How to get it right: Localised underpinning

One of our surveyors was alarmed when called out on site recently for an excavation inspection. Above is what he saw.

The Building Regulations application was actually for a new foundation to connect the back of the house to the detached garage, but the builder had taken the opportunity to strengthen the house foundation at the same time.

This left more than 2m of masonry unsupported and both the first floor and roof spanned onto it. 

Other than holding brickwork in position, the makeshift propping offers no real support to the corner and poorly fitted waste pipes and a rainwater pipe were leaking water into the excavation and saturating the supporting ground.

In reality there were around five tonnes of load transferring through a timber prop that needed to be removed before the concrete could be placed. The excavation base had to be cleaned, widened and concrete poured as quickly as possible to minimise the possibility of settlement.

All's well that ends well - luckily enough

Fortunately the householder had a lucky escape, no thanks to the builder who could have caused serious damage to the property - albeit well intended.

Underpinning should never be carried out without expert advice in bays wider than 1m and the bays should always be bonded together with reinforcement.

The best way to ensure full contact between the old and new foundation is to place the new concrete 50mm below the existing foundation to permit a dry sand cement packing the following day.

Always talk to your building control officer about what you want to do and they’ll be able to offer guidance on how best to tackle the issue. (Find their contact details here.)

Further information

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Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.