Do you need to build a below-ground substructure that definitely won’t leak – and are wondering if such a thing is even possible?

If you’re a main contractor, or part of a main construction team, you’ll know that the biggest issue when constructing below-ground is going over programme – and once the work has been completed, there’s no guarantee it won’t leak. Well, at least if you’re using in situ or precast concrete.

A lot of clients come to us asking if we’ll be able to provide a final product that’s superior to traditional methods, and eliminate the risk of leaking. As a subcontractor who specialises exclusively in prefabricated steel substructures, I’m about to tell you there is way.

Below, I’m going to tell you more about how it works, and how you can build a substructure that’s proven not to leak.

What are prefabricated steel substructures?

Essentially, prefabricated steel substructures are a newer, more innovative and pretty much revolutionary method of below-ground construction. They’re manufactured entirely out of steel, via an off-site factory environment.

The manufacturing process itself? It involves the constructing of steel plates, and the necessary strengthening to build substructures in an off-site facility that’s been certified to BS EN 1090 (i.e. a Construction Product Regulation hEN1090) with code 2 certified welders.

Once your prefabricated steel substructure is complete, it’s transported to the site, and fixed into position. It’s then back-filled with concrete between the earth and your substructure.

TIP: Check out our blog: ‘What is Steel Prefabrication Below-Ground?’ to find out more.

How can they eliminate the risk of leaking?

As I mentioned above, prefabricated steel substructures are made using fully-welded steel plates. So, once your substructure is complete, it’s similar to the hull of a boat. This makes it impossible for water to get in, eliminating the risk of water ingress over time.

It goes without saying that if you do opt for prefabricated steel, your chosen subcontractor should be certified to CE certified to BS EN1090-1:2009 (as mentioned above). This will ensure your prefabricated steel substructure will be (and continue to be) 100% watertight and built to the regulation required for a steel structure.

TIP: Take a look at our blog: ‘How Can You Guarantee a Prefabricated Steel Substructure to be Watertight?’ to find out more.

How long can they stay watertight for?

Unlike more traditional substructure methods, prefabricated steel substructures don’t have joints. These joints can eventually fail – it’s actually quite common for them to do so. This leads to long-term issues with water ingress. In fact, in situ or precast steel using Sika concrete or a waterproof membrane will only last 10 – 30 years.

However, with prefabricated steel, you can be fairly certain that they should stay watertight for over 100 years in the ground. How do we know this? Well, we thought we’d employ an (unbiased) expert to find out for us!

Dr. J Bloomfield is an expert in the corrosion of steel in concrete, and he produced a report for us which was able to confirm that the way prefabricated steel is constructed, means that they will last over 100 years – even against the most aggressive soil conditions.

So, theoretically, anyone providing this method of prefabricated steel should be able to reassure clients it’ll last the distance.

Advice from a specialist subcontractor

If you think prefabricated steel may be the best option for your project, I’d recommend getting in touch with a subcontractor who specialises in prefabricated steel as early in the process as possible. Or if you’re still unsure, speak to some of the main contractors who have used prefabricated steel for their own projects, to get a better idea of what’s involved.

When the majority of our clients approach us, they’ve never heard of prefabricated steel. We had a client who we’d been installing vehicle inspection pits for throughout the ’90s. In 2004, they were approached by British Gypsum to install two large substructures in their factory during a Christmas shutdown.

The design was specified using reinforced concrete. However, the director of Northfield Construction contacted our managing director to discuss the programme, and the available options. This project was absolutely programme critical. It was vital they had the factory back up and running by January 2nd.


By now, you should have a better idea of how you can build a substructure that’s proven not to leak. Prefabricated steel is the only way you can completely eliminate the risk of your below-ground substructure leaking. Despite some very loose promises you hear from companies offering other methods.

Do you have any specific questions about prefabricated steel substructures? Let me know in the comments’ section. I’m here to help!