A rooflight upstand is a raised perimeter kerb that sits between the flat roof surface and the rooflight frame. It lifts the rooflight above the roof deck to prevent water ingress, allow correct drainage, and provide a sound fixing point for the frame. Without a correctly specified upstand, even the best rooflight will underperform and almost certainly leak.
Most people buying a rooflight focus on the glass. They compare U-values, frame colours, and glazing specs. The upstand barely gets a mention - yet it is the single most important factor in determining whether a rooflight installation succeeds or fails long term.
This rooflight upstand guide explains exactly what an upstand is, why it is required, what the correct specification looks like, and what goes wrong when it is done poorly.
1. What Is a Rooflight Upstand?
An upstand is a raised box-like structure that sits on the flat roof deck around the rooflight opening. The rooflight frame sits on top of it.
Think of it as a plinth. It creates a step up from the roof surface to the bottom of the rooflight frame. This step is critical. It is what keeps rainwater, standing water, and debris away from the frame junction.
Upstands can be constructed in several ways. They are most commonly built from treated timber, but can also be formed using structural insulated panels, steel, or concrete blockwork depending on the project type and roof construction. Some rooflights are supplied with a pre-formed upstand as part of the product. Others require a site-built upstand to be constructed before the rooflight is fitted.
2. Why Is an Upstand Required?
There are three core reasons an upstand is required on any flat roof rooflight installation.
Water management
Flat roofs are not truly flat. They are designed with a fall - typically a minimum of 1:80 - to direct rainwater towards drainage outlets. Even with correct falls, water moves slowly across a flat roof and can pool temporarily during heavy rainfall.
Without an upstand, the bottom of the rooflight frame sits at or below the roof membrane level. Water pools against the frame. It works its way under the sealant and flashing. It finds its way into the building.
The upstand raises the frame above the roof surface. It creates a physical barrier between the waterproofing layer and the frame perimeter. Even if water pools around the rooflight during a downpour, it cannot reach the critical junction between flashing and frame.
Structural fixing
A rooflight needs a solid, level fixing point. The roof deck itself - whether timber joists, concrete, or metal decking is not always the right surface to fix directly to. The upstand provides a consistent, purpose-built fixing surface at the correct height. It also allows the installer to pack and level the rooflight frame accurately, regardless of any minor variation in the roof structure below.
Thermal and vapour performance
A correctly constructed upstand incorporates insulation within its structure. This prevents the upstand itself from becoming a thermal bridge - a cold path through which heat escapes from inside the building. Without insulation in the upstand, condensation can form on the inner face of the kerb, leading to damp and mould around the rooflight curb internally.
3. What Is the Correct Upstand Height?
This is where many installations go wrong. The upstand is built too low.
The minimum upstand height for a flat roof rooflight in the UK is 150mm above the finished roof surface. This figure appears in both rooflight manufacturer installation guides and in the guidance produced by the Single Ply Roofing Association (SPRA) and NFRC (National Federation of Roofing Contractors).
The 150mm minimum exists for a clear reason. It provides enough clearance to allow the flashing to lap correctly onto the upstand face, while keeping the frame junction well above any likely water level on the roof surface.
In practice, 150mm is a minimum - not a target. On roofs with slower drainage, larger catchment areas, or any risk of debris accumulation near the rooflight, a taller upstand of 200mm to 250mm provides additional protection.
4. Upstand Construction - What It Should Look Like?
A correctly built timber upstand for a domestic flat roof rooflight typically consists of the following:
Outer structural frame: Treated C16 or C24 structural timber, typically 47mm x 100mm or 47mm x 150mm depending on the required upstand height. The timber is fixed to the roof structure at the base and forms a rigid box around the opening.
Insulation infill: Rigid insulation board - PIR or phenolic - cut to fit within the upstand frame. This is what prevents cold bridging. The insulation should be continuous with the main roof insulation layer so there is no gap in the thermal envelope.
Capping or flashing: The rooflight manufacturer's flashing system is then applied over the top of the membrane turn-up, overlapping it by a minimum of 75mm and sealed at the top edge with a compatible sealant or mechanical fixing.
The rooflight frame then sits on top of this completed upstand assembly and is fixed through the capping into the timber below.
5. Pre-Formed Upstands vs Site-Built Upstands
There are two ways to provide an upstand for a flat rooflight installation.
Site-built upstands are constructed on site by the roofing contractor or carpenter. They are flexible - they can be built to any height and adjusted to suit the specific roof construction. The quality of a site-built upstand depends entirely on the skill and knowledge of the installer. Done well, they are excellent. Done poorly, they are the most common source of rooflight leaks in the UK.
Pre-formed upstands are supplied as part of the rooflight product or as a separate accessory from the manufacturer. They are factory-made to the correct height, pre-insulated, and designed to integrate directly with the rooflight frame and flashing system.
Our Flat Rooflight is available with compatible upstand and flashing accessories, designed to simplify the installation process and ensure a watertight result. For bespoke opening sizes, our Glass Rooflight in Fixed and Custom Sizes can be specified to match your exact structural opening dimensions.
6. The Upstand and U-Value Performance
The upstand is part of the thermal envelope. Its performance contributes directly to whether your rooflight installation meets Part L building regulations.
A poorly insulated or uninsulated upstand acts as a thermal bypass. Heat escapes through the upstand walls even if the glazing above it has an excellent U-value. In a worst-case scenario, an uninsulated timber upstand can raise the effective whole-installation U-value by 0.3 to 0.5 W/m²K compared to a correctly insulated one.
This matters particularly on new builds where the rooflight must achieve a whole-unit U-value of 1.6 W/m²K or below. Building Control assessors are increasingly aware of thermal bridging at upstand junctions and may ask for psi-value calculations or thermal modelling to confirm that the junction detail meets the requirements.
For a full breakdown of U-value targets and glazing specifications for Part L compliance, read our guide on Meeting Part L 2026: Thermal Requirements for Flat Rooflights.
