A flat rooflight upstand is normally built so the top of the kerb stands at least 150mm above the finished roof surface, measured from the waterproofing rather than the structural deck. It can go higher where the roof is thick, where it falls steeply towards the rooflight, or where snow and standing water are likely. The exact height comes from three things together: the fall of the roof, the depth of insulation and covering, and the instructions for the particular rooflight system.
That is the short version. The longer one matters because the upstand is the one part of a flat rooflight that is hidden from the street and hidden from the room, yet it decides whether the roof stays dry and whether the opening looks square once you are standing under it. This page goes through each height, each fall and each trade-off in the order you meet them on a survey.
What the upstand is there to do
No flat roof is level. It carries a gentle fall so rain runs to an outlet, a gutter or the edge, and even a shallow roof will hold a film of water for a while after a shower. The upstand lifts the rooflight clear of that film. Without it, the joint between glass frame and roof covering would sit in the wettest place on the roof.
The upstand also gives the waterproofing something to climb. Whatever covers the roof, whether GRP, EPDM, felt or a single-ply membrane, has to turn up the outside face of the kerb and finish under the rooflight frame. The higher that upturn, the further any splash or standing water has to travel before it reaches the joint. Roof covering manufacturers set their own minimum upturn for this reason, and the rooflight installer works to the larger of the two figures.
Finally, the upstand is structure. It is a small box, usually timber, fixed down to the deck or to trimmed joists, and it has to carry the weight of the glass unit, take the force of wind trying to lift it, and stay square for the life of the roof. A kerb that twists by a few millimetres will rock the frame, stress the seals and show up later as a gap in a corner.
How high a flat rooflight upstand should be
For most domestic work the practical starting point is 150mm of upstand above the finished roof surface. Many roof covering and rooflight makers quote a figure of that order as a minimum for the waterproofing upturn, and it is a sensible rule of thumb when you are first sketching a job. Treat it as a floor, never a target.
The figure is measured from the top of the finished covering at the kerb, not from the plywood. On a warm roof with 120mm of insulation board and a membrane above, a timber kerb that is 150mm tall measured from the deck would leave almost nothing standing above the surface. That is the commonest way an upstand ends up too low: someone cuts the timber to a round number before the roof build above the deck has been added up.
The height can go up for good reasons. A roof that falls hard towards the opening, a roof with a parapet that holds water back, a site where the covering needs a deeper upturn, or a location where drifting snow is a real prospect. On the Dartmoor edge and the higher ground inland, a little extra margin is cheap protection.
There is also a ceiling on how tall the kerb should be, which is set by planning and by looks. We come back to both below.
Falls, and the uphill side of the kerb
A flat roof is laid to a fall, and the rooflight does not share it. The glass sits level, or close to it, while the deck beneath slopes. On a roof with a gentle fall, the upstand is built level on top, so one side stands a little taller than the other. Across a rooflight a metre wide, that difference is small. Across a wide rooflight on a roof with a decent fall it can be noticeable, and it is one reason a kerb is set out from a level line rather than measured down from the deck at each corner.
The uphill side is where the trouble starts. Water running down the roof meets the kerb head on and has to go round it. If the kerb is tall and the fall is gentle, that is fine. If the upstand on the uphill face is low, or the roof has been laid with a dip against the kerb, water slows, pools and sits against the joint. This is why we look hard at the way flat rooflights collect water when we survey, and why the answer is usually found in the kerb and the roof build, not in the glass.
The cure for water gathering behind the kerb is a small, shaped drainage detail, often called a cricket or a saddle. It is a wedge of insulation or timber laid on the uphill side so that water splits and runs round both flanks of the kerb instead of stopping against it. The covering is then dressed over the wedge. On a large rooflight, or one set across the line of the fall, a cricket is as much part of the job as the kerb itself.
How steep the roof falls is its own subject. Our answer on what pitch a flat rooflight needs sets out how the fall of the deck and the fall of the glass relate to one another.
Insulation depth decides the build, and the build decides the kerb
The kerb has to be tall enough to clear the roof build, and the roof build is thicker than most people expect. Under the covering sits insulation, a vapour control layer, the deck and the joists. Thermal performance under the Building Regulations drives how much insulation there is, so a roof renewed alongside the rooflight is usually thicker than the one it replaces.
There are two broad ways the build can go, and they change how the upstand is formed.
- Warm roof
- The insulation sits above the deck, under the covering. The upstand stands on the deck and is wrapped in insulation on its outer face, so the kerb is tall in total even though the part you see above the roof surface may be modest. This is the usual build for new flat roofs and for extensions in places like Cranbrook, Exmouth and the newer parts of Exeter.
- Cold roof
- The insulation sits between the joists, with a ventilated void above it and the deck on top. The kerb stands on the deck and the void has to stay clear around it, which affects where the trimming goes and how the lining is formed beneath. It turns up more often in older flat roofs on Victorian and 1930s houses, and in roofs where the insulation has been added later from below.
In both builds, the point is the same. Add the layers up from the deck, decide the finished surface level, and only then cut the kerb. If the kerb is also wrapped in insulation, that wrap needs space of its own so it can run continuously from the roof insulation up the kerb face. A gap there is a cold spot, and condensation will find it from the room side.
