Upstands, falls and coverings: how a flat rooflight is fitted

A flat roof is not quite flat. It is laid with a gentle fall so that rain runs to an outlet or a gutter, and a rooflight must live with that water. The upstand raises the frame above the roof surface so that the joint between glass and frame never sits in a film of water.

It does three jobs. It lifts the unit clear of the water. It gives the roof covering something to turn up against, so the covering can be sealed to a vertical face instead of ending at a hole. And it carries the weight of the rooflight down to the structure around the opening. Get those three right and the glass is the easy part.

There are two ways to have one. A maker can supply an insulated upstand with the unit, delivered ready to fix. Or a carpenter can frame one in timber on the roof and insulate it there. Both work; the choice depends on the unit, the roof build-up and the covering, and the survey is where it gets settled.

People ask for the height of an upstand as if there were a single answer. There is not. The height depends on the covering, on the maker's fixing instructions for the rooflight, and on how much fall the roof has near the opening. Covering manufacturers and rooflight makers can each set minimums for their own products, and the upstand has to satisfy both.

Taller is not simply better. The upstand and frame together decide how far the finished unit projects beyond the plane of the original roof, and that projection is what the planning limit measures. Class C, the part of the planning order that takes in rooflights, usually leaves a house in England free of an application if the unit stays inside its limits, so the height of upstand plus frame is checked at survey. You will find the limits under flat roof skylight planning permission, and the wider rules are in the skylight planning guide.

A low upstand looks neater from the garden and from the room. A taller one gives the covering more height to turn up against. The best answer is the lowest upstand that satisfies the maker's instructions and the covering's, and that is a figure we take from the products chosen, never from a rule of thumb.

Water on a flat roof follows the fall. An upstand set across that fall acts as a small dam: water runs to the uphill face, stops, and has to find its way around. If it lingers there, grit and leaves collect, the covering stays wet and the glass gets a dirty tide line along its upper edge.

Good practice is to set the position so the water can pass. A rooflight with its long side along the fall disturbs it least. Where the unit has to sit across the fall, the covering is formed so water is steered round both ends, sometimes with a small raised shape behind the upstand called a cricket or a tapered insulation board. These are details that depend on the roof, so they are looked at during the survey, on the roof itself.

It is also why a photograph cannot give a price. From a photo we cannot see which way the water runs or where it will stop. A WhatsApp photo to 07880 284133 is a good way to begin a conversation, and the visit does the rest.

Under every upstand is a frame in the roof structure. Joists carry the deck, and an opening cut through them takes support away, so the cut ends are carried on new timbers across the opening, called trimmers, which pass the load to the joists either side. Which way the joists run decides which sides of the opening need that extra timber.

A narrow rooflight can sit between two joists with very little disturbance. A wider one means cutting a joist or two, and the timbers around the hole can be doubled or otherwise strengthened to take the load, as the design requires. The rooflight, the upstand and any snow or water that gathers on the glass all add weight, so the structure is checked at survey and the opening is sized to what the roof can carry.

The deck, the layer of boards or sheet that the covering is laid on, has to be firm and flat right up to the foot of the upstand. A soft or uneven deck spoils the tie-in however good the covering is, so we look at the deck as well as the covering. Building control cares about the framing, since the Building Regulations generally call for approval of a new rooflight. The Building Regulations guide goes further.

The covering is carried up the outside face of the upstand and finished under the frame's trim or flashing. How that is done depends on the material, so we look at what is on the roof before we say how the joint will be made.

Felt

Felt is a layered covering, built up in sheets with joints lapped and sealed. At an upstand, the layers are carried up the face and finished under the frame. The laps have to be sound, which is why the condition of the existing felt matters.

EPDM

EPDM is a single rubber membrane. It is turned up and bonded to the upstand's face, with the corners formed and sealed using the membrane maker's own accessories. Its few joints are an advantage; the membrane maker's instructions decide what to use at each corner.

GRP

GRP is fibreglass laid in resin on the deck, so it cures into one continuous skin. Taken up the upstand, it becomes part of the same surface, with no lap to open. Forming a neat corner takes skill, and a good surface needs dry weather when it is laid.

Fitting a flat rooflight runs in a set order, and the order matters because each step relies on the one before it.

The covering is cut back to the size of the opening. The deck is cut, the joists are trimmed, and the new framing is fixed before anything waterproof goes on. The upstand then goes up, insulated, and the covering is turned up it. Only after that does the rooflight itself sit on its upstand and get fixed according to the maker's instructions.

Inside, the shaft between ceiling and glass is lined with insulation and board, then plastered or finished to match the room. Settle beforehand where the finish stops: bare plaster, or painted.

An uninsulated upstand is a cold bridge around the whole opening. Where the outer face meets the room, cold air can sit on the lining and moisture can collect on it, so insulation in the upstand matters as much as the glass.

In England, a new or replacement rooflight in an existing home should have a U-value no worse than 2.2 W/m²K. Treat it as the target for the whole unit. A rooflight's U-value is based on its whole outer developed surface area, which is often larger than the hole in the roof. Where the kerb comes as one assembly from the same supplier, the 2.2 limit covers the kerb too; an upstand built on site should be no worse than 0.35 W/m²K.

The two routes differ in practice. A maker-supplied assembly arrives with its performance already worked out. A site-built upstand is only as good as the insulation placed in it. For more on glass and warmth, see energy-efficient rooflights.

When a flat rooflight is swapped for a new one, the existing upstand can sometimes take the new frame. That depends on three things: whether it is the right size, whether it is sound, and whether it is high and insulated enough for the new unit and covering.

If it is, the new unit sits on it and less work is needed. If not, a new upstand is formed and the covering is tied to that. The survey is where we decide, because nothing can be judged from the ground. The energy figures apply to a whole new unit, which counts as a controlled fitting, and the new one should do at least as well as the old.

For which type to choose, read the flat rooflight choosing guide.