Meta description: Learn how a VELUX flat roof kerb works, how to plan its size and height, and what to consider for skylight installation, flashing, insulation, and maintenance.
A VELUX flat roof skylight needs a stable, weather-resistant connection between the roof opening and the skylight. That raised frame is commonly called a kerb in the UK and a curb in the United States. Its dimensions, height, construction, and flashing details affect fit, drainage, energy performance, and leak resistance. Because VELUX products and installation requirements vary by model and market, the product instructions—not a generic kerb detail—should guide the final design.
What Is a VELUX Flat Roof Kerb?
A roof kerb is a raised frame built around a roof opening. A curb-mounted skylight sits on top of this frame, rather than fitting directly into the roof deck. The kerb creates a raised perimeter for the skylight and provides a surface where the unit can be secured and integrated with the roofing membrane or flashing system.
In the United States, curb is the usual spelling; kerb appears in VELUX literature and other markets. The terms generally refer to the same raised support, but they do not guarantee that every curb-mounted product has the same dimensions or installation method.
A kerb is not the skylight itself, and it is not simply a decorative trim. It is part of the roof assembly. Its construction must support the unit, maintain the intended opening, and help direct water away from the penetration. It also needs to work with the roof’s insulation, air barrier, vapor control strategy, and waterproofing.
How the Kerb Fits Into a Flat Roof Skylight
A typical flat roof skylight installation includes the structural roof opening, a framed kerb, the skylight unit, and a compatible roof membrane or flashing detail. The exact sequence depends on the product and roofing system. In many curb-mounted installations, the kerb is built first, the roof covering is dressed up and over its exterior, and the skylight is then fastened to the kerb as directed by the manufacturer.
The term “flat roof” does not mean the surface is perfectly level. Roofs need drainage slope, and skylights must be installed in a way that does not create a ponding-water problem. The unit’s approved orientation and minimum roof pitch, if specified, should be checked in its current installation instructions. A kerb cannot compensate for inadequate roof drainage or an incorrectly oriented skylight.
Some VELUX flat roof products are designed to be installed on a purpose-built curb, while other configurations or regional product lines may use different mounting details. For that reason, identifying the exact model is essential. The product’s label, order documents, or installation guide can help confirm the model and the required mounting arrangement.
Choosing the Correct Kerb Size
Kerb sizing is one of the most important planning steps. The opening in the roof structure, the outside dimensions of the kerb, and the skylight’s required mounting dimensions are related, but they are not necessarily identical. A rough opening that matches a product’s nominal size may still be wrong if the instructions call for specific clearance, framing, or curb dimensions.
Before construction, compare the current VELUX installation instructions with the project drawings and the actual product. Confirm the required:
- Model and mounting type: Check that the skylight is approved for the proposed curb or kerb installation.
- Rough-opening dimensions: Verify the framed opening in the roof structure, not just the visible skylight size.
- Kerb dimensions: Use the manufacturer’s specified inside and outside dimensions, where provided.
- Kerb height: Confirm the minimum and recommended height for the product, roof assembly, and local conditions.
- Fastener locations: Follow the specified attachment points and fastening requirements.
- Roofing compatibility: Check that the kerb detail works with the selected membrane, flashing, and insulation system.
There is no safe universal VELUX kerb size. Dimensions can vary by product family, size, mounting configuration, and country. A measurement found for one model should not be applied to another without checking its product documentation.
Kerb Height, Drainage, and Local Requirements
Kerb height affects how the skylight meets the roof and may influence resistance to water entry, snow buildup, and debris. A low kerb can leave the installation more exposed to standing water or snow, while an unnecessarily tall one can affect the appearance, interior shaft, and detailing. The appropriate height depends on the manufacturer’s instructions, roof design, local building requirements, and exposure conditions.
In some jurisdictions, building codes or roof-system specifications set requirements for skylight curbs, flashing, wind resistance, or roof drainage. Requirements can differ by location and building type. An architect, roofing professional, or building official can clarify which rules apply. The kerb should not be raised or modified in a way that conflicts with the skylight’s tested or approved installation.
Drainage deserves particular attention on low-slope roofs. Water should flow away from the skylight and toward the roof drains without collecting behind the kerb. The roof plan, insulation layout, cricket or saddle details where appropriate, and the position of nearby drains all affect performance. A skylight should not be used to solve a drainage issue by itself.
Kerb Materials and Construction
Kerbs are commonly framed from suitable structural lumber or constructed using another approved material and system. The material must be strong enough to support the skylight and resist movement under expected loads. It also needs to suit the roof assembly and the exposure to moisture. The correct choice depends on the design, product requirements, and local construction practices.
A well-built kerb is square, level, securely attached to the roof structure, and sized to the manufacturer’s specifications. Its top surface should provide a sound, continuous bearing area for the skylight. Uneven or twisted framing can make it difficult to seat the unit properly, compromise seals, or place stress on the frame and glazing.
Thermal performance matters too. The connection between the kerb and roof should be considered as part of the insulation and air-sealing plan. Uninsulated or poorly sealed framing can create a cold bridge, increase heat loss, and contribute to condensation. The design should maintain continuity between the roof insulation and the skylight assembly as much as the product detail allows.
