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sub floor ventilation

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  1. #1
    huc is offline
    Apprentice (new member)
    Join Date
    May 2005

    Default sub floor ventilation

    Hi all,
    first time posting, I have a Victorian terrace house, in which I have just ripped up the ground floor. I have some rising damp which has rotted out some floor boards, joists etc plus some termite damage. The house foundations are large sandstone blocks and the floor is level where the brick work starts. I wish to add some more sub floor ventilation.

    What would be the best way to cut into the sandstone blocks so I could add some aleta 230mm by 160mm vents? The sandstone is about 240 to 250mm thick.

    Is there any pest control measures I should take before I put the floor back in?

    Also any suggestions for a DIY solution for the rising damp through the sandstone?


  2. #2
    Apprentice (new member)
    Join Date
    Oct 2004


    Quote Originally Posted by huc
    Is there any pest control measures I should take before I put the floor back in?
    Have a pestie have a look at it. It is probably the best chance you're going to get to have a preventative treatment while the flooring is up.

  3. #3
    Join Date
    Aug 2003


    Try here.

    I have used this for the house we are in at the moment.
    The brickwork had no dampcourse in it, and the damp used to rise 500mm up the outside brickwork.

    Bloody brickies.


  4. #4
    Senior Member
    Join Date
    Mar 2004


    Stone and rubble foundations have worked well, provided they haven't been interfered with too much. Many of the damp problems we have are related to what we have done to the foundations, rather than an inferior building practice of past times.

    Sandstone foundations were used in Sydney because (a) it was readily available and (b) because it was relatively porous and dried out quickly - damp from under the house could pass through the sandstone and evaporate. So we paint it - and the damp is trapped.

    Major causes of damp problems also include - building on additions (traps the damp with a similar effect to painting the sandstone), alterations to exterior soil levels (which happens over time anyway, but can be hastened by gardening, concrete paths, etc.) and using the sub-floor area as a handy place to dump rubble (a house we bought at Lilyfield had had a trapdoor cut in the floor and all of the rubble from demolished fireplaces and chimneys dumped into it - it reached up almost to the floor).

    We currently live in a late 1800's house with a rubble foundation. The only damp problem was in one room to which an addition had been added and a concrete path laid up the wall. The damp had been "repaired" once - plaster removed from lower part of wall and replaced with concrete render. No problem - the damp rose above the render and affected the plaster above it.

    I guess your Enmore terrace (I grew up in one of those when it was definitely not a trendy place to live) has at least one adjoining neighbour - the neighbouring houses and their history can also impact on your problems.

    You will find some general information on rising damp at -

    And to treat it - while the floor is up, you might get a few quotes from professionals. You obviously don't have to have them do the job, but in the process of quoting they will point out to you where the problems are.

    DIY treatments seem to be a thin on the ground. You will find info on Ab-Tech Damp Proofing Liquid at -

    There was another product released a few years ago - tubes of silicone with a long thin nozzle which were inserted into drilled holes and left there until their contents had been discharged into the wall. Can't remember the name of the product which is pretty sad because we used it on our damp corner (along with lowering the ground level outside to where it originally was).

    You are probably limited in where you can place vents (front and back?). It might be an idea, especially if the rear has been build over, to consider a sub-floor fan to encourage air movement.

    And before you do much else, just check that it is rising damp, and not falling damp (from yours or neighbours roof, guttering, etc.), seepage, etc.

  5. #5
    Join Date
    May 2005


    I had similar problem a few years ago. There was a DIY product that involved drilling and injecting silicone. In the end I sub contracted that part out because I thought the pros had a better product.
    I had little air space under house and to try and reduce dampness as far as possible I installed fans.I levered out a sandstone block using large hired saw and chisels. Cut block in half and installed a fan and replaced the other half with the fan alongside and some coloured mortar to fill gap. To create sufficient draught I installed another fan on the other side of house and put both on timers. There's a bit of an art in the placement of the fans, - if considering best get advice. Some advise one to push air in, and other to draw it out.

  6. #6
    Apprentice (new member)
    Join Date
    Mar 2006


    Not sure about cutting holes in the sandstone without an expert involved but installing an active ventilation system is a good idea and you could do this easily yourself.

    Use a combination of an inline fan and ducting to spread the circulating air and make sure there are no "dead" air spots under the floor. Unless you get good crossflow ventilation (there are Australian Standards for this) with passive (ie vent openings only) you might be wasting your time cutting holes in lovely sandstone...

    Takes some thought but in my situation (Surry Hills: 1880s terrace, no subfloor ventilation!) we had two vents installed, one intake with no ducting attached and one output with ducting and fan attached where the ducting/fan takes air from the farthest points underneath the subfloor from the intake vent. This causes air to travel from the intake vent right through the entire subfloor until it gets sucked up by the ducting at the back off the subfloor (oppposite end of the subfloor from the intake vent).

    In my house I got someone to install this...wasnt that expensive but I could have halved the cost if I had done it myself...although I wouldnt have felt confident about cutting the vent holes in the sandstone footings, particularly as the floor level is lower than the street and had to go for a periscope type job.

    So try the active vent method and make sure you have a good subfloor access hole so you can keep an eye on how things are going down there once it is done.

    You will be suprised at how effective it is..keeps all the sub floor timber and floor nice and dry, primarily as it stops moisture condensation, which I think casues more damage to these timbers than rising damp itself (although the two are obviously related).

    And here is a potential bonus that is related to the porous nature of sandstone discussed previously...., if you make sure the inside surfaces of the sandstone underneath the floor is clean and debri free, you might find that the moisture that has been rising up through the sandstone to the brick work above (ie rising damp) is being evaporated directly out of the sandstone first...via all the fresh air that will be circulating under your house!

    As suggested above timers are the way to go..depending on conditions and how long you need to run the fan this should only costs a few cents a day to run and as you now know, this is a bit cheaper than replacing floors etc. every few years.

    I'd have a go at the active ventilation...not expensive to run and even if you think passive methods (ie a couple of new vents) will do the job you might find some additional benefits from active methods. If you still have a prob then maybe start looking at the more expensive options. (Inline fan and ducting is suspended from joists by 1" plastic tape)....only your dog should be able to hear it when running. )

    PS the website wombat47 posted above is probably one of the best websites I've seen for info on causes of damp....although like me, dont be suprised if your damp problems are caused by more than one type of damp!

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