I'm planning to build a ground level deck, approximately 8ft x 10ft long. I'm going to use deck concrete blocks for footing, e.g. :
I want to place 4 of those in the corners and probably two-three in the middle on left and right sides:
A B
X-------X
X-------X
X-------X
X-------X
D C
My question is about framing: would it be OK to use 2x6 boards, or it is too weak and I should rather pick 2x8 instead ?
Thanks.
2x6 lumber should never be used for joists that span open air. Even on a deck. They're fine for fully supported floors, such as with full-contact rock or gravel bed foundations.
Then, since you're using those footing blocks it's not so much a matter of lumber size, but of footing interval. (You can't double the rim joists.) I'd use 2x8 for the whole thing, with the common joists on 16" centers (partially for structural reasons and partially to give you flexibility in decking).
Since you're using single rim joists (to fit the footings), I'd use four footing blocks on each of the 10' sides. This will ensure stiffness appropriate for the load of the common joists. There's really no reason to use more on the 8' rims, as they don't carry any more than the common joists in the middle.
I won't quote load tables or the like. I'm speaking from 30 years of construction experience. If you're going to need to pass inspection, run your plan by your city office for review.
You say 8' x 10' deck.
The code requires a deck to support 40 lbs. per square foot. (ICC Table 301.5.) So, live load plus dead load (about 6 lbs. per square foot) equals 46 lbs. per square foot total load MINIMUM.
If the joists/beams are 4' apart, the ones on the edges will need to support 92 lbs. per linear foot and the middle beam will need to support 184 lbs. per linear foot.
A 2x6 spanning 10' can support about 60 lbs. per linear foot. A 2x8 can support 140 lbs. per linear foot.
I'd use 1 - 2x8 on the edges and 2 - 2x8's in the middle.
Now, make sure they're fastened to the posts to support 500 lbs. at the corners and 1000 at the middle beam. (Usually we like to place the beams on top of the posts and then just use a clip to keep the beam from moving off the post.)
Also, if it's more than 30" off the ground, the deck will need guardrails and if it's adjacent to the house, it'll need lateral resistance attachments to the house.
By the way, 2x6 decking can barely span 4'. It will feel springy when walking on it. You could run the beams the other direction, add one more set of concrete footings, and use 2x6 beams (spanning 8', which will support 90 lbs. per linear foot.). Then, the decking will only span about 3'-3". Much stiffer.
The IRC has a section dedicated to exterior decks. Your deck can be entirely constructed using tables from this section. No engineer. No faith in anybody's authority but the IRC's.
All of your foundation blocks appear intended to minimize the size of framing, but differential settlement, construction imprecision, etc. will ultimately result in a wavy deck surface. Just 4 supports is fine and are possibly still cheaper than all of that foundation work.
The IRC beam tables prescribe that "beam cantilevers are limited to the adjacent beam's span divided in 4." Rather than put blocks at the corners, then, it's most efficient to locate them at (8'-0")/6 from the ends of the 8'-0" beams. This results in 1/6 cantilever lengths adjacent to the 4/6 central span of 5'-4". A single 2x12 meets this maximum central span length for all of the IRC's tabulated species, where I'm assuming that you live in a typical region with snow load less than or equal to the 40 psf live load.
The IRC joist table requires 2x8s for 16" joist spacing and 2x10s for 24" joist spacing. That's given your 10'-0" joist span and 40 psf live load.
