Can permeable paving be installed near a building?

Monday 29th November, 2021

When it comes to choosing and installing a permeable paving system, several myths can affect decision-making. This includes whether permeable paving systems can be installed near a building, as well as questions over how long permeable paving lasts.

We asked Design Technician Benjamin Birkhead to explain the rules and regulations around installing permeable paving close to a building wall – keep reading to find out more.

Can permeable paving be installed near a building wall?

At Marshalls, we are often asked whether it’s possible to install an infiltrating permeable system within 5m of the wall of a building. The answer is yes, but there is some confusion around this that I hope to help clear up here.

The confusion is often due to the wording used in approved building regulations documents, which focuses on safeguarding a building’s foundations. There are many risks to a building’s foundations that need to be assessed, amogst them high saturation which can cause the subgrade to become soft or cause settlement.

Building regulations Approved Documents1 paragraph 3.25a states:

“Infiltration devices should not be built: within 5m of a building or road or in areas of unstable land”

and paragraph 2.9 states about the design of a permeable paving subbase:

“The design of the storage layer is undertaken on a similar basis to the design of the storage volume in soakaways.”

It’s easy to see how this would lead someone to question whether infiltrating permeable systems should be installed within 5m of a building, however, clause 2.9 also states explicitly that:

“Where infiltration is not possible they may also be used with an impermeable barrier below the storage layer as a detention tank prior to flows discharging to a drainage system.”

In simple terms, if an impermeable barrier, such as the Marshalls MM380 Tanking membrane, is used, it is fine to install permeable paving within 5m of a building wall.

However, that isn’t all, and this is why there is the confusion. Further guidance goes on to explain that you don’t actually need a tanking membrane within 5m of a building’s walls, a Type A Infiltrating system can be used which doesn’t include a tanking membrane.

Installing a Type A infiltrating system less than 5m from a building

We have seen the guidance from the Building regulations approved documents but Interpave’s permeable design guide2 also addresses this question. Paragraph 5.4 states that, since permeable pavement systems allow dispersed infiltration over the area, rather than from a concentrated source, the 5m soakaway rule need not apply to Type A infiltrating systems, as has been clarified explicitly by the UK Government.

This is expanded upon in the Susdrain fact sheet “Using Suds close to buildings”3:

“Permeable paving that only collects and drains rainwater falling directly on it can be used against any building providing there is no point source of water from any other impermeable surfaces connected into it...”

and that

“Because infiltration from a plane feature is much more dispersed, has a shallow height and has a short retention time there is less potential for flow to occur laterally in any significant quantities.”

In addition, any concentrated flows that occur in permeable paving systems, outlet pipes, or from inlets into the subbase from additional catchment areas, need to be considered. Outlets should be located more than 5m away from the walls of a building. Any inlets into the subbase should be detailed to distribute the flow across an area of the subbase.

In terms of best practice, when you consider allowing water to flow close to foundations, we would always recommend you consult a geotechnical advisor or registered ground engineering professional for advice.

In summary

Whilst it is easy to understand where this myth around permeable paving comes from, hopefully this explanation is helpful when determining what you can and can’t do. In summary:

You can install a permeable paving system within 5m of a building wall, including Type A systems, because:

  • It allows dispersed infiltration mimicking natural vegetation
  • The catchment area to infiltrating area ratio is much lower than that of a soakaways

But, make sure that:

  • Inlets from additional catchment areas are not a point source
  • Concentrated outlets are located further than 5m from a building

Benjamin Birkhead works for Marshalls Design Team; working with landscape architects and engineers and offering specialist support to help bring customer’s landscape designs to life.

References

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1010058/Single_stitched_together_pdf_of_all_ADs__Jul21_.pdf
https://www.paving.org.uk/essential-permeable-paving-for-suds-guide
https://www.susdrain.org/files/resources/fact_sheets/09_12_fact_sheet_suds_close_to_buildings.pdf

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