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  • Writer's pictureSam Costigan

New Build Waterlogged Garden

Updated: Oct 14, 2021

How many times have you heard horror stories about new builds? You‘ve heard of the new build snagging surveys that many surveying companies offer, but because it’s a new build, everything should be fine right? Wrong.

We have carried out many new build snagging surveys and on all of them we find over 50 different snags, sometimes reaching into multiple hundreds of snags, some even major building regulations breaches. However, there’s one thing that crops up more times than anything else… Waterlogged gardens.

Recently we were appointed by our client in Leeds to investigate the cause of ponding water on a newly laid lawn and flooding of the patio area.

As part of our investigations we carried out desktop research and established that the geology of the land in the area is thick, slow draining clay subsoil.

During our investigation on site we dug a small trial hole to confirm our assumptions. We noted that the subsoil was predominantly thick, saturated clay.

One of the properties of clay is that it is incredibly good at retaining moisture; not so good for draining that rainwater from your garden though. Topsoil on the other hand, is vital for improving drainage of the lawn.

We identified within our investigation that the flooded areas of the garden were happening because the turf had been laid directly onto the clay soil without a sufficient depth of top soil.

The NHBC standards section 10.2.8 states the following:

“waterlogging of gardens within 3m of the habitable parts of the home should be prevented by appropriate soil selection…”

On this basis, the home builder has failed to comply with this section of the NHBC standards.

Furthermore, the NHBC standards section 10.2.9 states:

“garden areas within 20m of habitable accomodation shall be adequately prepared… appropriate action undertaken to restore physical condition and drainage characteristics of the topsoil and subsoil that has been compacted during construction. This should include subsoil decompaction to a minimum depth of 300mm and topsoil cultivation to the full depth of compaction… a minimum thickness of 100mm topsoil provided”

We noted from images provided by our client that the subsoil had been incredibly compacted prior to laying the minimum amount of topsoil and was not de-compacted to a minimum depth of 300mm. We also established that a minimum thickness of 100mm topsoil had not been provided.

Image provided by our client

Lack of topsoil at 100mm depth

Our client was also experiencing flooding of the patio. We lifted a paving flag to investigate the base in which it was laid. We identified that the paving had been laid onto a hardcore sub base with a sharp sand blinding layer. This is common practice and to be expected however, we also noted that various areas of the sand blinding layer included large chunks of clay soil. We established that the sand layer had been contaminated with the subsoil which was leading to water retention in the base.

Sand blinding layer laid directly onto clay subsoil in areas

Sand blinding layer contaminated with clay soil

Our conclusion in this instance is that poor workmanship and preparation has led to the defects at the property. A portion of this can be attributed to poor supervision also as the property should not have been signed off with these defects present.

Our recommendations to the client and builder is that the garden area should be excavated, the subsoil de-compacted to a minimum depth of 300mm before laying a minimum of 100mm topsoil. We further recommended that the patio should be taken up, the sand layer removed along with any areas of clay soil before re-instating the patio correctly.

If you‘re having problems with your new build or in the process of purchasing one, why not get in touch to see how we can help?

Sam Costigan | MCIOB

Chartered Builder

0113 887 1941

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