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Pouring concrete over concrete slab – should I put with plastic in between?

canada_deck | Posted in General Questions on

This is for the energy efficient shed project.  I didn’t want to mix 2 yards by shovel and I can’t get a full size cement truck down my driveway.  I’ve now discovered a local firm that will deliver just under 2 yards with a lighter truck.  I’ve also discovered that the driveway slabs is further out of level than I had anticipated (about 8 inches across the length of the shed.)
As a result of all of this, I’m thinking about ditching the skids and shims and floor and just pouring a pad over the existing driveway slab to give a level working surface and then using that as the floor and footing.  Note that under 108 sq ft, I do not need a permit for any of this and the building codes don’t apply.

If my goal was to heat the shed, I appreciate there would be a lot of heat loss through the floor.  Since my actual goal is temperature stability with temperature never dropping below freezing, I am hoping the concrete floor will help keep the shed above freezing when temperatures outside get below freezing. I am in a similar climate zone to Seattle.

I’m wondering how this change in direction may impact moisture and how I should deal with that.

My plan is for the shed to be air-tight with a vapour barrier on the inside wall. 
My big questions are:
A) Will moisture be coming up through the concrete and into the shed? 
B) If yes, could I stop that with epoxy on top?  Should I put a sheet of plastic down on the existing driveway shed before I pour the slab on top?
C) I have room to put down rigid foam before I pour this slab.  Given that my goal is to use the earth to stabilize the temperature at 5-15 degrees, would that be counter productive?

The previous plan was 6*6 rafters with blocks on the low side and then a floor made of 2*6’s with PT plywood on the bottom, normal plywood on the top and 6″ of Rockwool in the middle.

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  1. Expert Member
    BILL WICHERS | | #1

    Note that you can rent small power mixers to mix loads of concrete that are too big for shovel-style mixing but are also too small for a delivery. This might be a good option for you here. The power mixers have a motor driven drum and are commonly used to mix small batches of concrete or mortar on job sites.

    I would not put poly between the old and new slabs. You should put some pins to lock the old and new slabs together. There are various ways to pin the slabs together, and if it’s a small enough job you could probably get by with some large (3/8”) tapcons driven into the old slab with an inch or more of head sticking up into the new slab. The idea is to lock the new slab to the old slab so that it acts like one piece.

    Epoxy on top of the new pour will probably be enough for basic moisture control. Remember that you have to wait long enough for the concrete to cure before putting the epoxy on, since the concrete needs to pass the moisture test first. This can take several months depending on conditions.


    1. canada_deck | | #2

      Thanks Bill!

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