Removal of structural rear wall

Creating open plan layout has become widely popular among homeowners, who want to knock down as much of the rear wall as possible when building a rear extension. However achieving this,
requires careful considerations and consultations with structural engineers to choose the most suitable method of construction, as structural failures are not only caused by vertical forces.
Removing a substantial part or whole of the rear external wall and many times the spine wall, can result in existing party walls become laterally unrestraint. To ensure masonry buildings maintain the required lateral stability (subject to wind force), the amount of external or buttressing wall that can be removed should be limited to the guidance and recommendations set in the Approved Document Part A of the building Regulations.

There are mainly three options for removal of a rear wall such as;

Option 1 – Horizontal steel beam

Maintaining a minimum of 550 mm wall (internally) at either side of the opening. A steel beam will be designed to support the vertical load from the floor and wall above and bear on padstones.
This method benefits from low construction cost and minimum disruption to the existing structure. The disadvantage would be the limitation in the size of the opening and therefore in many cases not desired by homeowners.

Option 2 – Goal post frame

This option consists of a horizontal steel beam on vertical columns. The size of the opening achieved with this option can be as wide as possible and the whole of the rear wall between the two flank
walls can be removed if required. Consideration of the temporary support is vital and should be maintained by the main contractor until the permanent structure has been installed. Additionally
the frame has to be tied to the existing structure to provide the required lateral stability.

A disadvantage with this method would be the need of underground works, which could be time consuming and expensive. As the new steel columns have to be positioned at the edge of the party
walls, they will require new foundation in many cases or it could be that the existing foundation simply would not be adequate to support new localised point loads from the steel columns, resulting
in new pad foundations needing to be constructed.

Option3 – Steel Box frame

Similar to option 2, the full width of the rear wall can be removed, obviously after the builder provides a suitable temporary support to the existing structure above. A steel box frame is
composed of a top steel beam supported on two vertical steel columns which respectively are supported on a bottom steel member. The bottom beam has to be encased in concrete to provide
protection against corrosion being positioned below ground level and will be supported on the existing foundation which was originally under the removed wall.

The great advantage with this method in addition to the flexibility in the size of the opening, is the principle of utilising the existing foundation resulting in very limited works to be carried out
underground. The disadvantage would be the higher cost in steelwork, and the disruption to the ground floor along the removed wall, which may not always be suitable in all cases. As a conclusion, the ultimate choice will remain with the homeowners who should carefully consider their options with professionals to achieve a cost effective, safe and successful structural project.

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