How Do You Build a Timber Kerb for a Rooflight?

Build a rooflight kerb by fixing doubled trimmers first, then framing a box in 47mm treated timber down into those trimmers, bracing the corners, clad the outer face with rigid insulation continuous with the roof insulation, and finish at 150mm above the finished roof measured at the lowest point of the fall. Square within 3mm, level within 2mm across any span.

The sequence, in order

Order is not a preference here. Several of these steps are impossible to correct once the next one is done.

1. Trimmers in and fixed, before any joist is cut. A pair of trimming joists parallel to the existing joists on each side of the opening, generally doubled and bolted, with trimmer members across each end. Props go under from below where the span calls for it. Only then is the deck cut and the interrupted joists hung off the trimmers on galvanised hangers.
2. Square the opening and check it. Diagonals measured both ways and agreed. A kerb built onto an out-of-square opening ends up out of square itself, and a flush glass unit made to a rectangular size will not sit evenly on it.
3. Cut the kerb stock. 47mm treated softwood is our normal stock. Depth is whatever gets you to 150mm above the finished roof surface once the deck, insulation and covering build-up are all accounted for, which usually means a taller kerb than people expect. On openings over about 1.8m we go to 47 x 150mm or double the stock on the long sides.
4. Assemble and fix down into the structure. The kerb is screwed or bolted into the trimmers and the deck. It is a structural element carrying wind uplift on the glass, not a trim sitting on the insulation. A kerb perched on rigid board and pinned through it will work loose.
5. Brace the corners. Angle blocks or plywood gussets at each internal corner. A 2m unbraced timber span will bow enough under its own load and thermal movement to break the bedding line under the rooflight frame.
6. Check level and fall. The top of the kerb should be level in both directions so the glass unit sits true, while the roof below it falls away on all four faces. That combination is the point of the exercise, and it is why the 150mm is measured at the low corner.
7. Insulate the outer face. Rigid insulation to the outside of the timber, thickness continuous with the roof insulation, butted tight with no gap at the base. This is not an upgrade item. It is what stops the kerb acting as a thermal bridge round the whole perimeter.
8. Then the covering. Up the insulated face, over the top edge, detailed for whichever membrane is on the roof. The kerb is not finished until that is done.

Typical timber sizes and tolerances

Auditing a kerb that is already built

Most people reading this are not about to build one. They are trying to work out whether the one on their roof was built properly, usually because something has gone wrong or because they are about to spend money replacing the unit sitting on it.

Do this from a window, from a ladder at the eaves, or from photographs taken by somebody with proper access. Never walk out onto a wet, frosty or elderly flat roof to inspect anything.

Six things to look for

• Visible height at the low corner. Sight along the roof to the base of the frame. If one corner reads much shorter than the diagonally opposite one, the kerb was built level on a roof with a fall and the 150mm was taken at the wrong end.
• Where the covering stops. You want to see membrane wrapping over the top edge and disappearing under the frame. If you can see bare timber, a painted timber edge, or a line of mastic between the covering and the frame, the top edge detail is missing.
• Corners of the membrane. On EPDM, a folded corner shows as a raised crease. Preformed corners are flat and rolled. On felt, corners filled with black mastic instead of cut and torched are visible from a few metres away.
• Any insulation on the outer face. On kerbs built before roughly 2005, usually none. Bare timber straight from the warm ceiling to cold outside air is why the reveals stream with water every January, which is condensation and not a leak.
• Bowing along the long sides. Sight down the length of the frame. A visible dish in the middle of a 2m run means the kerb was not braced.
• Water standing against any face. Two days after rain in drying weather. A puddle against the kerb is a ponding problem, and it will outlive any rooflight you set on top of it.

Local stock and what turns up

Across the 1960s to 1980s estates at Kesgrave, Rushmere St Andrew, Chantry and Whitton, garage and utility roofs carry domes on plain unbraced timber boxes with the felt stopped short of the top edge. That was normal practice at the time and none of those kerbs are insulated. Behind the Victorian terraces in central Ipswich the problem is different: the rear addition joists are irregular and often slightly sagged, so a kerb built square to the deck is not square to anything else. We set out from a laser, not from the boards.

On the coast at Felixstowe and Harwich the kerb goes to 200mm and the covering laps are mechanically terminated rather than adhesive only.

This page is the short sequence and the audit. The full construction detail, including deck junctions, vapour control layer position and warm versus cold roof build-ups, sits on our Knowledge page for flat roof rooflight kerb construction. For preventing leaks across the whole installation see fitting a flat rooflight without leaks, for the fall figures see minimum fall for a flat rooflight, and for diagnosing water already coming in, flat rooflight kerb leak. The parent page is flat roof skylight installation.

Send us a photograph of your existing kerb and roughly where the property is and we will tell you what we can see from it. Request a fixed quote and the survey will confirm the rest, with a 10-year workmanship guarantee on whatever we build.