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16 Feb 2024 < Back

Navigating Ground Floor Waterproofing: A Critical Perspective

This article was written by Ben Hickman, Technical Director of CLW. In this article we delve into an increasingly common challenge in UK residential buildings – waterproofing the ground floor. While it might seem straightforward, the nuances of this task reveal significant implications for structural integrity and design. 

The evolving landscape of ground floor waterproofing

As many surveyors and those in the construction industry will know, traditionally, buildings were constructed with a step up into the structure. 

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However, modern designs often feature the building set down, aligning the finished floor level with the external ground level. 

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This shift, while subtle, introduces new risks including: surface water flood events leading to water running over thresholds into the home, greater risk associated with rainwater splashback and risk of groundwater entry into the floor build up. 

Case study: A real-world example

To help demonstrate the issue, I thought it would be useful to draw in a real world example. The following image shows a changing room construction on a field in Cambridgeshire.  

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The following section details show that the finished floor level is at the same height as the external ground level. Also, the designed floor build up shows the Damp Proof Membrane (DPM) under the insulation some 250mm below finished floor level.

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This detail was present at entrances in order to comply with Building Regulations Part M, but it was also present around the full perimeter of the building. Furthermore, the hydrogeology meant that the ground was fairly saturated. As a result, water was coming to bear against the DPM. 

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Because the DPM is not waterproof (due to defects in the membrane, lack of seals at joints etc.) water entered into the floor build up resulting in a saturated screed and damage to floor finishes. 

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The invisible risk: Moisture and structural integrity

What this highlights is when the external and internal levels match, waterproofing must be a design consideration. Typically, this sort of detail is the prevue of the architect who adopts typical details using a DPM without consideration given to hydrogeology and structural waterproofing. Overlooking waterproofing design considerations leaves many residential properties at risk of water ingress.

Conclusion

So, what does this now mean? Is it now necessary for a waterproofing specialist to consult on the ground floor damp proofing designs where internal finished floor level is not greater than 150mm above external ground level for the majority of the building’s perimeter?  

I am not suggesting in this article that all ground floors need waterproofing. However, the decision to not waterproof ground floors where the building is set down is one which should be taken by a trained and competent specialist.

Your thoughts

We would like to see a wider discussion on this topic and, if warranted, see the industry (likely through the Property Care Association) publish a position. If this is an issue worth of wider consideration, latent defect insurers are likely to introduce more content in their technical manuals and that would see a shift in industry practices.

Comments

 

Paul Green

Well said / presented Ben, Delta have promoted your comments / type of design for many years now, and like you, we don't like risk, so will always strive to reduce them wherever possible, after all, as PCA members, we're all in this together, and are here to hopefully ensure that all structures, not just those below G/L, are kept dry.

Anthony Gooch

Good Article Ben , this often gets overlooked on ground floor designs. I am currently working on my own project with the same scenario on a sloping slope site that is just below ground at the rear with 200mm above at the front . I have opted for an external sheet membrane system with geo drain to Remove any risk.

Terry Smith

Once the line of defence needs to be positioned below ground (albeit only a little), the requirements of BS8102:2022 may need to be implemented as there is a potential risk of hydrostatic pressure acting on the system at some point during the design life of the structure... In these situations, the need for a waterproofing solution should be assessed based on a geology report. In the absence of a report, it must be assumed that at some point during the design life of the structure the system is going to be subject to hydrostatic pressure so a waterproofing system implemented regardless; not just relying on typical DPM/ DPC arrangements that would pose a greater risk of failure in this situation (as evidenced in your article). However, the problem arises (taking your detail as an example) that once this becomes a waterproofing design, not damp proofing, simply switching out the DPM for a post applied Type A waterproofing membrane to extend up to ground level and link in with DPC (as most would think logical) and/ or utilise the cavity for detailing up to ground, would not technically be possible as membranes are no longer deemed suitable for internal sandwich tanking arrangements, as were removed from BS8102 during the latest update in March 2022. This leaves a hole in in the waterproofing design which I think would not easily be filled by internal waterproofing systems without a complete redesign.

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