Understanding why Concrete Boot Lintels fail
Whilst carrying out a recent building condition survey in Nottingham, I encountered a case of concrete boot lintel failure, and thought it would be useful to outline the visual defects seen and the mode of failure.
What are Concrete Boot Lintels?
It is first necessary to understand that when looking at cavity wall construction, the inner leaf of masonry forms the structural load bearing element of the building, whilst the outer leaf of masonry forms the weatherproof outer envelope of the building shell. Concrete boot lintels were designed so that the lintel bearing surface only rests on the inner structural leaf of masonry. The outer face of the lintel is shorter and has no bottom bearing surface on the outer toe, with the head of the lintel supporting the masonry above window and door openings.
Boot Lintels were primarily installed in the 1970’s, and gave the opportunity to provide a cleaner design to the external facade, as the face of the lintel could be hidden behind bricks slips, though often. the concrete face of the lintel can clearly be seen seen above window and door openings. Where this is the case, then they are easily identifiable as the lintel does not extend into the masonry as either side of the opening. Thermal imaging often alerts us to the presence of concrete boot lintels, as we see direct cold bridging above the internal window openings, and of course, these areas can be a focus for cold surface condensation.
The image above shows how the lintel sits flush with the opening below, and does not extend into the masonry at either side of the opening. Typically, we’d expect to see a minimum of 150mm of bearing surface at either end of the lintel.
Visual Signs of Boot Lintel failure
Typically, we may see an open horizontal bed joint above the lintel, caused by the lintel dropping slightly, and the subsequent lack of support to the head of masonry above.
Internally we may see cracking to internal plasterwork to the head of window and door openings, as seen in the following image.
In these cases, we can’t fully understand the scope of cracking to the underlying masonry, without first removing the cracked plasterwork, though often this isn’t necessary due to the obvious cause of failure.
More commonly, we see the classic sign of boot lintel rotation, whereby stepped cracking appears at either side of the lintel to the head of masonry above.
Why do Concrete Boot Lintels Rotate?
Once you see the ‘boot’ shape of the lintel when viewed in profile, and further understand that they are only supported by bearings on the inner structural leaf of masonry, with the outer toe being unsupported, then it is easy to understand the forces at play, which cause the lintel to rotate, but this is best illustrated in the two technical detail drawings below.
Recommendations for Repair
There are specialist contractors who have developed remedial systems for repairing concrete boot lintels in place, which includes anti-rotation of the boot lintel, back into place, prior to installing a wall tie system that permanently secures the lintel. This wall tie system is often installed in conjunction with a helical steel bars, which form a composite masonry beam below the front toe of the boot lintel, thereby providing further support to prevent future rotation.
In. this particular property, the home buyers were looking to renew all external windows and doors, therefore, it would most likely have been more cost effective to simply swap the boot lintels for concrete lintels, which have a bearing surface to both the inner and outer leaf of masonry, or for a more discreet steel cavity lintel. For both cases, temporary support, such as ‘Strongboy’ props, would need installing to support the head of masonry above the openings, whilst remedial works are underway.
If there are no plans to replace the windows and doors then you should carry out a cost benefit analysis, on lintel renewal versus lintel repair, before making a decision on the best course of action to take.
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