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

What is a roof lantern kerb or upstand?

The kerb is the raised structure around the opening in the flat roof. It forms the interface between the building and the roof lantern.

The kerb itself is normally constructed as part of the roof works by the builder or roofing contractor. It must provide a suitable structural base for the lantern while also allowing the roof insulation, waterproofing and internal finishes to be completed correctly.

The precise construction will depend on the roof build-up and the roof lantern system being installed.

Why accurate kerb dimensions matter

One of the most important things to understand is that roof lantern dimensions are not always measured in the same way.

A lantern manufactured to the wrong kerb dimensions may simply not fit the prepared opening, which is why the builder should work from the dimensions and upstand information supplied for the particular lantern rather than relying on assumptions or dimensions used for another system.

At Just Roof Lanterns, there is also an important distinction between our timber and aluminium collections.

Timber and aluminium lanterns are measured differently

Our timber roof lantern collections are generally specified using the internal kerb opening. This is the structural timber-to-timber opening and does not include internal finishes such as plasterboard. Current Classic and Linear product pages use this internal kerb measurement as the critical setting-out dimension. 

Our aluminium roof lantern collections are generally specified using the finished external kerb size. The outside dimensions of the prepared kerb are therefore the critical dimensions and must correspond with the size shown for the chosen lantern or approved drawing. 

This difference is easy to overlook, so it is important that the correct measurement convention is established before the kerb is constructed.

Timber roof lanterns — internal kerb opening
Aluminium roof lanterns — finished external kerb size

Link timber roof lanterns here to your timber upstand-detail page and aluminium roof lanterns to the aluminium upstand-detail page.

The kerb should be square, level and structurally suitable

Whichever roof lantern system is chosen, the supporting kerb should be square, level and structurally suitable for securely fixing the roof lantern.

This is important both for the installation itself and for the alignment of the finished lantern.

The size should therefore be checked in more than one place, and the builder should confirm that the opening is square before installation. Where appropriate, checking the diagonal measurements is a useful way of confirming that a rectangular kerb has been constructed square.

For aluminium lanterns in particular, JRL's current installation guidance states that the finished kerb should be square, level and suitable for securely fixing the aluminium ringbeam.

Kerb height and width

There is no benefit in treating every roof lantern as though it requires exactly the same kerb construction.

The required width and build-up depend on the lantern system, roof construction, insulation and weatherproofing detail. For example, our current timber upstand detail illustrates a typical supporting frame based around a 100mm-wide timber upstand, together with exterior-grade ply, insulation and internal finishes. 

The relevant product drawing or upstand detail should therefore be followed rather than constructing the kerb to a generic dimension.

This is also why I would remove the present article's statement that Approved Document C universally requires a 150mm roof-lantern kerb. The current article presents that as a blanket Building Regulations requirement, which is too broad. 

Weatherproofing and insulation

The upstand forms part of the wider flat-roof construction, so its weatherproofing and insulation need to be considered as part of the roof design rather than as part of the roof lantern alone.

The external roof covering should be properly integrated with the kerb, while the build-up should allow appropriate insulation around the opening. The exact materials and detailing will vary according to the roof construction.

Our aluminium upstand guidance deliberately treats the supporting construction, insulation and weatherproofing as works by others, because these details need to be appropriate to the individual project and roof system.

Your builder, roofing contractor or other appropriate professional should therefore determine the structural, insulation and waterproofing construction required for the project.

When should the kerb dimensions be confirmed?

Ideally, the roof lantern and kerb should be considered together rather than independently.

For a standard-size lantern, the builder can work from the published upstand dimensions for the chosen model. For a made-to-measure lantern, the required dimensions should be agreed before the lantern is manufactured.

This avoids one of the most common problems on site: a roof opening being constructed first and the roof lantern being expected to fit it later.

Where an approved production drawing has been issued, that drawing should take precedence over assumptions or preliminary measurements.

Common kerb and upstand mistakes

The problems we most often want to avoid are straightforward:

• confusing an internal kerb size with an external kerb size;
• measuring between finished plasterboard surfaces where a timber-to-timber measurement is required;
• allowing external roof finishes to alter a critical finished external dimension;
• constructing the upstand before confirming the lantern specification;
• an opening that is not square or level;
• or an upstand that is unsuitable for securely fixing the lantern.

A few checks before the roof lantern is ordered or installed can prevent considerably more difficult alterations later.

Before your roof lantern is installed

Before installation, check that the kerb dimensions correspond with the dimensions supplied for the roof lantern, that the opening is square and level, and that the supporting construction is ready to receive the lantern.

For more detailed setting-out information, see our timber roof lantern upstand detail or aluminium roof lantern upstand detail.

You can also browse our full range of roof lanterns or contact us if you are unsure which dimensions apply to the lantern you are considering.