I am in the process of constructing an extension and would like some advice please on how to waterproof part of the external wall which will be below the level of the adjacent garden. To clarify, the garden slopes upwards away from the house and I have dug out to allow for footings etc. and am now laying the first courses in engineering brick up to DPC/DPM level. Part of the extension will be around 1.2m below the level of the garden. As this section will never be seen, my thoughts are that in order to reduce moisture ingression, I could use engineering bricks in this location and then place a DPM against the finished wall prior to backfilling. Is this an acceptable method or am I being too simplistic?
Waterproofing an external brick wall built below garden level
- Thread starter craig1234
- Start date
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Surely you dig the soil away from the building and have a pathway round the extension?
No, it's a very small garden so I don't want to lose any more of it. The area in question will be a utility room and there used to be a "potting shed" in that location that had the same thing (soil (flower bed) against the wall but without any protection). Adjacent to the utility room there will be a set of patio doors leading directly up some steps to the top of the garden so obviously in that location there will be a path with a retaining wall.
Retaining wall 150mm away with an acco finishing 150mm below dpc at the the bottom - then you have a "normal" build - otherwise take your chances with polythene sheets, internal tanking etc - and don't skimp on the detail.
Your polythene on the outside will just trap any water entering the outer leaf higher up the wall unless you put a cavity tray above it.
Your polythene on the outside will just trap any water entering the outer leaf higher up the wall unless you put a cavity tray above it.
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CDBE - is the aco strictly necessary as it is a small wall (I think maybe 2.5m in length) and at the depth of the footings it is pure sand so if I made provision, any water that did get down to the bottom would I think drain away.
Woody - I'm not a builder but to my way of thinking, isn't it better to keep the water out of both internal and internal walls? As I am constructing the walls now then I have the opportunity to do that .
Woody - I'm not a builder but to my way of thinking, isn't it better to keep the water out of both internal and internal walls? As I am constructing the walls now then I have the opportunity to do that .
No, you will never keep the external face or the wall dry unless you create a continuous barrier down the walls and under the foundations.
You either design a barrier into the cavity or on the internal face
If you can avoid water collecting at ground level you wouldn't need accos, at its lowest point my internal floor level is about 250mm below external ground level and it's impervious clay so I was faced with lots of water standing against my wall (before the extension we pretty much had a small pond at the back of the house - so for belt and braces I also continued the floor DPM up inside the cavity against the inner skin:
As Woody says, a cavity wall is designed for a bit of water getting through the outer leaf - it just collects in the "trough" at the bottom of the wall and harmlessly evaporates or drains away below DPC level - you get problems when this can't happen and you've got excessive ground water getting in.
You might get away with something less if your soil is free draining but there is an element of risk.
As Woody says, a cavity wall is designed for a bit of water getting through the outer leaf - it just collects in the "trough" at the bottom of the wall and harmlessly evaporates or drains away below DPC level - you get problems when this can't happen and you've got excessive ground water getting in.
You might get away with something less if your soil is free draining but there is an element of risk.
Coffee stain optional...
Coffee stain optional...
I hope the DPM holds up, I like strong coffee!
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