Are you a main contractor or part of a preconstruction team who needs to guarantee your substructure will be watertight?

As you’re probably aware, the biggest concern when constructing below-ground is the risk of it taking longer than expected – and once completed, there’s always the fear it may leak, leaving the client with a long term problem.

Many of my clients ask me:

“How can you ensure I’m not going to get a final product that won’t perform properly, or even start to leak over time?”

As a subcontractor who specialises exclusively in prefabricated steel substructures, I feel this is such an important question to answer.

Which is why below, I’m going to talk about why prefabricated steel substructures can completely eliminate the risk of leaking..

What makes a prefabricated steel substructure completely watertight?

The way prefabricated steel substructures are constructed involves making it out of fully-welded steel plates. Once the substructure is built, it is comparable to the hull of a boat. The plus side of this is that it will be impossible for water to penetrate.

It goes without saying that many construction products are BBA approved. However, to ensure a prefabricated steel substructure is built to a higher standard, the factory should be CE certified to BS EN1090-1:2009, with code 2 certified welders – like ours.

The standard set in these facilities is a regulation managed by law for all structural manufactured items, and if possible, your chosen subcontractor should be certified to CE certified to BS EN1090-1:2009.

So, although you probably won’t find a written warranty for prefabricated steel (due to insurance purposes), there’s no better way to ensure there’ll be no leaks for your below-ground substructure.

How long will a prefabricated steel substructure stay watertight for?

That’s a very good question. Due to the longevity of a mild steel prefabricated substructure being posed as an issue with this method of construction, here at Premier Substructures we decided to try and get a definitive answer.

We employed Dr. J Bloomfield, an expert in the corrosion of steel in concrete. He produced a report which confirmed that the way we construct our substructures will ensure they last over 100 years in the most aggressive soil conditions (saline soil, which is rare to be found in the UK), before the steel has been corroded sufficiently to leak.

This is in contrast to the fact that a Sika concrete or a concrete with waterproof membranes attached will offer a lifespan of just 10 – 30 years (depending on the product selected).

So, anyone else producing prefabricated steel substructures in the same way, should theoretically be able to provide the same long-lasting results.

Advice from a specialist subcontractor

Concerned about water ingressing into a substructure? I’d suggest that the only way to get a long-term watertight structure would be to use a prefabricated steel option, like one of our clients who had to manufacture a water interceptor tank at a project in Southampton.

The structure was over 3.5m deep, and although their specification was designed for an in situ concrete substructure, they were able to contact us for a quote instead, in order to reduce risk both long-term with the client getting a substandard product, and short-term (with regards to the time spent on-site).

They were aware that digging this deep and trying to construct with tight tolerances would be a big problem, especially when they realised there may be a long-term water ingress issue from a high water table in the area.

In order to help mitigate this risk, they eventually opted for a prefabricated steel approach.

It’s worth bearing in mind here that some in situ methods of constructing below-ground have “guarantees” which are sometimes very loose, and may include waterproofing products that have a minimal lifespan.

The nature of how more traditional substructures are constructed means they have joints, which can lead to long-term water ingress issues if the joints fail. Unfortunately, this can be quite a common occurrence.

If you’re concerned about leaks, the best thing to do is to discuss your options with a construction professional (e.g. a building surveyor, architect or engineer) and also complete your own market research. There are several ways of construction below-ground level.


I hope you’ve found this blog helpful when looking into how you can guarantee a prefabricated steel substructure will be watertight. You may not get a written warranty, but it is the only way you can completely eliminate the risk of leaks in your below-ground substructure.

This is despite some of the very loose “guarantees” you may see from other methods.

Do you have any specific questions about watertightness for prefabricated steel substructures? Let me know in the comments. I’m always happy to help.