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Damp clay soil under house should I take action?

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  1. #1
    Apprentice (new member)
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    Apr 2007
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    ACT
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    Post Damp clay soil under house should I take action?

    Hi, I have a 1977 brick veneer house on brick piers. We have owned the house for seven years and I go under the house every 6-12 months looking for termite activity.

    Today during my inspection I noticed much of the clay soil under the house was moist/damp. Almost muddy in parts but no puddles thankfully. Previous inspections I have done after some rain there has been only a small area (5 sq meters) of damp soil. Whereas now around 50% on the soil under the house is dampish.

    Like many places in Australia we have had heaps of rain over the past year the most since we have had the house but none for about week. Should the soil have dried in that time? Could the soil dampness be a result of rising water table?

    The house is on a very minor slope and there are numerous ventilation grills throughout and no raised garden beds. Under the house smells like wet soil (like when your gardening) but not musty. All the subfloor timber looks normal no signs of fungal attack/rotting/decay. And the brick piers look stable.

    I am wondering whether I should do something immediate- like get an exhaust fan under the house and/or install an agricultural drain to the back of the house to prevent run off or should I just monitor the situation more regularly every month or so in the hope that it dries out?

    Any advice would be appreciated thanks

  2. #2
    GeoffW1
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    Jul 2008
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    Hi,

    You mention clay underfloor, on a slope. Clay will normally have an underlying sandstone shelf at some depth, and I think water is seeping downhill on this stone, to you.

    The first thing I would have said would be to put in bigger or more wall vents, but you have those. So that's step 1. An ag drain on the uphill side should help, if you can run it to a good spot. I had to do that at a previous house.

    Permanent damp underfloor is never desirable in the long term, so I would say monitor it now, and if it persists, you could also put in some forced ventilation. You can get a wind driven extractor

    Wind driven turbine system cost nothing to run and that is good for your pocket and for the environment

    which I think is ugly, but you might have an out-of-the-way spot for it

    Cheers

  3. #3
    Apprentice (new member)
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    Thanks for the reply Geoff.

    In terms of monitoring how frequently should I check on it - weekly, fortnightly, monthly..?

    How quickly can timber start to suffer the ill effects from excessive moisture? - I don't want to obsess over it - nor do I want to be too relaxed about it either...

  4. #4
    GeoffW1
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    Hi,

    Well, you want to be able to see a change for the better. If you put in an ag drain on the high side, it should have an effect pretty quickly, say after a month.

    If you have not done that, and it is the same in a month or two, then you would probably be considering forced ventilation.

    So have a look each month or so. Dry rot is a fungus which actually needs wet conditions. It would take longer than a month or two to develop from nothing.

    Cheers

  5. #5
    Apprentice (new member)
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    Quote Originally Posted by DIYwannabe View Post
    Like many places in Australia we have had heaps of rain over the past year the most since we have had the house but none for about week. Should the soil have dried in that time?
    Sorry I can't offer much advice re the underfloor ventilation, but I can say that when we removed our old garage slab from atop the clay soil, it hadn't rained for several weeks, yet by the time the bobcat had run over it a few times it became like a mire, so much so that we had to delay work for a week or so to let it dry out.

    I guess the point I'm trying to make is don't underestimate the water retention of clay soil, so anything you can do to assist drying has to be helpful. Unfortunately *too* dry is probably not desireable either!

  6. #6
    Old Chippy - 4K Club Member
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    Canberra
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    This is the ACT where we have had the longest and largest rain spell after 9 years of below average rainfall and drought. ACT has heavy reactive clay soils that are now pretty much saturated - water will eventually migrate from exposed soils to any dry areas under houses. I moved in to my current dwelling in the mid-90s and have never any sign of moisture under the house - this year it has become moist about 1 metre or so in (2m in on the side with sloping ground above it - that's gravity for you) - so far. The saturated soil in the ACT has been topped up every week to ten days all summer and we have had low daytime temperatures and higher than usual night time temperatures.

    What to do - nothing!

    This is a dry cool temperate climate with generally low humidity. By next season if it returns to normal (we have wet autumn still ahead probably as the El Nino is still in place) the under-house area will dry out again. Wouldn't even bother with any monthly inspection - just your annual or 6 monthly (which is what I do) termite patrol. Of course it makes you feel better then do it more frequently, but it isn't needed.
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  7. #7
    Apprentice (new member)
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    Thanks Geoff and Bloss.

    Bloss you make a valid point about the ACT weather conditions.

    I will take your advice and keep an eye on things but not stress too much about it.

    Thanks again to all for advice - this forum is fantastic for such discussion.

    Cheers

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