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I'm looking to add a wood/composite deck over an existing concrete slab that has heaved/cracked and looks unsightly. We've only owned the house for a year so I don't have the full history but we have resolved some drainage issues that I assume caused the cracked slab in the first place. Now we'd like to create a more visually appealing back patio.

Some quick info:

  • Slab size = 12' x 14'
  • Slab thickness = 4"
  • Slab is covered by overhanging shingled roof with eaves
  • Vertical clearance to sliding door threshold = ~2-1/2"
  • Slab heave = 3/4" ~ 1" at the center of the slab

Option 1:

I've considered floating deck tiles directly on top of the slab but I'm concerned about the slab not being level enough for this, and also about how well the floating tiles will hold up over an Ontario winter.

Option 2:

The second option is to lay down some 2x2 joists (16" OC) and put cedar/composite decking on those. This is my preferred method but I'm not sure what the best way to support the joists is. I'm thinking of using plastic shims to level & support the joists and some steel angle brackets to secure joists to slab.

I haven't seen any information on how much unevenness can be tolerated by floating tiles, so if anyone has experience with this that would be great. Otherwise, I'm mostly looking for suggestions on how to best support the 2x2 joists that the decking will sit on.

  • Have you considered removing the slab entirely? Four inch concrete comes up no problem at all. – Martin Bonner Jun 10 at 20:52
  • What's the advantage of this? – spizzak Jun 11 at 11:51
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    It gives you enough space above the ground to install proper bearers for the deck. I thought your big problem was that there was only just enough room over the concrete for the decking itself. – Martin Bonner Jun 11 at 11:59
  • Right, that would solve that issue. It just seemed like a waste to get rid of a fairly solid foundation. I will add this to the list of options- thanks! – spizzak Jun 11 at 12:09
  • if it's breaking up it's not partcularly solid – Jasen Jun 11 at 18:50
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You are going to building a series of joists that presumably terminal at rim joists and then shim this floor assembly level. Are you putting plywood on top of it so that your tiles sit on the plywood?

If you make a 12'x 14' floor assembly out of 2x2 joists with rim joists and this is for the most part sitting on the ground, you could shim with composite shim or make your own from decking offcuts. Another option is you could use a non-shrink structural grout in any space between the joist and the slab and use get a mortar bag to apply it. If the heave in the slab is making the assembly not level chip/grind it out.

Your deck is covered, the concrete base is porous, assuming water is not pooling under the deck in a storm I don't think you have to worry too much about drainage. If the water has previous pooled on the concrete then you might want to worry more about it.

To keep the shims in place you could either use the grout or use an adhesive (PL Premium). Direct contact between the PT 2x2 and the concrete is fine assuming there is no puddling of water.

  • Standard deck boards on the joists is what I meant by option 2. I've edited the question to clarify that. That is the direction I'm leaning in but looking for guidance on how to lay down the 2x2 floor – spizzak Jun 10 at 18:09
  • Assuming I was able to grind the slab down is it okay to have the PT framing on the slab or should it always be raised for drainage? Also, if I raise it using shims, do I need to ensure the shims stay in place somehow or just the weight of the deck on it is enough? – spizzak Jun 10 at 19:14
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The question is a bit vague. I'd be setting landscape blocks around the slab, filling it with sand to the appropriate height, and setting pavers flush with the blocks. Done and done.

The framing thing will be very putzy since you can't span any appreciable distance with a 2x2. You'd have to support every sleeper on 16" intervals or less, with every support being a custom thickness. You'll end up with a "tappy" deck that that makes noise when you walk around on it. It'll never really finish settling due to moisture cycles and warpage.

  • That's an interesting idea I hadn't considered. I'm not sure I could fit pavers in 2-1/2" of vertical clearance though. I guess another option me be to pour another 2-1/2" on top of the existing slab as well – spizzak Jun 11 at 11:50
  • I just learned about thin pavers- sounds like this would be a good option as well :) – spizzak Jun 11 at 12:16
  • Yeah, many patio pavers are 1-1/4" to 1-1/2" thick. – isherwood Jun 11 at 18:11
  • A good option is you have the vertical space. If the patio attaches to the house ideally the drainage plane is 4" below the top surface of the impermeable layer of the door opening - probably most older construction doesn't follow this. You don't want to setup the scenario where the exterior patio is higher and encourages drainage under the door to the house. – Fresh Codemonger Jun 12 at 16:55

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