What is a rooflight upstand?

Get the upstand right and the rooflight above it has an easy life. Get it wrong, too low, poorly insulated or badly detailed, and the unit sitting on top of it can only do so much. On a flat roof the upstand is the part that keeps water out and warmth in, and it does most of that work before the glazing ever comes into it.

A pitched roof sheds water by angle. A flat roof does not, or at least not much: it is laid to a shallow fall of one or two degrees and rain moves off it slowly, so at any given moment there is standing water somewhere on the surface. If you set glazing straight into that plane, the joint between the glass and the roof sits in the wettest place on the whole building. The upstand solves that by raising the glazing clear of the water line.

It does three separate jobs at once. It holds the glazing up above where water collects and where wind drives rain into a corner. It gives the flat roof covering, whether that is felt, single-ply membrane, GRP or a liquid coating, a vertical surface to be turned up and bonded to, which is how the waterproofing carries on past the opening instead of stopping dead at a flat edge. And it provides the solid, level, square base that the rooflight is bolted or clamped down onto. A rooflight is only ever as true as the kerb underneath it.

There are two ways the upstand arrives on your roof, and the difference matters when you are reading a quote.

With a kerb-based rooflight, the builder or installer forms the upstand on site as part of the roof structure, usually a timber kerb built off the joists and insulated, and the rooflight is a separate item that sits on top of it. This is the common approach for flat roof lanterns and for larger fixed and opening rooflights. You control the height, and the waterproofing is dressed up the kerb before the unit goes on.

With a self-upstand or builder’s-kerb rooflight, the unit is supplied with its own moulded upstand already attached, often a GRP or PVC skirt in one piece with the frame. It drops over a plain opening and the roof covering is dressed up the moulded skirt. These are quicker to fit and are common on smaller flat roof rooflights, but you are tied to the upstand height the manufacturer gives you, which is often shallower than a site-built kerb.

Neither is right or wrong on its own. What matters is the finished height above the roof surface and how well the thing is insulated, and both routes can get there or fall short depending on how they are detailed.

The number you will hear most often is 150mm, measured from the finished roof surface to the top of the upstand. That figure is not arbitrary. Most flat roof covering guidance asks for the waterproofing to be turned up at least 150mm at any abutment, so that driven rain and any temporary ponding cannot reach the top edge and track behind it. An upstand shorter than that is asking water to find the one weak line on the roof.

The same 150mm turns up in the planning rules, from the other direction. A rooflight on a house is generally permitted development only if it projects no more than 150mm above the plane of the roof slope, sits below the highest part of the roof, and meets the obscure-glazing and restricted-opening conditions on side elevations. On a flat roof there is no pitched plane to measure against in the same way, and flats, maisonettes, listed buildings, conservation areas and Article 4 zones are all exceptions, so treat this as general guidance and check with your local authority before you assume. In Southend the city council is the place to confirm it, and along the Leigh cliff top the Article 4 Direction changes the answer entirely.

There is a genuine tension here between the two 150mm figures: the waterproofing wants height and the planning envelope may cap it. On a well-insulated flat roof the covering is thick, so a 150mm structural kerb can end up giving you rather less than 150mm of visible upstand once the insulation and covering are built up around it. This is exactly the sort of thing worth resolving on paper before anyone starts cutting joists.

A flat roof points at the sky, not at a compass direction, so it collects the high summer sun almost square on whichever way the house is turned. That makes every flat roof rooflight a south-facing problem for solar gain, even one on the north side of the building. Around here that matters more than most places, because Southend faces south across the estuary and the light off the water adds to an already open sky.

The upstand is where two of those concerns get settled. First, its insulation: a bare timber kerb is a straight thermal bridge, a cold ring around a warm opening, and on a poorly detailed kerb that is where condensation shows up in winter. An insulated upstand keeps the warm line continuous. Second, the upstand is what carries the glazing, and the glazing is where summer heat is won or lost. A flat roof rooflight over a hot room, a kitchen especially, wants glass with a low G-value, in the region of 0.3 or below, so that it lets the daylight in without turning the room into a greenhouse. The kerb holds the glass; the glass spec decides how the room feels in July. It is worth settling both together, which is the thinking behind our solar-control glazing work.

If you are planning a flat roof rooflight, decide the upstand before you decide the unit. Work out the finished height you need for the covering to turn up properly, check whether a site-built kerb or a self-upstand unit suits the opening, and make sure the kerb is insulated so it is not a cold bridge. Then choose the glass to match the orientation, which on a flat roof means treating it as south-facing whatever the house is doing.

If an old rooflight is coming out, that is the moment to look at the kerb underneath it, because a replacement is the one time the opening is open and the upstand height and insulation can be corrected without extra disruption. If the roof covering itself has failed and is leaking, that is a roofer’s job first, not ours, and worth sorting before any new unit goes in.

When you want a specification, our flat roof skylight installation page walks through the whole build-up, and you can tell us the opening size and orientation and we will come back with a kerb detail, a glass spec and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the hole in it.