Structural Steel for a House Extension

If you are considering a structural steel for a house extension, this blog examines a few options. Steels facilitate open-plan living. The choice is usually between setting the steel below the ceiling line or fitting it within the floorspace above.

Wherever possible, we would set the steel within the ceiling void and avoid a downstand.  This gives a nice, clean line, and you feel the extra height as soon as you walk in the room.  A steel below the ceiling reduces the sightline and gives a less impressive feel.

In this example, with the steel set within the first floor depth, you can see the sharp edge formed between the ceiling line and what was the original first floor outside wall. You can read more about this in the glass extension project.

Sometimes it is not practical to fit the steel in the ceiling void. In this example we were faced with two original steels below the ceiling, supporting first floor walls. It was not worth the technical challenge and cost of raising those beams, so we set our new steel in line.

A structural engineer produces calculations and a method statement for installing the steel and its supporting piers. In many cases, he or she would need to assess the ground conditions before finalising the design. For this, we would dig a pilot hole 250mm square in stated locations to check depth of footings and ground stability.  In several cases, the engineer required us to underpin  supporting structure. He or she may also specify padstones to spread the new load on existing walls. In this photo you can see a padstone above the bricks on the far pier.

Here is the same room when nearly finished, and you can read more on this home renovation in Bristol.

The method statement might require temporary support to the floor above, while installing the steel and removing the wall below. This is done with acrow props either using needles (short steel I-beams) or strongboys depending on the load.

In the construction above, you can see strongboys slotted into pockets; the steel is then inserted beneath. This requires careful planning to allow sufficient access for positioning and manoeuvring the large, heavy steel in a confined space. Here, a hoist lifts the steel into position but, from experience, 6-8 people are still required to carry, drag, lift, manoeuvre and offer helpful comments (!)

Here is how this extension turned out; because of the existing steel, the new steel was also fitted below the ceiling.

In this construction, the contractor used needles with acrow props on either side to support the two skins of the floor above. You can just see the needles protruding on the far side, each supported by two acrows. The ground floor wall is still in place at this stage.

The steel was then installed and the wall removed – here is a view from the other side.

Once the steel is in place, the contractor will wedge lead slate (or ram dry mortar) into any small gaps to prevent the masonry above from settling and cracking. Here is an image of this finished extension.

To maintain stability from lateral loads, particularly for a long wall, a structural engineer will require a masonry support pier of around 600mm internal projection onto which the steel will bear. For a kitchen, this is quite convenient as a 640mm return will cover the cabinets and worktop, as in the photo above.

However, if you want to minimise any projections to maximise an opening, space or view, you might consider a sway frame design. This has a smaller section and hence is less obtrusive, but is constructed from steel to which the overhead steel is bolted or welded. This example shows where we wanted to maximise the opening for new French doors in this end elevation. You can see the galvanised sway frame in situ. Note the returns of just 300mm either side of the doors, maximising the glazed sections and views of the garden. You can read more on our home remodelling portfolio page.

A recent project saw us install a cranked steel to be “lost” in two ceilings and a wall. Most steels are single piece and straight, but if you are opening up an area with a difference in ceiling levels, you can install a cranked steel. An example is shown in Section A-A below. A fabricator will make one for you by welding together three or more steel sections.

Here is the cranked steel being hoisted into position. Joists at the two different floor heights will sit in the webs of the two separate horizontal steel sections.

And here it is in situ:

This current project is also particularly interesting because of the H-shaped configuration of steels required to hold up the first floor (see partial plan view below). We are taking out three ground floor walls to open up the space completely.

The three steels were bolted together as shown in the connection detail drawing below.

Here you can see the 3 steels in position; the front beam is galvanised; the cranked and central beams are finished in red lead; when viewed in plan, they form the letter H:

Here is the near-finished renovation:

If you need the steel to reach further, the structural engineer may specify a ledger plate – a flat section welded to the base of the main structural member. This would be required, say, to support additional stonework, which is then built up off the plate. You can see a ledger plate on the galvanised beam (right hand) below. Incidentally, this beam was galvanised, since part of it would remain exposed to the elements. Galvanizing is the means of applying a protective coat of zinc to the substrate metal to prevent corrosion.

Over the years our contractors have offered optional prices for installing the steel below or within the ceiling. Typically, installation within the ceiling space may add around £1500 + VAT. You should then factor any additional cost for making good the first floor room if needles are used. This can add £500 or so.  However the flush design gives a more satisfying finish to the project and is the look you will live with for years to come.