Laying 4x4 over a cement slab

Roman RytovMay 25, 2009

I have a cement slab (11'x25') which I'm building a wood deck on top and extending it on the ground so the total deck's size will be about 500sq. ft. To make the entry door opening and avoid any steps the height for the joists over the slab is exactly 3.5" (on the ground it's about 8"). I'm building a compound frame consisting of 2x6 over the ground and 4x4 laying just on top of the slab and connecting with the 2x6 over the ground. I'm to anchor the 4x4 but my concern is water and moisture contact with the slab. I thought to use some mastic insulating the wood from direct cement contact but not sure whether it's a pragmatic approach. What do you advise? I probably can insert thin plastic shims between the 4x4 and cement.

p.s. I've been in this project for some time now so things seem obvious to me. Let me know if something is unclear or I missed anything.

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I would use some pieces of flat stainless steel or aluminum approx 1/8" or 3/16" thick to get that wood up off of the concrete slab to allow for ventilation and prevent water from wicking up from the slab.

I wouldn't use mastic. That will only wick the water up just like the concrete or itself. You need something that is impervious.

The other thing you're going to have an issue with is warping/twisting of the 4x4s. You will have to anchor it to the concrete every three feet or so minimum otherwise you're going to have a mess.

    Bookmark   May 27, 2009 at 9:16AM
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Roman Rytov

numerous deck builders have to me me that the treated 4x4 is designed for water contact. although ideally to prevent it in my case I have little choice. i have no gap to provide real space but I'm assuring ventilation under the deck and keep the incline so water is not supposed to stay there. I'm anchoring 12' long 4x4 by 4 5" lag screws each (so it's roughly 3.5" distance). It seems quite secure. I'll post pictures later when I'm done with it. Drilling 2" long 5/8" radius holes in concrete is no fun...

thanks for your input!

    Bookmark   June 4, 2009 at 6:29PM
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Sure, they can get wet, but it's helpful for them to dry evenly. Without elevating them, the bottoms of the 4x4's will always take longer to dry than the tops. There can be some issues down the road, esp. with dirt accumulating.

    Bookmark   June 5, 2009 at 8:28AM
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