Footings for single skin small block shed

Discussion in 'Builders' Talk' started by KratomMich, Jun 21, 2021.

  1. KratomMich

    KratomMich New Member

    I am currently building a small 10x9ft small block shed that will have a pent roof with the highest point at 8ft in height and lowest 6.8ft. It will have a rear block wall to the 8ft height with a pillow either side and then a side wall on either side with the front being a wooden frame and wooden roof structure with felt. I am wondering about the footings. I started to dig the footings and have leveled the ground out removing 10" of soil until level with a small extension wall 3 course high in brick that was on top of a concrete end slab as the garden is raised around 10" from the floor of the house. The footings I have dug down then from this to 550mm in depth at 440mm wide. Though the soil below is clay, firm clay IE if you push on it it doesnt really give and feels compact unless wet then it will indent slightly, will also indent with a good small mallet blow but nothing to bad. Now I am worried will this be strong enough for my 3 wall single skin block building? The concrete for footings will be 12" thick and there are no trees around the area at all and being in manchester it doesn't ever dry out to bad so the ground should stay relatively damp hopefully stopping the chance of the clay drying. The ground has been tampered with a long time ago as these houses are built in 1930's and so the ground is a mix of clay, brick dust and coal dust though on doing a sample the clay goes down substantially into the ground probably over 2m or so.

    The flooring in center will be hardcore and MOT1 mix, thin layer of sand, damp proof, insulation and 100mm concrete.

    Will these foundations be ok for this size of shed? I believe the houses themselves are not that deep in terms of their foundations and speaking to building control for advice they stated clay was ok if relatively compact though heard of a lot of contradicting statements about clay for footings.
     
  2. Severntrent

    Severntrent Screwfix Select

    Notwithstanding the issue of the proposed depth and possible frost damage issues Table 10 in Approved Document A gives you the necessary guidance on foundation size relating to soil properties. You will find from a bearing point of view you proposals are more than adequate.
     
  3. KratomMich

    KratomMich New Member

    My understanding for frost permutation was around 350mm deep minimum is this not correct? would it need to be deeper? The ground around the building once finished will be back filled then soil back then flagged. I am hoping I will only need 2 coarses of block from concrete base to dpc level going off the depth possibly.

    So in terms of strength the size seems to be ok?
     
  4. Severntrent

    Severntrent Screwfix Select

    A 440 wide foundation on firm clay can take a load of 4 tonnes per metre run which is greater than your shed will impose so ok.
    Water pipes are buried a min of 750 deep to stop them freezing but its all down to risk and consequence, i wouldn't be to worried about a shed
     
  5. stevie22

    stevie22 Screwfix Select

    It's change of moisture content leading to settlement that is the prime leader in determining foundation depths. Trees nearby -> go deeper, higher clay plasticity ditto.

    NHBC guides are for houses where you want certainty of no possible movement. Massive overkill for a shed.
     
  6. KratomMich

    KratomMich New Member

    Luckily manchester being so rainy i dont think it will ever fully dry out, ors ill water the surrounding land if we ever have prolonged heat to keep it moist. So 550x440 with 300mm of concrete is a good bet then.
     

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