When the design exists but not for steel

The common situation

A house is permitted for masonry, and the client later decides they would build in light gauge steel after all. It is one of our commonest jobs — and not because the drawings received are wrong.

Why shop design is not enough

Because the structure follows a different logic:

• the wall thickness differs, so the internal dimensions of the plan change,
• the bracing works on a different principle, so the position of continuous wall lengths becomes critical, Designing a shear wall,
• the floor has a different depth and a different build-up, Floor build-ups,
• the transfer members around openings are different, Openings and lintels,
• and the build-up is entirely different — the cabling does not run in the structure. Service cavity.

What that means in practice

The second and third steps have to be worked through again: we redesign the structural part, and the shop drawings are produced from that. The three steps.

The architectural concept generally stays — the massing, the orientation and the room relationships do not change.

What it means for the permit

If the structural system changes against the permitted design, the permit has to be amended. The content of the construction documentation may not depart from the final permit. The content of the permit design.

We settle this at the start, because it affects the programme too.

What can be gained

The wall thickness drops, so usable floor area is freed up — on a plot with a fixed permitted footprint that is several square metres. Comparing wall thicknesses.

And the self-weight drops too, which shows in the foundations.

The point

Moving from masonry to steel is not a change of format but a redesign. The architectural concept stays; the structure and the build-up do not.