Concrete garage base thickness.

Discussion in 'Building' started by Adam68, 12 Apr 2017.

  1. Adam68

    Adam68

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    We are having a block built garage and I want to know what depth of concrete is needed for base on a soil/ hardcore bottom .
    The builder has dug 1.5ft trench foundation. He will pour concrete in there and build 4 walls on the perimeter.
    My question is how deep does the base have to be to allow a vehicle to be parked on it. He was not thinking of digging much but raising the slab slightly . We have a sloping garden downward towards the rear . I thought u dig for base and pour concrete 6 inches to level with ground?
    Any help appreciated
     
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  3. ^woody^

    ^woody^

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    100mm hardcore, 100mm concrete
     
  4. Adam68

    Adam68

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    I assumed it was 6 inch concrete for vehicle. Will 4 inch be strong enough? He has dug a shallower hole at entrance where there is no blocks. The concrete here will be Deeper than the base.
     
  5. oldbutnotdead

    oldbutnotdead

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    100mm will be fine as long as the hardcore is whacked down properly. 500mm is a bit shallow for foundations unless you're in the deep South on firm clay. And yeah, no point putting foundations where the door will be. Is this edifice needing building regs (> 30 sq m) - if so the nice BCO will tell you how deep he wants the footings and the slab
     
  6. Adam68

    Adam68

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    No it's just 18ftx10ft.
    The foundation will have concrete and then blocks put in. How deep should they be. ?
    The garage is going to be 2.5 high max.
     
  7. oldbutnotdead

    oldbutnotdead

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    Up here 700mm is about standard for strip in solid ground (heavy clay mostly), really depends where you are and what the ground is like. If there's heavy clay or something solid at the bottom of the trenches at current depth then it'll work. And having the slab above ground level saves you money on dragging spoil away, also means any water hitting the slab can run off into the garden
     
  8. Adam68

    Adam68

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    It's soil. Not sure what u mean by heavy clay. When he dug up there were quite a few bricks dug up and some stones . We also had a few tree roots to dig out. Depth is 45-50cm max.
     
    Last edited: 13 Apr 2017
  9. oldbutnotdead

    oldbutnotdead

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  11. Adam68

    Adam68

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    It mentions 0.45m which is what he has dug. It talks about clay soils. When digging they had to use shovel with some force. However I'm not sure what kind of soil it is. The garden had not been dug for 15 years. Is the depth of 0.45m to alleviate frost. How would that impact the structure of the wall or garage ? Will it be safe or worse case scenario sink into ground over the years.
    Is it normal practice to pour concrete into trench then build on this with blocks.?
     
    Last edited: 13 Apr 2017
  12. Ian H

    Ian H

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    Crap soil will be a big neat pile of black. Heavy clay would be a scruffy pile of big yellowy lumps.

    I poured a single 150mm thick slab and the bricky blocked up directly off that.
     
  13. Adam68

    Adam68

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    So you poured 15cm of concrete into trench and put bricks on that?
    So if he pours more concrete or thicker concrete it will be more stable ?
    The soil is not black when dry just grey.
     
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  14. oldbutnotdead

    oldbutnotdead

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    That handy document I linked to also gives the width of the strip foundations that will be deemed to comply (not that you have to comply but it isn't a bad guide). Your soil sounds very soft if they were shovelling it straight out (usual deal is spade or pickaxe or mattock to break the soil up then shovel to shift it). Have you tried hammering a stake in or pushing your finger in at the bottom of the trenches?

    If you're not in an exposed wilderness and you're in the south of the UK then 500mm deep will be adequate to mitigate frost heave. You mentioned tree roots- are there any trees nearby? Minimum width for the strip would be twice the thickness of the blocks, softer ground=wider strip. And yes, totally normal to chuck 150mm or 200mm deep concrete footings down then put blockwork on top a few days later (7 days is safe, this time of year worth watering the strip every morning and evening of those 7days as well). Don't forget DPC in the wall.

    To be honest, your blockwork 2500mm shed- you could use blocks on their sides in the trench as foundations and most likely it would go nowhere. Have faith in your builder- if you don't have faith in him/her then get another builder.
     
  15. Ian H

    Ian H

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    No there was no trench. Our ground is solid, I put a 150mm shuttering around and area 8mtrs x 3.5mtrs, laid in some mesh and poured the concrete. Then built off that.
     
  16. ^woody^

    ^woody^

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    You built a raft foundation then. That can work for some ground types
     
  17. Adam68

    Adam68

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    Had a go this morning and the stake does go in a few inches with a stone banging on it. They were using spades with weight of foot on it and it was coming out. Where there were bricks it was difficult obviously. Top soil came off very easy( lawn). The trench base seems quite damp not dry.
     
    Last edited: 14 Apr 2017
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