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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

isherwood
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James
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  • Where in the world are you? I doubt this is up to building code in the US. – Tester101 Jul 01 '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 01 '16 at 19:38
  • Decks have pretty strict codes, what for preventing folks from getting hurt when they collapse. – Tester101 Jul 01 '16 at 19:41
  • @Tester101 The idea here is this thing is one step off the ground. Basically the frame under the deck boards would keep it square, I just want to keep the wood off the ground and have some cinder blocks leftover from some other work. – James Jul 01 '16 at 19:46
  • The box stores sell concrete piers that just sit on prepared ground. So, a partially buried concrete block is not such a crazy idea. However, Tester is correct that you should check your local building codes for a detached deck. They are surely more strict for an attached deck especially a deck that is further above the ground. This idea is more like a slightly raised patio than a deck. The single 2x4's will not carry much weight for a long span without deflecting too much. They will need a lot of support. Consider 2x6's or 2x8's for longer spans. – ArchonOSX Jul 02 '16 at 00:22
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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 02 '16 at 00:29
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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 03 '16 at 19:21
  • To clarify, a "ground-level" deck is structurally no different from one two stories up. You can only get away with small "sleeper" joists if they're continuously supported by a crushed rock bed. – isherwood Jul 03 '16 at 19:26

1 Answers1

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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.

Community
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Mazura
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  • 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 02 '16 at 00:35
  • @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 02 '16 at 00:37
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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 03 '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 03 '16 at 19:25