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Raft or Other Foundation?

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  • 05-12-2005 10:07am
    #1
    Calluna
    Closed Accounts Posts: 102 ✭✭


    Hello,

    I'm looking for a bit of advice with regard foundations for a new house I'm hoping to self build in the coming year (or two!). Am currently at planning permission application stage, but have been doing a bit of 'sussing out' with regard aspects of the build and have got different information from different people. I was just wondering what are people's opinions on the use of a raft foundation versus the traditional foundation dug under the walls only? The site is quite good land and I don't think that there is any fear of sinking/shifting, which I thought was the main reason for rafts, but people have told me that they are the same price or you save on time? Or that the main costs are for the broken stone fill and so it works out the same anyway cost wise? There'll be about 1,300 square feet of a ground floor area , but a two storey, if that makes any difference. Also wondering about hollowcore ceiling but will start another thread for that.

    Thanks


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    #2
    JohnBoy
    Registered Users Posts: 2,343 ✭✭✭JohnBoy


    my understanding is that a raft is substantially more expensive, you've the cost of formwork, a lot of steel and a hell of a lot of concrete, parents house is on a 18+ inch slab, cos the site is pure sand and i think it added 12k to the build 4 years ago


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    #3
    tcoen
    Closed Accounts Posts: 32 ✭tcoen


    Have your engineer do a site assessment. The most popular foundation type is strip foundation. Make sure you use at least 35N concrete, the difference between this and lower strength concrete is not huge overall. Most self build mags recommend this.
    Nowadays alot of builders are making internal wall strips 900mm wide by 300mm deep and the external walls the same or wider. Some walls in the house can be pulled through so that bother sides of the house are connected depending on the design. Strip founds are also steel reinforced.
    For extra peace of mind you can get a structural engineer to design the best foundation for your site soil type etc. Trees / railway lines etc near house can influence found design.
    Raft may be more forgiving if you want to change internal layout during the build.
    Hollow core floor is good for sound insulation at low frequencies and if you put underlay and carpet on top then the high frequency sound insulation will also be good. Would be a good idea if you are going for UFH upstairs.
    Blocks hve to be built on there side usually to support the extra weight which can also add to the expense.
    Look into Insulated Concrete Formwork house build type if you are an early adopter type.


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