What Is a Rooflight Upstand? Height, Pitch and Insulation Explained
A rooflight upstand is the raised, insulated kerb built around a roof opening that supports the rooflight. It lifts the glazing clear of the finished roof surface and carries the weight of the unit. For most flat glass rooflights, the upstand should stand at least 150mm above the finished roof covering and be set with a slight fall so rainwater runs off the glass rather than collecting on it.
Why Flat Rooflights Sit on an Upstand
Mounting glazing directly at deck level would leave the frame seals exposed to standing water, wind-driven rain and lying snow, which is why anyone fitting a skylight on a flat roof starts with the kerb rather than the glass. The upstand creates a vertical step that the roof membrane can be dressed up and over, moving the critical junction well above the point where water gathers..
How High Should a Rooflight Upstand Be?
The accepted minimum is 150mm, measured from the top of the finished roof covering rather than from the timber deck. Measuring from the deck before the insulation and membrane are laid leaves the finished kerb noticeably lower than intended. Taller upstands of 200mm to 300mm are common on refurbishment projects, where a new insulated roof is laid over the old one and wherever heavy snowfall or exposed sites justify the extra margin.
Planning a Flat Rooflight for a New Upstand?
The Rooflights & Skylights UK team can confirm the right rooflight size for any upstand opening, including custom sizes for non-standard kerbs.
Explore Rooflight Collection →Building in the Correct Fall
Flat glass needs a gradient to drain, typically a minimum of 3 degrees with around 5 degrees widely recommended. Where the roof deck is close to level, the upstand is built taller on one side to create that fall. Where the roof already has sufficient pitch, the upstand can follow it, provided the top remains true and flat so the rooflight frame beds evenly along its full perimeter.
Getting the Opening Size Right
Flat rooflights are usually sold by internal size, which corresponds to the clear opening inside the upstand. A unit listed at 1000×1000mm therefore needs a 1000×1000mm opening, with the kerb walls built outside that line. Framed glass rooflights are typically designed around a set kerb width, so the supplier's stated wall thickness matters as much as the opening itself.
Before ordering, check that both diagonals match to confirm the opening is square and that the top surface is level across every side. Detailed advice on measuring a skylight opening covers the common errors, such as confusing the rough opening with the glass size.
For frameless rooflights, accuracy is even more important, since the glass overhangs the kerb and any irregularity in the upstand shows clearly at the edges.
Insulating and Waterproofing the Upstand
Upstands are generally formed in treated timber, with rigid insulation fixed to the outer face and the roof membrane carried up the sides and across the top. Inside, the plasterboard lining should finish neatly at the top of the kerb so the reveal is insulated all the way to the frame. Leaving this area uninsulated creates a cold bridge and the resulting condensation is often mistaken for a failing seal.
Most problems traced back to a poor upstand involve membrane that stops short of the top edge or flashing that has pulled away from the kerb. Anyone repairing a leaking skylight should inspect this junction before replacing sealant on the glass. The same principles apply to roof lanterns, which also bear on an insulated kerb, although their greater weight makes a structurally sound upstand even more important.
Choosing the Right Rooflight for Your Upstand
A correctly built upstand keeps a rooflight watertight and thermally efficient for decades. Rooflights & Skylights UK supplies Made in Britain, UKCA-marked flat glass rooflights weather tested to BS 6375, with internal sizing that matches standard upstand openings and custom sizes available for non-standard kerbs. The team can confirm opening dimensions and kerb requirements before you order.
Frequently Asked Questions
- What is the difference between a builder’s upstand and a supplied kerb?
- Can an existing upstand be reused for a replacement rooflight?
- Who normally builds the rooflight upstand?
- Can a rooflight upstand be lower than 150mm?
- Does a rooflight upstand require Building Regulations approval in the UK?
- What size rooflight fits an upstand opening?
- What fall should a rooflight upstand have?
- What is a rooflight upstand made from?
A builder’s upstand is constructed on site in timber and finished with the roof covering. A supplied kerb is a factory-made base delivered with the rooflight, usually in insulated uPVC or aluminium and is common on refurbishment work where speed matters.
It can, provided the timber is sound, the dimensions match the new unit and the kerb is adequately insulated. Rotten or undersized upstands should be rebuilt rather than patched.
The roofer or main contractor usually forms it during roof construction, before the membrane is laid. The rooflight installer then fixes and seals the unit to the finished kerb.
In the UK, a 150mm upstand above the finished roof surface is generally recommended. Lower upstands may increase water ingress risks and should be assessed by a roofing professional.
Approval may be required if installing the upstand involves structural roof alterations. The installation must also meet applicable UK Building Regulations for insulation, structural safety and weatherproofing.
Flat rooflights are sized by the clear opening inside the upstand, so a 1000×1000mm opening takes a 1000×1000mm rooflight. Standard sizes from Rooflights & Skylights UK run from 500×500mm to 1000×3000mm. Custom sizes for non-standard openings are delivered within 7 to 10 working days.
Flat glass needs a minimum fall of 3 degrees to drain, with around 5 degrees widely recommended. On a near-level roof, the upstand is built taller on one side to create the fall.
A builder’s upstand is usually formed in treated timber, with rigid insulation on the outer face and the roof membrane dressed over the top. Supplied kerbs are factory-made, usually in insulated uPVC or aluminium.