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I'm building five platforms that are 32'x16.5'. These platforms are for large Safari wall tents.

I'm using something called Tuff Blocksᵀᴹ as deck supports (instead of cement blocks) and they have a cradle for 2x joists and single-ply beams about 2 1/2" off the ground, so for a ground level deck no need for posts.

All lumber will be pressure treated Southern yellow pine (ground contact rated).

We will be as low to the ground as allowable using a drop beam design. By doing a drop beam I significantly lower the Tuff Blocks required from ~70 to ~20, saving ton of money and time leveling out each block. The problem I'm having is that with any deck I've ever built, I've used multiple ply lumber to create beams but these Tuff Blocks only hold one board single-ply.

The joists will be 2x8s set at 16" on center down the 32' run of the deck, supported by a single-ply 2x10 beam. Maximum span of joists including cantilever will be 8'.

My plan is to have a beam every 8' or less (2 @ 8', 2 @ 7') down the 32' run with a total of 5 beams made of single ply 2x10 and those beams will run across the deck a total of 16.5' (1 board) with Tuff Blocks supporting the beam every 5' maximum. I'd like to cantilever those beams 9" on each end.

Can I use a single ply 2x10 of Southern yellow pine ground contact? I think my spans are fine but I'm not sure and I want to make sure this thing doesn't have bounce - there's not going to be a lot of weight on it, only two or three people max in the tent at any time. I will also have a shower and a toilet built on the deck.

Can't find any good charts for single-ply deck beams. Again, the beams will be 2.5" off the ground.

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    Don't know tuff blocks, but the cradle is maybe 1.5 inches deep? If so if you want extra stiffness, a 2x10 with a 2x8 should work. You want to prevent the beam from flopping over mainly. A 2x10 with blocks/angles should be enough. A low deck means about the worst that happens is drinks get spilled.
    – crip659
    Nov 2, 2023 at 22:13
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    Adding the drawing of your deck framing would help this question immensely.
    – FreeMan
    Nov 3, 2023 at 12:14

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A single number 2 grade or better Southern Pine 2x10 beam is good for a 4'-3" span if the joists are all 8 ft long and come from both sides of the beam. The beams at the ends that are only loaded from one side are good for a 6'-0" span. See IRC R507.5. Note that the table is for beams that are loaded from only one side. For a beam loaded from both sides, you have to add the joist span contributions from both sides. 8 ft joists on one side and 8 ft joists on the other side, for example, implies that you would look up the beam corresponding to 8 ft + 8 ft = 16 ft joists in that table.

Your 2x8 joists are unnecessarily deep. Based on 8' length and 16" spacing, the table of IRC R507.6 indicates that 2x6 joists are adequate.

The literature for that Tuff Block product says that they're designed for 1700 pounds. Taking dead load at 10 psf and live load at 40 psf, that implies a supportable area for each block of

1700#/(10 psf + 40 psf) = 34 ft².

This is basically a 6ft by 6ft square of deck. Your deck then needs a minimum of (32 ft)(16.5 ft)/(34 ft²/support) = 15.5 supports, so your 20 supports per deck sound reasonable. Beware of supporting a rigid beam on more than two supports, though. Often a low support will pick up zero load. Galileo wrote about this.

Drop Beam Problems:

An honest reading of the IRC's deck section doesn't cover drop beam decks unless they're braced against toppling by a ledger anchored to another structure, where that structure provides the deck's lateral restraint. Just like joists get blocking at their bearing points, each of your foundation support positions should probably have similar bracing. The code doesn't prescribe such bracing, though, so free standing drop beam decks seem non-conformant to me. If you really want the drop beam deck's height (because of piles of snow, for instance), then you should hire a geotechnical engineer. Instead of the blocking at each foundation position, I think that the cheapest and easiest detail would be four 4x4s set in concrete and sticking up 12" to brace the top of the drop beams against horizontal movement.

And if you work with an geotechnical engineer, ask him for a cheap alternative to these Tuff Block supports. With a compacted base of crushed rock which you should probably be doing anyway, I suspect that a chunk of pressure treated 4x6 laying on its side could replace these supports for less than half the cost. You might actually break even with an engineer, (10 USD/block)(5 deck)(23 block/deck) = 1150 USD.

Joist Bearing Problems:

Your 32' deck length with 8' joists worries me a little. Each joist requires 1-1/2" bearing when sitting on wood. This implies that 4 spans of 8'-0" length lumber will not add up to your 32'-0". At most they would add up to 32'-0" - 3(1.5") = 31'-7.5". Be sure to get that 1-1/2" of bearing.

Block Layout:

The beam span lengths or the weight at each block will prescribe block layouts. First I'll prescribe a layout based on span lengths, and then I'll check that layout against the 34 ft² maximum tributary area that was computed earlier.

The end beams have 6'-0" maximum span lengths, so 2 spans over 3 supports implies (16.5' - 2(6'))/2 = 2.25' long cantilevers on both ends. Footnote g from the table of IRC R507.5 rejects these cantilevers that are longer than 25% the span length of adjacent beams. Increasing the number of supports to 4 provides a conformant layout, with spans of 16.5'/3 = 5.5' length working with no cantilever. The tributary area of a non-corner support is (5.5')(8'/2) = 22 ft² which is less than the 34 ft² maximum.

The interior beams have 4'-3" maximum span lengths, so 3 spans over 4 supports implies (16.5' - 3(4.25'))/2 = 1.875' long cantilevers on both ends. Footnote g from the table of IRC R507.5 rejects these cantilevers that are longer than 25% the span length of adjacent beam. Increasing the number of supports to 5 provides a conformant layout, with spans of 16.5'/4 = 4.125' length. Again, no cantilever. The tributary area at a non edge support is (4.125')(8') = 33 ft² which is less than the 34 ft² maximum.

I count 23 total supports per deck.

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