Three ways to make the upstand
The kerb can be made in more than one way, and each has a different relationship to height and finish.
- Site-built timber
- A box of treated timber and plywood, fixed to the deck and trimmed to the opening. It is the most flexible choice, because the height can be set on the day to suit the actual roof surface. It is also the choice that depends most on the carpenter, since it will only be as square and level as the person who built it.
- Factory-made insulated kerb
- A made-up box, often with the insulation already bonded into the sides. The sizes are set, the corners are true and the insulation is continuous, which makes for a quick, repeatable fit. The trade-off is that you choose from the heights the maker offers, and an awkward roof may need packing underneath to suit.
- Kerb supplied with the rooflight
- Many flat rooflights are designed as a unit, with a matching kerb or upstand frame the glass clips or bolts to. The glass and its seals are tested together with that kerb, and the instructions for height, fixing and weathering follow from it. If you choose a unit like this, the maker's information leads on height.
Whichever route is chosen, the written quote names it, and the height of the finished upstand above the roof surface is put on the same page. A vague line that reads simply as "kerb" does not tell you what you are getting.
Height against planning limits
An upstand sits above the roof plane, so it counts when a rooflight is checked against permitted development. The General Permitted Development Order gives a house its rooflight rights in Class C, provided it rises 150mm or less above the roof surface, stays below the roof's highest point and satisfies the side-facing glazing conditions. Householder rights do not stretch to flats or maisonettes, and any external change to a listed home needs its own consent first.
On a flat roof that 150mm limit is the awkward one. A kerb of 150mm plus a glass unit on top can easily exceed it, because the measurement is taken to the highest part of the rooflight, not to the top of the kerb. A low-profile unit on a shallow upstand may fall inside the limit, while a taller kerb, a dome or a roof lantern is likely to go beyond it and need an application, or at least a conversation with the local planning authority first.
Planning authority varies across our area: Exeter City Council in Exeter and Topsham, East Devon District Council around Exmouth and Sidmouth, Teignbridge District Council from Newton Abbot to Dawlish, and so on. Conservation areas can have Article 4 directions that remove Class C, and Dartmoor and Exmoor are national parks with some rights restricted. If you are near the 150mm line, a Lawful Development Certificate confirms in writing whether the work is permitted. We check this at survey, before the quote is written, because it can change the height we propose.
What the kerb looks like from inside the room
The upstand does not end at the roof. Its inside face continues down through the deck and the joists to become the shaft you look up into. A kerb that is square and true makes a crisp lining possible. A kerb that is out of true will show through every coat of plaster, however good the plasterer.
This is where the height decision reaches the ceiling. A tall kerb on a thick roof build means a deep shaft, with a lot of lining to form and finish. A deep shaft with straight sides throws away a share of the daylight, so it is often worth splaying the sides outwards towards the ceiling. We explain the choice on how a rooflight is finished inside.
The inside face of the kerb also needs its own insulation, taken down continuously from the roof build, and a vapour control layer lapped onto the frame so warm, damp air cannot get into the roof. Where either is missing, the lining becomes the coldest surface in the room and the first place a damp stain appears. Painting over it brings the stain back, so the insulation is dealt with before the plaster goes on.
On a period house, such as a Georgian terrace in St Leonards or a Victorian villa in Torquay with a flat-roofed rear addition, the new lining has to meet a lime plaster ceiling. That is a finishing question rather than a height question, but the two meet at the kerb: the lining depth is set by the kerb and roof build, and the finish is chosen to suit the ceiling it meets.
Upstand questions to settle at survey
A good survey measures more than the opening. For the upstand, these are the points that decide the height and are worth asking about before you agree a quote.
| Point | Why it matters | What we record |
|---|---|---|
| Roof covering | Sets the minimum upturn and how the kerb is wrapped | Material, age, whether it stays or is renewed |
| Fall of the deck | Changes the height on the uphill side | Direction of fall, and whether a cricket is needed |
| Build above the joists | Decides the total kerb height | Insulation type and depth, warm or cold roof |
| Kerb type | Controls the heights available and the finish | Site-built, factory-made or supplied with the unit |
| Planning position | The 150mm projection limit applies to the highest part | Authority, designation, whether a certificate is advised |
| Shaft depth inside | Sets how the lining is formed | Depth, straight or splayed, finish |
The same questions help whichever firm you ask. If a quote does not answer them, ask for the missing line before the date is booked.
Where to go next
Upstand height is one decision inside a larger job, and the rest of it is covered in our guide to flat roof rooflight installation in Exeter. The roof covering decides how the kerb is wrapped, which is why we also wrote about flat rooflights on GRP, EPDM and felt roofs. If you are still choosing the type of glazing, our page on fixed or opening flat rooflights shows how an opening unit changes the kerb.
Every Brightview quote includes a written finish schedule that names the kerb height, the roof build and how the lining below is formed and finished, so none of it is left to be decided on the day. When you are ready, ask for a quote and we will survey the roof and the ceiling together.