealant rather than a lapped, dressed membrane will fail within a few years regardless of how well the sealant was applied on day one.

Stage four: levelling and squaring the top

Once the kerb is built, insulated, faced and weathered, it has to be checked for level and square before the lantern goes anywhere near it. The tolerance we work to is level across the diagonal to within 3mm, and square, meaning the two diagonal measurements corner to corner match each other, to within 5mm. That sounds tight for a timber structure, and it is, because the lantern frame above it is rigid aluminium or engineered timber and has almost no ability to twist itself to accommodate an out-of-tolerance base.

Checking is done with a spirit level along each edge and a tape measured corner to corner on both diagonals. Where the two diagonal figures differ by more than the tolerance, the kerb is out of square and needs correcting with packers or by adjusting the frame before the lantern is set on it, not after.

This check happens twice on our jobs, once when the timber frame is built and again once the insulation, facing and covering have all been added, because each of those layers can introduce a millimetre or two of its own if it is fitted unevenly. Catching a drift at the covering stage, before the lantern is on site, costs nothing to correct. Catching it after the frame has been fixed down means taking the lantern back off.

How the lantern base sits and drains

The lantern’s eaves beam sits on top of the finished kerb, on a bedding strip or setting blocks that take up any minor unevenness within tolerance, and is fixed down at 300 to 400mm centres around the full perimeter. On coastal work every one of those fixings is 316L stainless, because a standard grade fixing corroding at one point around the perimeter is exactly how a level kerb starts to twist years after installation.

Drainage at this junction matters more than it looks. Any water that gets past the outer gasket line, which happens occasionally in driven rain regardless of specification, needs somewhere to go rather than sitting on top of the kerb against the frame. The base detail includes a weep path, small gaps or channels built into the sill profile that let incidental water drain back out to the roof surface rather than pooling against the timber. A kerb built without that drainage path traps the very water the rest of the detail is designed to shed, and damp collects exactly where you do not want it, against the timber under the frame.

What changes on a coastal job

The build-up described above does not change fundamentally near the coast. What changes is the height, taken to 200mm rather than 150mm because wind-driven rain climbs a vertical kerb face further at Felixstowe, Harwich, Aldeburgh, Southwold, Frinton and Clacton than it does inland, and the fixing specification, moved to 316L stainless throughout rather than standard grade. Estuarine locations such as Woodbridge, Wivenhoe and Manningtree sit between the two, with upgraded fixings a sensible precaution without needing the full 200mm coastal height in most cases. The full reasoning behind that grading sits on our roof lantern installation page, alongside the published price table.

Where an existing kerb was built short of these figures, particularly the height and levelling tolerance, it tends to show up later as a twisted frame, gapping gaskets or a cold-bridged reveal, which is covered in full on when a roof lantern upstand is the problem, not the lantern.

Where this fits in the wider installation

Building the kerb is one stage in a longer sequence that starts with the survey and structural opening and ends with the water test at handover. The full stage-by-stage process, including how the roof stays watertight overnight while the rest of the work continues, is set out on the roof lantern installation process, stage by stage.

Common questions about roof lantern kerb construction

Every kerb we build is checked against these figures before the lantern goes on, and the survey confirms the exact specification for your roof. Request a fixed quote for a new lantern or a full replacement, kerb included.