What is a 'Knock-Through'?

A knock-through is one of the humblest aspects of the Structural Engineer's work, but none the less important for that.

Our brief would include a site visit partly to discuss the owner's intentions with regard to removing the wall and altering their living space and partly to gather information to use in design, such as a measured survey of the property and assessing the load paths affecting the area in question. We would also discuss whatever options are available that might offer alternatives for a lower-cost solution or one which might avoid party wall or foundation issues – this depends on each situation and the client’s wishes.

If the wall is not a supporting wall then the Engineer would normally write a letter containing a memorandum to the effect that the wall can be removed without requiring structural intervention, so that this can be kept as a record in the event of selling the property or renewing insurance etc. This is usually accompanied by a simple diagram indicating the location of the wall in question. In this event, Building Regulations approval is not required.

If the wall is a supporting wall, Building Regulations approval is required and the owner would need to make an application to the Building Control service – either the local authority or an approved private-sector inspector. This is a legal duty as the property owner and of course insurance cover will depend on complying with this requirement.

The building inspector will follow up the application with an inspection to witness the beam (or whatever supporting device) is installed, before it is covered in. He would need a specification to check against, and this would be supplied by the applicant. Normally a Structural Engineer would be engaged to design the works, but this is not (in England) a statutory requirement.

DESIGN

If we were commissioned to produce the design, we would analyse the combinations of loads on the wall according to appropriate British or European Standards for design – Eurocodes have recently been introduced but at present British Standards remain valid and are preferred due to their long history of use in this class of work. Typically a beam would be designed with a view to bending and shear strength, buckling resistance and deflection according to the standards, and one of a range of standard beam or lintel sizes selected.

Equally important are the supports; the bearing stress imparted by the beam on the supporting wall is calculated and if necessary the area is reinforced by specifying and engineering brick or concrete padstone for the beam to rest on. Sometimes, due to the presence of doorways in the adjoining walls the slenderness of the supporting wall is insufficient and a steel post may be required, typically at the hallway end of a beam replacing the spine wall separating the front and rear living rooms of a 19th or early 20th Century house. This depends on the client’s choice of opening width vs budget as well as the structural design.

We would provide drawings comprising a general arrangement plan showing the location of the beam in the building, and details of the beam and its supports, together with any other significant features necessary to the structural design. This would be sufficient to obtain building regulations approval, as well as allowing your builder to price and execute the work correctly.
When commissioning building alterations such as this, make sure your builder is aware of the statutory requirements and if in any doubt as to the status of the wall being removed, give your local structural engineering expert a call!