Registered Construction Detail - Independent concrete lintel

The fire stop should achieve the required fire resistance and include a cavity tray (where required) and weep holes above. The minimum projection of door and window frames into the cavity is 30mm to ensure cold bridging doesn’t occur.

For partial fill, this may leave an uninsulated void that requires closing with a suitable cavity barrier and/or fire stop. Whilst it’s not a requirement of Approved Document B, a fire stop is recommended by MIMA and most warranty providers if the cavity is fully filled.

How to get it right

• In certain situations, the lintel may also require fire resistance
• Use a flexible sealant between all interfaces of the internal air barrier and the window/door frame members
• Cavity barriers around openings may be formed by the window or door frame if the frame is steel (0.5mm thick) or timber (38mm thick)
• Ensure the cavity closer is in contact with the insulation within the cavity and the window/door frame
• Keep cavities clean of mortar snots and other debris during construction
• Cavity barriers may require an additional vertical DPC and/or cavity tray
• Fire rated cavity barrier/closer may be required depending on the position and type of window and construction of soffit.

Find out more by downloading the Independent concrete lintel fire detail.

Further information

LABC Registered Construction Details (RCDs) are a set of thermal details designed to minimise heat loss through weak points in the building envelope. They are free for everyone who works with local authority building control.