When a flat rooflight installation goes wrong, the rooflight upstand is almost always where the problem started. It is the structural interface between the roof deck and the glazing unit - the component that determines whether a rooflight sits correctly, drains properly, and remains watertight for the life of the building. Yet for something so critical, the rooflight upstand is often treated as an afterthought, specified too late or built incorrectly on site.
This guide is written for installers who want a clear, practical understanding of what a rooflight upstand is, the different types and materials available, the correct minimum heights, and the installation steps that make the difference between a trouble-free job and a costly callback.
What Is a Rooflight Upstand?
A rooflight upstand - also referred to as a skylight kerb or rooflight base frame is the raised structural surround that sits on the roof deck and upon which the glazing unit is fixed. Its primary purpose is to lift the bottom edge of the glazing above the level of the finished roof surface, preventing water from pooling against the frame and finding a way in.
Without an upstand, a flat rooflight would sit flush with the roof membrane, making water ingress at the junction almost inevitable. The upstand creates a positive weathering detail: water running off the roof membrane is directed away from the glazing frame rather than towards it.
The terms rooflight upstand, skylight kerb, and rooflight base frame are often used interchangeably across the industry. In practice, they refer to the same component, though some manufacturers use specific terminology for their proprietary systems.
Rooflight Upstand Types: Which Is Right for the Job?
Upstands are not one-size-fits-all components. The right type depends on the rooflight product being fitted, the roof construction, and whether the job is a new build or a retrofit.
Manufacturer-Supplied Upstands
Most quality rooflight manufacturers supply purpose-built upstands engineered to interface precisely with their glazing units. These are the safest choice for installers because the tolerances are designed to work together - the rebate depth, fixing centres, and weathering details are all matched to the rooflight product. For flat rooflights, manufacturer-supplied upstands are available in standard sizes and are by far the most reliable route to a watertight installation.
Refurbishment Kerbs
Where an existing rooflight is being replaced and the original upstand is sound, a refurbishment kerb can be fitted over the existing structure. These are aluminium or GRP kerb sections designed to clad the original upstand, providing a clean and correctly profiled substrate for the new glazing unit without requiring full structural works. They are particularly useful when replacing older dome skylights or polycarbonate units with modern flat glass rooflights on occupied buildings where disruption needs to be minimised.
Proprietary Aluminium Kerbs
Aluminium kerb systems are available as factory-fabricated components and are widely used on commercial rooflight installations. They are thermally broken, dimensionally accurate, and designed to integrate with the roofing membrane system via standard upstand flashing details. For commercial rooflight projects where multiple units are being installed, aluminium kerb systems offer consistent quality and rapid installation on site.
Upstand Materials: Timber, Aluminium, and GRP Compared
The choice of upstand material has a direct bearing on thermal performance, durability, and compatibility with the roofing system.
|
Factor |
Timber |
Aluminium (Thermally Broken) |
GRP |
|
Thermal performance |
Good with insulation |
Excellent |
Good |
|
Durability |
Moderate — requires treatment |
Excellent |
Excellent |
|
Weight |
Light |
Moderate |
Light |
|
Compatibility with membranes |
Good — easily bonded |
Requires correct flashing detail |
Good — easily bonded |
|
Bespoke sizing |
Fully flexible |
Factory-fabricated to order |
Moulded — less flexible |
|
Typical application |
New build, extensions |
Commercial, high-spec residential |
Refurbishment, replacement |
|
Risk of cold bridging |
High if uninsulated |
Low |
Low |
Minimum Upstand Height: Getting This Right
Upstand height is one of the most common points of failure on flat rooflight installations. The minimum upstand height for a flat rooflight is 150mm, measured from the finished roof surface (after membrane and any surface finish) to the underside of the glazing unit.
Installing a rooflight on an upstand shorter than 150mm is one of the most reliable ways to generate a warranty claim and a callback. Some manufacturers will not honour their product warranty if the upstand height cannot be demonstrated to meet the minimum.
On roofs with limited falls or where ponding is a known risk, increasing the upstand height to 200mm or above provides a sensible additional margin. For further context on why flat roof drainage details matter so much at the glazing junction, the guide on why flat skylights need a pitch covers the drainage principles that underpin correct upstand specification.
Rooflight Upstand Installation: Step-by-Step
The following sequence covers the installation of a site-built timber upstand for a flat rooflight on a new build or extension project.
Step 1: Form the Structural Opening
The structural opening in the roof deck must be correctly trimmed with a header and trimmer joists. The opening should be sized to allow for the upstand frame plus any insulation layer around the outside of the frame. Do not cut the opening to the finished rooflight size - allow for the upstand construction.
Step 2: Build the Timber Upstand Frame
Construct the upstand frame from pressure-treated structural timber to the required height, ensuring all corners are square and the top face is level. The internal dimensions of the upstand frame should match the rooflight manufacturer's specified kerb size.
Step 3: Insulate the Upstand
Fix rigid insulation board to the outside face of the timber upstand to a minimum depth matching the insulation level of the surrounding roof. This prevents the upstand from acting as a thermal bypass and eliminates the risk of condensation on the internal face of the frame. The insulation should be continuous with the main roof insulation layer with no gaps at the junction.
Step 4: Overlay with the Roofing Membrane
The roofing membrane - whether single-ply, GRP, or felt is dressed up and over the face of the upstand and turned down into the internal rebate. The membrane must be fully bonded to the upstand face and taken to a minimum height of 150mm above the finished roof surface. Any laps in the membrane at the corners of the upstand must be correctly welded or bonded and inspected before the rooflight is fitted.
Step 5: Fit the Rooflight
Once the membrane is complete and the upstand has been checked for level, square, and minimum height, the rooflight unit can be fixed. Follow the manufacturer's fixing specification exactly - incorrect fixing centres or inadequate fasteners are a common cause of frame deflection and subsequent seal failure. Ensure all joint sealants are applied to the manufacturer's specification before glazing units are lowered into position.
Wrapping Up
A correctly built rooflight upstand is the foundation of every reliable flat rooflight installation. Get the height, insulation, and membrane detail right at this stage and the rooflight will perform for decades. Get it wrong and no amount of quality glazing will save the job. At Skylights Roof Lanterns, our UK-manufactured flat rooflights are supplied with full installation guidance - contact our team if you need upstand specification support on your next project.
