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I have a rough plan for a ground level deck. The deck will not be attached to anything though it will sit under and extend a bit from a free standing pergola.

My plan is to bury most of a cinder block underground as a base. The hole will be filled with a little gravel to level, the block placed on top and then filled with dirt.

On top of the blocks I plan to run AC2 2x4's. The outer most (the picture is off) 2x4's will be two screwed together and then a single 2x4 for the inner joists.

So the joists would look like this:

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  • Is this a sufficient base for decking?
  • How far should the joists be spaced?

enter image description here

  • Where in the world are you? I doubt this is up to building code in the US. – Tester101 Jul 1 '16 at 19:37
  • @Tester101 Is code particularly relevant if it is not attached to the house? But, Nebraska to answer your question. – James Jul 1 '16 at 19:38
  • Decks have pretty strict codes, what for preventing folks from getting hurt when they collapse. – Tester101 Jul 1 '16 at 19:41
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    You're supporting the 2x4 post with the weakest part of the block: an open span with hollow space behind it. Unreinforced concrete is very weak in a span like this since it puts the concrete under tension. It'd be much better if you supported it on a wooden cap that spans the entire top of the block. – Johnny Jul 2 '16 at 0:29
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    Johnny's right--concrete blocks have no strength at all in span orientation like that. You're asking for disaster. Even if you fill them with sand, they'll crack and settle. Use them like they're designed to be used (as they are in a foundation wall) or don't use them. Also, 2x4s and 2x6s are studs, not joists, and certainly not beams. – isherwood Jul 3 '16 at 19:21
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A plain old cinder block will deteriorate within a matter of years. Use blocks designed for the purpose, or pour footings like you're supposed to, following code to the letter.

Concrete Deck Block:

enter image description here

Other than as being part of the railings, 2x4s don't belong anywhere in the construction of a deck.


I've never used these, but a key rule to follow would be placing them on undisturbed soil (don't add dirt back into the hole to make it level). Remove the sod and dig your hole as perfectly as possible.

  • What makes a cinder block degrade when used like this? I've seen plenty of unpainted cinder block walls that are exposed to the elements. – Johnny Jul 2 '16 at 0:35
  • @Johnny - Walls usually have roofs to shed the majority of the water. And in constant contact with the earth, those blocks will degrade rather quickly. I'm not exactly sure how they make CMUs, but the picture above is a solid chunk of concrete. – Mazura Jul 2 '16 at 0:37
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    The cinder blocks I have were underground for 15 years and are in perfectly good shape. The deck blocks are a good suggestion though. – James Jul 3 '16 at 14:33
  • There are unprotected block foundations in use by the millions around the U.S. Deterioration is not a concern. I'd use something solid for a minimal cost increase, though. – isherwood Jul 3 '16 at 19:25

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