Concrete base for wooden cabin

Feb 25, 2006 3 Replies
U

After lots of useful replies about a gravel path I thought I would ask some questions about another project I have been "thinking" of doing for some time. I say "thinking" as I haven't done anything like this before and what I have read various sites either conflicts or leaves some questions.



For information the wooden cabin is approx 13' x 15' and I am told in total weighs approx 3 tonnes. I have also been told by the building supplier the concrete base needs to be a 6" subbase (hardcore?) and 6" concrete due to the weight of the building. I have got as far as clearing the grass from the area and digging to the necessary depth for the base so that the base is just above ground level. I was intending on putting gravel around the outside of the building to prevent water splashing up onto the cabin.



On a friends suggestion I also have begun to collect a small amount of hardcore consisting mainly of house bricks. Although this may prove to be a waste of time and effort!



These leaves me the following questions?


  1. From reading various sites it seems that although the traditional hardcore (Bricks, broken concrete) is cheaper than specific sub grade material e.g. DTp1. It is considerably more work as it needs to be broken down small enough, doesn't compact as well and obviously needs collecting. The ideal subbase seems to be DTp1 compacted with a whacker plate but will be expensive.

Is DTp1 overkill for this type of base or is there another material other than traditional hardcore that would be suitable?


  1. I don't seem to be able to see a definitive anwser on whether a base this size and loading will need steel re-enforcing mesh to prevent cracking?


  1. As access to the site is through the house I was considering having the concrete pumped through the house. I realise that this will cost more but will reduce the amount of work mixing the concrete and will mean it is more consistent i.e. not mixing in slightly varying quantities (also bearing in mind my experience in mixing concrete!).

  2. Is it recommended to put a membrane material between the sub grade and the concrete? I read that this will stop water permeating from the ground up and prevent the sulphates in the subbase attacking the concrete?

I have researched this a fair amount on web and in books but anwsers or hints on the best way to build this base would be appreciated.



Thanks Martin



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R

Broken brick/concrete hardcore will be just fine, I'm sure. The whacker plate should compact it sufficiently.

The reason it will crack, if it does, is because the foundation subsides slightly and in an uneven way. Compact the harcore well and I wouldn't expect a problem even if you don't use reinforcement.

Remember, if you ask a structural engineer whether you need reinforcement, or the extra nail, he'll always say yes, because, well, you might. But generally you don't if you've got the mixture and the thickness right. But nothing is finally certain in this area, so if you want belt and braces, do it all.

If the concrete itself is below ground level then the plastic won't have any effect in stopping the groundwater rising into the wood of the cabin and

*sulphates in the subbase attacking the concrete* sounds rather like a statement made by plastics manufacturers. Does it matter if they do? I expect that most concrete is simply poured onto the subbase without plastic sheet. Your cabin will have rotted away before the concrete does, I bet.

Rob Graham

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I sent this out yesterday but it doesn't seem to have got into the system: sorry if you get it twice!

Most important, check with the manufacturer and ask if it can be supported on pads/concrete posts. Most can and this could be much, much cheaper than putting in a concrete base. You can also gain a handy space underneath... Bear in mind that a lot of these cabins get put up in places where a concrete base would not be practical. Having a void under is great for running wiring, water pipes, network/tv cables, etc.

If you have to go for the base, ask the manufacturer for their designed base. Typically they have thicker sections where they have to be loadbearing and thinner where they are not.

Adding reinforcement (and expansion joints if big enough/recommended by the manufacturer) is one of those things whre the cost of getting it wrong and putting it in is so, so much less than the cost of not putting it in and finding that you should have..

You may want to put in some thermal insulation in the base, if you plan to use it year round. But the manufacturer will advise on this.

-- Sue

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