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I am planning to build a ground level deck in bay area in northern California where we have pretty nice weather, and no snow in my living area. I am looking for feedback with following specs

  1. finish height 13"
  2. on existing uneven concrete patio (highest to lowest is around 1.5")
  3. composite deck 1" thickness
  4. 16" OC for internal joist
  5. most of the time it should be empty deck and no huge furnitures.

I used decks.designer.com to draw a fairly simple layout.

enter image description here

Questions

  1. is 2x8x10 PT Hem fir ok to span a little less than 10' for internal joists? I looked on decks.com and for 16" O.C, it claims 11'-1" for span.
  2. Rim beams: I am planning to use double 2x8x8 for the rim beam for long sides only (22'), which should give me around around 7' span for double 2x8. Or I need more footing to support shorter span?
  3. Is 4x4 posts enough? Or I need to use 6x6 posts? I know 6x6 will meet all codes, but wonder if I can just use 4x4.
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  • composite generally wants 12" oc. – Fresh Codemonger Mar 18 at 6:59
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    are you going to slope it away from any structures? I like storage so I typically 2% slope it, plywood the deck, torch on membrane, add sleepers for level, composite to finish. nice dry storage underneath. deck framing never gets wet should last 100 years. Most code says wood at least 6" off ground - are you doing concrete pillers to sit the posts? – Fresh Codemonger Mar 18 at 7:06
  • You write "on existing uneven concrete patio" - do you intend for your new deck to have free space between the joists and the existing concrete or would they be resting on and supported by the existing concrete? – brhans Mar 18 at 10:21
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    I note that you say you're using 2x8x8 for your rim joists, but your drawing doesn't show that - it shows one @ 2x ~14' and one @ 2x ~8'. Just checking that your words are off instead of the drawing being off. The way it's drawn seems like a much better idea than the way you have it worded. – FreeMan Mar 18 at 14:19
  • @brhans i would have free space between joists and existing concrete. – IHC_Applroid Mar 18 at 16:20
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Have you considered a floating deck - a.k.a. "Dek Blocks"? That makes a totally DIY project, and depending on location, either no permits or minimal permits. No concrete to pour, no digging. Deck sits on special blocks instead of being anchored to footings. Deck is not attached to house. There are height limitations, but 13" is effectively "nothing" (or ground-level as you stated).

I have no affiliation with the manufacturer - just built one myself (with my evil twin helping) several years ago, 14' x 14'.

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  • yeah, i thought about it. but I am still facing the same issues for the spans and rim beams...etc., right? – IHC_Applroid Mar 18 at 6:03
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    There is a difference because instead of depending on a few widely spaced footings, you can use a bunch of more closely spaced blocks. – manassehkatz-Moving 2 Codidact Mar 18 at 6:12
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You don't mention anything about the foundation. Unfortunately that's likely to be the most painful part of this project.

When you put a deck like this on top of a patio you are essentially putting all the weight that was evenly spread out over the patio on the 8 points supporting your posts. Unless the patio is very thick there is a danger that you could crack the patio.

The right way to do this would be to either remove or cut holes in the patio. Dig holes to the required depth and width and build the appropriate foundation (30" deep and I think 18" wide). If you don't do this you will likely have problems.

I haven't used them but the deck blocks could be better as you would likely have a lot more point loads. This would also minimize your worries about spans etc.

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  • I have concrete patio now (as pointed out in OP) but I forgot mention is thickness, i think it should be 4-5" thick concrete slab. – IHC_Applroid Mar 29 at 16:32

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