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cibula11 07-13-2007 10:10 PM

Decking over concrete

Can I install pressure treated decking over existing concrete slab? I have an old concrete slab where my stairs for my deck sit on top of. The problem is that the slab with the stairs on it is only about 18" wide and there is a sidewalk that runs perpendicular to it. Any ideas on how to make this work?

I thought about running 4x4 posts along the perimeter of the slab and then putting down p.t. decking on that. If this doesn't work, could I use composite shims under the decking to give the area some drainage? I can't raise it too high because then it would be an awkward step up onto my deck stairs.

Deacon 07-13-2007 11:24 PM

Yes, you can install pressure treated decking over existing concrete slab. If drainage is a problem, just use 2x4 pressure treated lumber and scallop the bottom that rests on the concrete slab. The water will just run underneath it.

cibula11 07-14-2007 07:36 AM

I don't think drainage should be a problem after thinking about it. The concrete slap is not level, so after I shim up the low side, any water would be able to run off the slab.

I had another question. The slab is about 36" wide. I am framing the perimeter of the slab with treated 4x4 and then laying the decking over that. To keep my heighth at a minimum, could I just screw into the 2 4x4 on the outside of the slab and then use some sort of exterior adhesive in the middle, so that I won't have to drill my decking into the concrete?

I wasn't sure if having tapcons or something else would look all that great in the decking.

Deacon 07-14-2007 08:41 AM

Use your 4x4 perimiter with at least a center 2x4 even with the top. I would not just depend on glue to hold the decking field in place though. Just predrill the 2x4 decking and screw down with galvanized screws and fill holes if desired. PT lumber is known to warp/cup/check etc. As far as attaching the perimeter, just drill some pins into the existing slab and "pin it" to the 4x4's. This way, the deck can be secure from movement/shifting but can be lifted if and when needed later on.

mudmixer 07-14-2007 09:36 AM

Even if your wood is PT, the life will be shortened substantially because of the moisture and lack of ventilation. - It is the moisture in the air around and under the wood, not necessarily the drainage.

Low wood decks have the same problem and that is why it is recommended to keep them up 18", apply a vapor barrier and have ventilation around them,

cibula11 07-14-2007 10:40 AM

Would plastic work as a vapor barrier? I could just lay it down and then add my lumber on that. The entire landing measures 3' x3', so if it rots out after a few years it won't be costly or time consuming to replace it.

Deacon 07-14-2007 10:57 AM

If a pressure treated deck ROTS after a couple of years, I would find a different supplier of pressure treated lumber. Pressure treated lumber will not ROT on ground contact just as long as you are using "ground contact" treated lumber - better known as CCA-C.40. If you lay a VAPOR barrier, just watch out for all the moisture/bugs/mold and everything else you will be growing underneath it. You better get out the bug spray because you will be growing all kinds of things underneath the vapor barrier.

cibula11 07-14-2007 01:09 PM

Instead of attaching the 4x4 posts for the perimeter to the existing slab, could I drive rebar through the posts and secure it that way? I don't really want to see a bunch of hardware since these posts will act as a step and be seen.

Deacon 07-14-2007 02:16 PM

Just drill into the existing concrete slab with ---- let us say, with a 3/8" concrete bit. After the 3/8" holes are drilled within the existing concrete slab - install some 3/8" pins -precut to be 1 1/2" above the concrete slab. Lay your pre-measured/pre-made Pressure Treated Step/Deck atop these pins and hit the deck so to leave the pin marks underneath the deck framing. Drill the perimeter with a 3/8" wood paddle bit where the indents are and set into place. This way, there is nothing shown on top. The deck can be LIFTED off the pins when and if ever needed in the future. You are basically doweling the concrete slab to the decks perimeter.

Deacon 07-14-2007 02:44 PM

P.S. You do not need to go overboard with the doweling -- one on the left and one on the right will suffice. The decks natural weight will hold her down.

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