Thinking about using a prefabricated steel substructure for your project, but need to know more about the tolerances it can be built to first?

It’s true that each project has specific needs and requirements that are dependent on the substructure and that its intended use. However…

If all you know is the +-15mm/20mm tolerances for in situ and precast concrete – as stated in most national building specifications (NBS – or the NSCS, for example) – then you might be surprised about the tolerances prefabricated steel substructures can withstand in comparison.

As a specialist subcontractor, I’m going to talk you through the tolerances a prefabricated steel substructure can withstand.

What tolerances can prefabricated steel substructure be built to?

Prefabricated steel can be built to much tighter tolerances than you can expect with in situ and precast concrete (+-15mm/20mm). In fact, they can be built to within +-3/4mm, which is a significant difference – although it can sound too good to be true to some people!

TIP: Check out our recent case study, where we built a prefabricated steel substructure for our client Winvic Construction, to a tolerance of just +-2mm!

Generally, main contractors and architects only tend to be aware of in situ and precast concrete approaches, and often don’t know they’re able to improve the tolerances down to this level using prefabricated steel.

When you see the final product and how prefabricated steel substructures are constructed using steel on calibrated jigs in a factory, you’ll see how the construction method is far superior to the alternatives.

When is prefabricated steel the best choice?

Sometimes the tolerance which is provided by precast and in situ is simply too large for the end use of the substructure.

If it’s a competitive tender between different methods of below-ground construction, using prefabricated steel will be favourable if the structure needs to be built to a tolerance tighter than +-15/20mm.

In fact, often the only way to completely de-risk the activity of building a substructure to tight tolerances is to use prefabricated steel. This is due to so many factors that can often go wrong whilst using insitu concrete and constructing the substructure on-site.

This is mainly due to:

  • In some cases, the shuttering isn’t correctly fixed, therefore will move during the concrete pour
  • During the pour, the shuttering can ‘bow’ or ‘belly’ under the pressure of the concrete due to there not being enough verticle and horizontal supports being installed

Advice from a specialist subcontractor

The tolerance required for your project is calculated depending on the end use of your substructure. This is usually agreed upon by the client, architect, and structural engineer. The method of how the structure is constructed will confirm the tolerance.

So, let’s say your below-ground substructure is to be constructed using in situ concrete. The tolerance of your substructure will be +-15/20mm.

As an example, when we work for vehicle manufacturing clients, they will stipulate a tolerance of +-4/5mm. This is typically due to the type of machinery they place within the substructure. Because of this, they need to be built incredibly accurately – precast or in situ just won’t work.

So, if you think prefabricated steel might be the best option for your next project, I’d recommend getting in touch with a contractor who specialises in this method of below-ground construction. They should be able to show you some case studies, and help you out with some free advice.


I hope this blog has helped shed some more light on the tolerances a prefabricated steel substructure is able to withstand.

Do you have any other questions about prefabricated steel substructures? Let me know in the comments’ section. I’d love to hear from you!