What is a Kerb (Upstand) and Why is it So Important?

1. Support and Stability

A kerb acts as the foundation for your roof lantern. It provides a solid and level base, ensuring the lantern is adequately supported and remains stable over time. Without a properly constructed kerb, the lantern may become prone to movement or sagging, particularly under the weight of glass or during adverse weather conditions such as high winds or heavy snowfall.

• Material and Construction: Typically, kerbs are constructed from timber, metal, or insulated composite materials. Timber kerbs are often the most popular choice for residential projects due to their ease of customization and affordability.
• Height Requirements: Building Regulations Approved Document A (Structure) outlines the importance of structural stability for roof elements, emphasizing the need for robust supports like kerbs. While the document doesn’t specify kerb design directly, compliance with BS EN 1991-1-4:2005+A1:2010 (Wind Actions) ensures that the base structure can withstand wind loads.

2. Waterproofing

One of the primary functions of a kerb is to elevate the roof lantern above the surface of the roof. This elevation is critical for waterproofing, as it prevents water from pooling around the base of the lantern, thereby reducing the risk of leaks.

• Minimum Height Standards: According to Building Regulations Approved Document C (Site Preparation and Resistance to Contaminants and Moisture), the minimum recommended height for a kerb is 150mm above the finished roof level. This standard helps protect against water ingress in flat or low-pitched roofs.
• Weatherproof Detailing: The upstand must be integrated with a high-performance waterproof membrane, such as EPDM or GRP, to create a seamless barrier against moisture. Flashings are often used to overlap the kerb and the roof lantern for additional protection.

3. Thermal Insulation

In modern construction, energy efficiency is a significant consideration, and a well-constructed kerb plays an important role in minimizing heat loss.

• Thermal Breaks: Many kerbs are designed with insulation built into their structure to act as a thermal break between the roof surface and the roof lantern. This prevents thermal bridging, which can lead to condensation and heat loss.
• Regulatory Compliance: Approved Document L (Conservation of Fuel and Power) specifies the need for effective insulation in roof constructions. An insulated kerb contributes to meeting U-value targets for roofs, which are typically 0.18 W/m²K for new builds and 0.20 W/m²K for existing roofs undergoing renovation.

4. Compliance with Building Regulations

Kerbs are essential for ensuring that a roof lantern installation complies with UK Building Regulations. Non-compliance can lead to structural failures, moisture ingress, and even legal penalties.

• Fire Safety: Approved Document B (Fire Safety) highlights that roof constructions, including kerbs, must be designed with fire-resistant materials where required, especially in proximity to boundaries or other buildings.
• Minimum Upstand Height: As noted in Approved Document C, a 150mm minimum height is a common benchmark, but for roofs with parapets or specific drainage designs, this height may need to be adjusted.
• Load Bearing Standards: Kerbs must be engineered to support the weight of the roof lantern and any imposed loads, such as snow or maintenance personnel, in line with Approved Document A and BS EN 1991-1-3:2003 (Snow Loads).

5. Aesthetic Integration

Beyond its structural and functional importance, a kerb contributes to the overall aesthetic integration of the roof lantern with the surrounding roof.

• Customizable Design: Kerbs can be designed to match the profile and style of the roof lantern, ensuring a seamless transition between the lantern and the roof. This is particularly important for architectural projects where visual appeal is paramount.
• Cladding Options: To enhance the appearance, kerbs can be clad with materials like aluminium, timber, or composite finishes that complement the roof's design. Decorative options can also be applied to blend the kerb with the surrounding architecture.

6. Additional Considerations

When planning and constructing a kerb for a roof lantern, additional factors may need to be taken into account:

• Drainage: Properly designed kerbs often incorporate slopes or tapered edges to encourage water runoff, preventing standing water on the roof.
• Kerb Shape: While rectangular kerbs are the most common, bespoke shapes such as square, octagonal, or lozenge designs may be required for certain architectural styles or roof lantern designs.
• Structural Testing: For larger roof lanterns, kerbs may require additional structural reinforcement or testing to confirm they can bear the weight and dynamic forces.

Conclusion

A kerb or upstand is far more than a simple structural addition to a roof lantern installation. It serves as a cornerstone for the lantern’s support, stability, waterproofing, insulation, and compliance with building regulations. Whether you are installing a roof lantern in a new build or as part of a renovation, investing in a well-designed kerb is essential for ensuring a durable, efficient, and visually appealing result.

For further guidance on kerb design and compliance, consult the relevant sections of Building Regulations or seek advice from a qualified structural engineer or architectural specialist.