Structural steels for home extensions

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steel columncranked steel supportnew steel slotted into first floor space

One of the most visited pages on our website looks at structural steels for home extensions. Many readers then contact us for advice, and I wanted to share two recent enquiries which might be of more general interest. When it comes to structural matters, we always advise people to consult a structural engineer for calculations and method statements for each specific situation.

new steel inserted

Can you move existing steels from below the ceiling line to the floor space above?

The first enquiry questioned whether it was possible to raise a steel into the floor space above once it had been set below the ceiling line. Our immediate answer was that it could. Bear in mind that a new steel simply replaces a structural wall. The method to lift the steel would be similar to replacing the wall in the first place. Using acrow props and spreaders to support upper joists both sides of the steel, you would cut the joists in line with the steel.  Manoeuvring the steel upwards, you would adjust the brickwork or padstones at either end, then re-hang the joists from joist hangers secured to timbers in the steel web.

joist hangers

Of course, this was the simple answer.  But then our enquirer sent a photo of the actual problem …

Structural Steels Too Low

They were clearly facing a significant challenge; a challenge that should never have come about if the design had been considered more carefully. You can see how low these beams are by comparing them with the doorway nearby. By the time they had finished the floor, the clearance would have been around 1.92m. Note also how they were fitting a kitchen on the right hand wall – someone just over 6′ standing at the worktop would be constantly brushing their head against the underside of the boxing; now and for the next 30 years! To make matters worse, the steels also block the skylights and make the space feel smaller. This could all have been avoided and we describe an obvious method below.

Use people in your scale drawing

A simple scale representation would identified this problem quickly and the clients would have insisted on a change to the design.  We always produce 3D images and elevations including representative people to show how they would fit and move around the space (see our blog on 3D visuals). Such a visual would have shown how close a tall person’t head would have been to the steel before work had started. I hope you will check that your designer has given you a space that works!

fireboard steel

Steels in the ceiling

Wherever possible, our designs incorporate steels in the ceiling space because it produces a much cleaner line. It is slightly more expensive with some disruption and redecoration to the room above, but you are going to live with the consequences for a very long time.  We were able to offer advice, options and moral support to our enquirer, recognising that a change would be quite expensive at this stage.

steels being fitted

Nevertheless, it sounds as though her builders are prepared to help and raise the steels – great news.  I hope that the additional cost will swiftly become a distant memory when they start to enjoy their new space.

Types of structural steel

A second query concerned the options for structural steel design.  In particular our enquirer wanted to understand two things:  1. the difference between a goalpost design and a steel with padstones; and 2. why one would use a traditional universal beam (I- or H-shaped beam) as opposed to a hollow square beam.

Goalpost steel design

The goalpost or sway-frame design provides additional stability if the aim is to minimise the sight lines.  A padstone on nib walls would take up more of the visual space.

sway frame structure

Our design here was to maximise the natural light streaming through new french doors with side lights, so the vertical steels reduced the side bulk from around 450mm to less than 250mm.  This achieved the following result:

french doors

The nib-wall with pad stones is a cheaper solution when the size of the opening is less critical. The padstone is usually a wide concrete block which spreads the load more broadly than brick.  It may be that the wider nib wall is actually an advantage when considering the depth of kitchen base units.  Below is an example:

steel I beam for kitchen diner

With the following finished result:

Why use a universal (I- or H-shaped) beam over a hollow beam?

Most structural engineers will specify universal beams for domestic applications. Universal beams (UB) are horizontal, and universal columns (UC) are installed vertically. They are I- or H-shaped (there are subtle differences between the two, but we’ll save that discussion for another time).  They may also be referred to as Rolled Steel Joists (RSJ). Here is an example:

new steel slotted into first floor space

Hollow beams are formed as round (CHS), square (SHS) or rectangular (RHS) hollow sections. They may be specified in domestic work but only as a column.  They can be very small in section to provide the required strength (100mm or so, square) but it is difficult to join horizontal and vertical square beams together, so the universal shape is much preferred.

cranked steel support

Another important feature of the universal beam is that you can fit joists into the web, or timbers for joist hangers.  You also have access to run pipes and electrical cables easily from the side.

Home extensions

We’ve installed structural steels for clients in many of our home renovations.  This blog has given you a few things to think about.  If you’d like to read more, please see our original page entitled Structural Steel for a House Extension.

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