Upstands and kerbs for flat rooflights

A flat roof is a stack of layers: the structural deck, a vapour layer, insulation, and the waterproof covering on top. A quoted upstand height only means something once everyone agrees where the measurement starts. The sound starting point is the top of the finished covering beside the kerb, because that is the surface rain runs across. Measured from the deck, the same kerb looks far taller than the water ever sees.

That is why the height cannot be read from a catalogue. Each unit is made to stand a stated distance clear of the finished roof, and the instructions for the unit being fitted give its minimum. On a thin new roof a modest kerb reaches that clearance. On a roof that was re-covered over older layers, or one with thick insulation above the deck, the kerb has to be built taller to reach the same clearance above the surface.

We write the build-up down at the survey, layer by layer. Where we can, we read it from the underside of the ceiling, and at a corner of the roof we lift the covering to see what lies beneath. The kerb height on your quote comes from those notes.

Clearance also matters in autumn and winter. Leaves, snow and standing water collect at the foot of the glass, and an upstand set too low lets them sit against the seal. A few extra centimetres in the right place keep that edge clear of debris, which is why the measurement is taken once the final covering thickness is known.

The box a rooflight sits on is built in one of two ways. A timber kerb is made on the roof from treated softwood or ply, cut to the opening and insulated on site. A ready-made kerb arrives as a finished insulated section, sized to the unit, and is fixed to the deck and the trimmed joists.

A timber kerb suits an odd opening: a long narrow slot over a hall, a rooflight set close to a parapet, a unit that has to avoid an outlet by a hand's width. The carpenter shapes it to the roof as it is. A ready-made kerb suits a straightforward rectangle on a roof with a regular build-up, and it brings the insulation already fitted inside the box.

What the kerb sits on

Either type bears on the structure. The cut joists around the opening are trimmed with new timbers so the load of the glass and the kerb passes down through the wall, not through the ceiling board. In a garage conversion the roof joists may span a wide gap with a lightly built ceiling beneath them, so we look at the joist size and direction before agreeing the opening.

Fixing matters too. The kerb is tied down to the deck so wind cannot lift it, and the joint between kerb and deck is sealed before the covering goes on. We choose between timber and ready-made at the survey, once we have seen the roof, and the quote states which one you are getting.

The glass gets the attention, but the joint where the roof covering turns up the kerb needs as much care. Felt, a single-ply membrane and a fibreglass (GRP) roof each form that turn-up in a different way. Felt is laid in layers and torched or bonded up the face. A single-ply sheet is welded or bonded and folded into the corner. GRP is laid wet and cures into one skin that wraps the kerb.

Whatever the system, the covering rises up the outside face of the kerb and is held under the cap or flashing that closes the top. Water running down the glass then lands on the cap, drops clear of the join, and runs away across the surface. The detail fails if the turn-up is too short, so it has to start from the finished roof level and reach high enough up the kerb to stay clear of splashing.

The condition of the old covering decides how much new work goes around the kerb. If the surface is sound, the new turn-up can be joined into it. If the covering is close to the end of its life, we tell you at the survey, and the quote can include a fresh area around the opening.

Corners and joints

The corners of the kerb get a preformed piece or an extra layer, because a flat sheet folded into a right-angled corner gathers into a pleat. On a membrane roof the corner is made with a shaped patch welded over the join. On GRP the corner is laid in with extra matting. This is detail work, and it is why the survey photographs the covering before a kerb is specified.

A kerb that is only a hollow timber box is a cold bridge, a path that lets heat leave the room around the edge of the glass. The kerb is therefore insulated, either by the ready-made section's own core or by boards fixed inside the timber frame, and the insulation has to meet the roof insulation below so the line of warmth is unbroken. A new rooflight of this kind takes the Part L limit of 2.2 W/m²K, and an unbroken insulated kerb is part of how the whole installation behaves.

Where the roof is a warm roof, with insulation above the deck, the kerb carries that insulation up its sides. Where it is a cold roof, with insulation between the joists and a ventilated void above, the opening has to leave that ventilation working. A boxed-in void with no air path sets up damp in the timbers, so we keep the vent routes open or move them to suit.

Inside the room the shaft below the kerb is lined and finished. A deep shaft in a thick roof makes the light travel further before it spreads, so the finish is usually splayed outward toward the ceiling to widen the patch of daylight on the floor.

Condensation on the inside of a rooflight is a question of surface temperature and room humidity. A warm, well-sealed kerb keeps the frame edge from running cold, which helps. Our guide to why rooflights mist up covers sealed units that have failed, which is a separate problem.

Flat roofs are built with a slight fall so that water travels to one edge. A kerb stands across that route like a small dam, and the water has to find a way around it. If the kerb sits where the fall is gentlest and the nearest outlet is on the far side, a shallow pool can form on the uphill face and sit there after rain. Ponded water ages the covering and reaches for any weakness at the join.

The answer is to read the roof before choosing the position. We trace where the water goes, mark the outlets, and shape the surface so the uphill side of the kerb sheds toward both ends. On some roofs a small wedge-shaped section on the high side, often called a cricket, splits the flow. On others the simplest fix is a rooflight position a little to one side of the original idea.

The high side of the kerb faces the oncoming water and takes the most work from the covering. The low side lets water leave freely. For that reason the turn-up on the high side is often finished a touch taller, and the cap is checked for a tight lap there.

A garage roof with almost no fall

Garage roofs and old porches sometimes sit almost level. There the first job is to see whether water already ponds. If it does, a tapered section is laid around the kerb, or the roof is re-laid to a better fall before the rooflight goes in. We would rather agree that at the survey than learn it from a stain on the new ceiling.

The kerb does more than keep water out. It adds to how far the unit stands above the roof, and that figure is what planning looks at. A rooflight on a house is usually permitted development if it projects no more than 150 mm beyond the plane of the original roof, measured with the frame included. A kerb and frame together can take a flat-roof unit past that figure, and a roof lantern normally stands higher still. Our guide to skylight planning permission sets out the full conditions, and the answer on planning permission for a flat roof skylight applies them to a flat roof. Flats and maisonettes have no householder permitted development rights at all.

The opening itself is building work, since it alters the roof structure and the energy performance of the roof. We handle the Building Control notification where the work is notifiable, and the requirements are explained in our skylight Building Regulations guide.

Swapping an old rooflight on a tired kerb

When a unit is replaced, the old kerb is the first thing we inspect. A sound, correctly sized kerb can sometimes take a new unit of the same footprint. A kerb that has softened, sits too low for the new unit or was never insulated is rebuilt, because the new glass has to meet the current energy limit and the clearance in its own instructions. Taking off the old unit also shows what the covering looks like at the turn-up, which can alter the quote.

If you are weighing a swap, our guide to the cost of a flat roof skylight lists the other lines that go into the figure, and a free survey puts the kerb on paper before anything is ordered. The fixed written quote and the 10-year workmanship guarantee then cover the kerb as well as the glass.