What is a rooflight upstand?

What it does

It lifts the frame above the water line. That lets the roof covering be dressed up the face of the kerb and terminated under the rooflight’s frame flange, so the junction between roof and rooflight sits well above any water crossing the roof.

Without it, the joint would be at roof level, in the path of everything the roof sheds.

What it is made of

Usually timber, built up from the roof deck, insulated on its outer face in a warm roof. Some proprietary systems supply an insulated composite upstand instead.

The requirements

Height: around 150mm above the finished roof surface at the lowest point, measured from the finished surface rather than the deck. Too low and the covering has nowhere to terminate.

Square: both diagonals equal. A made to size unit will not sit on a parallelogram.

Level along the top: the roof beneath falls, but the top of the kerb is level, so the kerb is deeper on the downslope side.

Insulated on the outer face: otherwise it is a cold bridge around the entire perimeter, losing more heat than the glass.

Vapour control sealed to it: carried up the inside and sealed to the frame, or moist room air enters the roof build up.

Who builds it

Often whoever builds the flat roof, which is frequently not the company supplying the rooflight. That division is where jobs go wrong, and it is worth agreeing responsibility explicitly at quotation stage.

The kerb the rooflight sits on

An upstand, or kerb, is a raised frame built up from the flat roof deck, on which the rooflight sits. The roof covering is dressed up its outer face, and the rooflight sits on top with its own flange overlapping.

It exists because a flat roof is not dry. Every flat roof holds some water after rain, and an upstand lifts the glazing and its seal clear of it.

The four things that make one work

Height. At least 150mm above the finished roof surface. Lower than that and a heavy shower, a blocked outlet or a ponding area puts water at the level of the seal.

Fall. The deck falling away from the kerb rather than toward it, so water cannot pond against the base. On an older or self built extension this is the thing most often missing, and it is the commonest cause of a leaking flat rooflight.

Insulation on the outer face. The item nobody mentions and the one that decides how the room feels. A 1200 by 1800 rooflight has six metres of perimeter, and an uninsulated timber kerb punches straight through the roof insulation around all of it, presenting close to a square metre of poorly performing envelope at a U-value several times worse than the glass sitting on it.

Insulating it is a sheet of rigid board and an hour, and it disappears under the covering, which is precisely why it gets skipped.

A continuous vapour control layer, sealed to the frame, so warm moist air from the room cannot reach the cold side of the construction and condense inside it.

Construction and materials

Usually timber, built up on site and finished square and level. Some manufacturers supply insulated composite kerbs, which arrive as a unit with the insulation already integrated. Those are neat, they remove the risk of the insulation being skipped, and they cost more.

On a lantern, the kerb also carries the structure, so it is doing more work than under a flat rooflight and it is built accordingly.

Sloped upstands

Some are built higher on one side so the rooflight sits with a modest pitch, typically around 5 degrees. That sheds water and dirt considerably better than a truly flat unit and it looks almost identical from the ground.

Where a roof has minimal fall and correcting it fully is impractical, this is the sensible compromise.

The question to ask any installer

Whether the kerb is insulated on its outer face, and whether forming or correcting the kerb is included in the price or excluded from it.

It is the fastest way to tell who has detailed the job and who is simply fitting a unit, and it is why we will not price a flat roof rooflight without seeing the existing kerb.

What it is

A raised frame, usually a timber or insulated box, built up from the roof deck around the opening, on which a flat rooflight sits. Also called a kerb.

The rooflight is not fixed to the deck. It sits on the upstand, which lifts it clear of the roof surface.

Why the height matters

It lifts the glass, the frame and the waterproofing termination above the level water can reach.

Water on a flat roof does not just run off. It moves slowly, it collects in low spots, and in heavy rain or with a blocked outlet it builds a depth. The upstand height is what determines whether that water reaches the junction between the membrane and the unit.

On a green roof it has to clear the substrate too, and by a real margin, so that wet growing medium is never in contact with the frame.

Insulating it, which is not optional

The upstand passes from the warm side to the cold side, so a bare timber box is a conductive collar around the entire unit.

It needs insulation on its outer face, continuous with the roof insulation, meeting the underside of the rooflight frame with no break.

Where that is missing, the internal face runs cold, sits at the top of a warm humid room, and produces condensation and staining that gets reported as a leak. It is the single most common defect on flat rooflight installations, and insulated kerb systems exist precisely to solve it.

Getting it square

The unit is made to the finished kerb dimensions, so the kerb has to be square, level and accurate. A unit on an out of square kerb does not sit down evenly, and the gap gets taken up in sealant, which is a temporary arrangement from day one.

Survey to the built kerb, then order.