Steel Beams in House Extensions: Specification, Padstones, and Bearing
The structural engineer specifies the beam. The bricklayer builds everything that supports it, surrounds it, and conceals it.
Almost every rear extension with an opening into the existing house involves at least one steel beam (RSJ or UB section). The beam carries the load that was previously supported by the wall being removed. On a recent kitchen extension near Sevenoaks, we installed a 203 x 133 x 25 UB spanning 3.6 metres to form the opening for bifold doors, supported on 450 x 450 x 225mm concrete padstones built into brick piers at each end.
Padstones
A padstone distributes the concentrated load from the beam end into the brickwork below. Without an adequately sized padstone, the point load can crush the bricks immediately beneath the beam. The structural engineer specifies the padstone dimensions and material (usually dense concrete or engineering brick). The bricklayer must ensure the padstone is perfectly level and positioned at the correct height so the beam sits true.
Bearing Length
The beam must bear onto each support by a minimum distance (typically 100mm to 150mm, as specified by the engineer). Insufficient bearing length means the beam is not fully supported, which is a structural failure waiting to happen and a guaranteed Building Control rejection.
Building Around the Beam
Once the steel is craned into position, the bricklayer builds the wall up and over it. The beam is usually wrapped in plasterboard and skim-coated internally, but the external brickwork above must be carefully bonded and may include a brick slip lintel face to conceal the steel from outside. The build sequence guide covers how this fits into the overall programme.
For structural brickwork more broadly, see our load-bearing walls article. View our extension services.