Flashing and Waterproofing Around a VELUX Kerb
Waterproofing is a coordinated roof-system detail, not just a bead of sealant around the skylight. The roofing membrane or flashing needs to connect to the kerb in the sequence prescribed by the roofing system and the skylight instructions. Water should be directed over the roof covering and away from vulnerable joints, rather than trapped at the kerb base.
Flat-roof assemblies can use different waterproofing materials, including single-ply membranes and built-up or modified-bitumen systems. Each material has its own compatible adhesives, primers, fasteners, and termination details. A roofing contractor should verify that the membrane manufacturer’s requirements and the VELUX product instructions can be satisfied together.
Sealant may be specified at particular joints, but it should not be treated as a substitute for proper flashing or membrane integration. Applying an incompatible sealant, covering drainage paths, or relying on surface caulk to bridge a poorly formed joint can lead to failure. If the installation instructions call for a specific accessory or component, use the approved part rather than improvising a replacement.
Installation Planning and Common Mistakes
Planning should begin before the roof opening is cut. Confirm the skylight size, structural framing, interior finish, roof slope, drainage route, electrical needs for an operable model, and the roofing contractor’s membrane details. For a replacement project, inspect the existing opening and kerb rather than assuming they meet the requirements of the new skylight.
Common problems include:
- Using the wrong dimensions: A nominal product size is not necessarily the required rough-opening or kerb size.
- Building a kerb that is out of square: Poor geometry can interfere with fit, fastening, and sealing.
- Ignoring roof drainage: Water can pond around a skylight even when the curb itself is sound.
- Mixing incompatible materials: Membranes, adhesives, and sealants need to be suitable for one another.
- Skipping insulation and air sealing: Gaps can contribute to drafts, energy loss, and condensation.
- Changing the product detail: Field modifications may affect performance, warranty coverage, or code compliance.
- Relying on caulk alone: Sealant cannot replace a correctly designed roofing and flashing connection.
Roof work also involves fall hazards and potential structural changes. Cutting an opening can affect rafters, trusses, or other framing, so structural modifications should be evaluated by a qualified professional. Electrical connections for venting or powered accessories should follow applicable electrical requirements and product instructions.
VELUX Flat Roof Kerb vs. Deck-Mounted Installation
A curb-mounted skylight is supported on a raised frame. A deck-mounted skylight, by contrast, is designed to attach more directly to the roof deck or a specified mounting surface. These approaches are not interchangeable: they use different product configurations, dimensions, and flashing details.
| Feature | Curb-Mounted Skylight | Deck-Mounted Skylight |
|---|---|---|
| Support | Installed on a raised kerb or curb | Attached to the roof deck or designated surface |
| Planning focus | Kerb size, height, strength, and membrane integration | Deck condition, product-specific flashing, and direct attachment |
| Replacement considerations | Existing curb may be reusable only if it meets current requirements | Existing opening and roof deck must suit the replacement model |
| Compatibility | Requires a product approved for curb mounting | Requires a product designed for deck mounting |
The best mounting method is the one approved for the exact product and roof design. Converting from one type to another may require changes to the opening, framing, insulation, and roofing details.
Energy Efficiency and Condensation
The skylight’s glazing is only one part of its energy performance. The kerb and its connection to the roof can also influence heat loss and air leakage. A continuous air barrier and well-planned insulation help reduce drafts and limit warm, humid indoor air from reaching cold surfaces in the roof assembly.
Condensation can occur when indoor moisture meets a sufficiently cold surface. Kitchens, bathrooms, and other humid areas may require effective ventilation and moisture control. If moisture appears near a skylight, the cause could be condensation, a roof leak, or both; staining alone does not identify the source. A professional inspection can assess the roof membrane, flashing, interior air seal, and indoor humidity conditions.
For model-specific energy ratings or glazing options, consult the product documentation and applicable U.S. energy-rating information. Performance varies by model, glazing, installation quality, and climate. Marketing descriptions should not be used in place of the ratings and specifications for the selected skylight.
Maintenance and Troubleshooting
Periodic inspection can help identify problems before they damage the roof or interior. Check the roof area around the kerb for loose or damaged membrane, open seams, debris, and signs of standing water. Inspect the interior for staining, peeling paint, drafts, or condensation, while keeping in mind that the visible symptom may appear some distance from the actual water entry point.
Remove leaves and other debris from drainage paths as part of routine roof maintenance. Do not puncture, cut, or coat the membrane with an unapproved product. If a leak develops, avoid applying a quick patch over the skylight without locating the source. Water may travel beneath roof layers before appearing indoors, and an incorrect repair can make diagnosis more difficult.
When investigating a leak, a roofing professional should examine the skylight, kerb, roof membrane, nearby seams, and drainage conditions. If the skylight or flashing is still under warranty, the owner should review the applicable warranty terms and contact VELUX or an authorized dealer before making modifications.
Where to Find the Right VELUX Kerb Details
The most reliable source for kerb dimensions and installation requirements is the current instruction sheet for the exact VELUX model and size. Product documentation may be available through the VELUX website, an authorized dealer, or the product packaging. If the model is unclear, photographs of the skylight label and installation can help a dealer or installer identify it.
Before ordering materials or beginning work, have the installer confirm the mounting type, opening dimensions, kerb height, roof pitch, membrane compatibility, and any required accessories. For a new build or major renovation, these details should be coordinated among the designer, structural professional, skylight installer, and roofing contractor. That coordination helps ensure the VELUX flat roof kerb is not only the right size, but also properly integrated into the complete roof system.