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Summer house on concrete base... advice pls

Discussion in 'Building' started by Dangerous amateur, 7 Nov 2020.

  1. Dangerous amateur

    Dangerous amateur

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    Hi all, having searched and searched for a clear answer I’m hoping some one can help.

    I’m going to try and build a 3m x 3m garden room over lockdown.

    There is already a concrete base down which I am going to put it on which was laid in 1950s I think, very flat and level.

    I planning 2” x 3” joist directly on the concrete to allow for insulation below the floor so I can use it all year round.

    My wife has given me orders to minimise the height of the step up into it... so I need to find the right combination from concrete up.

    I know ideally I’d have a course of blue bricks or raise it with blocks and suspend the floor etc but I’m trying to find a way to avoid this that won’t cause problems for 10+ years.

    Been thinking about
    1) Concrete
    2) one of the following:
    (a) wrapping joists in Damp course membrane
    (b) full 3x3 DCM layer below floor
    (c) paint timber with bitumen

    3) 2 x 3 treated timber joists & noggins - doubled up at perimeter to support walls
    4) 60/50mm PIR insulation (foil on both sides) & Seams taped
    5) no air gap above / below insulation
    6) plywood floor / T&G - tbc

    I’ve got no idea what is best for #2 or if an air gap is needed.

    The base will go to the edge of the concrete so the cladding will over hang it slightly so rain can run off. There will also be an over hang on the pent roof that slopes to the back where I’ll have guttering to keep water away.

    Any suggestions / info really appreciated.

    thanks
     
  2. sxturbo

    sxturbo

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    2, a

    Build base then timber frame 4x2 all-round, osb board the outer skin then clad, insulate the inside then put whatever skin you want, plaster board, osb etc, have a year round room then.
     
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  4. Dangerous amateur

    Dangerous amateur

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    Thx SxTurbo, appreciate quick reply.

    2a -DPC wrapped joists

    no need for air gap above/below the PIR and floor.

    more straightforward than I’d hoped for
     
  5. sxturbo

    sxturbo

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    Their will be a slight air gap between joist and floor, that's all that's required
     
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  7. Dangerous amateur

    Dangerous amateur

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    SOLUTION
    (BE WARNED STILL TO BE TESTED )

    After much googling, I found another article that hinted on close cells membranes. From that article I found out that in the US they put FoamsealR between concrete and timber to act as a weather / water / airtight barrier.

    it’s a barrier strip of closer cell 1/4 - 3/4” deep.

    They don’t have it in Uk, but found something very similar on ebay which looks equivalent, so going to try that as logically it ticks all the boxes.

    Article on closed cell membrane
    https://homeguides.sfgate.com/moisture-barrier-between-sole-plate-concrete-slab-88387.html

    Video on sill gasket / membrane


    What I bought
    https://www.ebay.co.uk/itm/183493549420

    Thanks for help and hope this helps others / remember untested!!!!
     
  8. DIYnot Local

    DIYnot Local

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