Much of what we’re undertaking layout-wise is fairly typical for a general remodel and extension project. We are making the living spaces more open plan and opening up the back of the house to connect well to the rear extension. This requires steelwork to hold up existing walls at first floor level.

A useful rule of thumb is that if you remove more than two-thirds of a wall it will lose its lateral stiffness and be likely to move under wind loads. As we were taking out pretty much all of the rear wall of the house, we needed to replace that ‘stiffness’ with a steel goal post frame with stiff joints at the corners to provide the loadbearing capacity for the wall above and to keep the rear wall crack-free.

support beams of extension

Paul’s team work to support the existing structure while fitting the new steel beams

When it comes to the extension’s foundations, I have found that good thermal detailing is generally easier when opting for a floating raft foundation. This is a stiff, reinforced slab, and it will provide both the floor structure and the foundation for the new walls to the two extensions.

As it is rigid we can sit this over a base of insulation so that the slab is on the warm side of the construction. We decided to remove the existing timber floor structure and to run the raft continuously through the house and out to the extensions. This ties everything together structurally and also provides a nice, neat airtight line across the whole ground floor.

This raft sits on insulation varying from 100mm of PIR [polyisocyanurate], a rigid foam board insulation, on foam glass aggregate in the house, to between 400 and 600mm of expanded polystyrene insulation under the extensions.

It’s also wise to keep our new steel frame on the warm side as well. This is as much about avoiding condensation risk as it is about keeping the house warm and cosy. Cold steel in a warm house would cause all sorts of problems.

extension on 1960s house

The steel column bases have been bolted down to a ground beam — a reinforced thickening in the raft slab, with insulation underneath.

The wall immediately above the beam has been reconstructed in skins of lightweight concrete block with insulation between to minimise heat loss at this structurally critical junction. We’ve also allowed space between the columns and the existing walls for a minimum of 50mm of insulation to avoid cold bridging.

Finally, we’ve been keen to make the most of the concrete floor slab as a finish and for its thermal mass. To that end we had it power floated after pouring to give a smooth, flat surface. We will grind that surface further to create a very smooth floor finish before sealing it later in the project.

All of this has added time and complexity to the build, as has often been the case on this project. But careful thought and preparation at this stage will give us a much more comfortable home. It’s also enabled us to have a cheap, attractive and robust floor finish for our ground floor.

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