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I'm planning on building a detached, floating deck, 18 ft x 12 ft, in Vancouver, Canada. Using 2x10s as the dimensional lumber for the joists. Joists would be 16" OC with composite decking on top (Timbertech).

I've found conflicting information on how many pier blocks I would need for the deck. Some sites say every 4-6 ft, others insist it's under every joist.

Is there a definitive calculation I can use, any guidance would be appreciated.

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  • ...what's the bearing capacity of the soil under the deck site? That, and the area of the base of each block tell you how much each one will carry. If you are not using blocks under each joist, what else is under the joists holding them up? You don't mention beams, only decking and joists - and in that case, you certainly need two or more under every joist.
    – Ecnerwal
    Apr 5, 2023 at 1:17
  • You're using pier blocks sitting on top of the ground?
    – Huesmann
    Apr 5, 2023 at 14:14
  • Not sure about the bearing capacity of the soil, my plan was to dig a slight hole, around 4-6 inches, fill with rock base, tamp and then place the pier block in there
    – Uppsilon
    Apr 6, 2023 at 3:25

2 Answers 2

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For that design, you'd typically go with (2) 18' beams and have joists in between the beams. the joists will span with hangers. the beams you need to support. i imagine you've run this through the prescriptive deck design wood span tables from the AWC. at 18' you'll need the two supports (one each end) and one in the middle. composite typically wants 12" oc for joist spacing.

I did a deck more or less this size but used a PSL that I sent out for pressure treating to get the full span with just two supports (I had 5' storage under my deck). I used plywood with a torch on membrane, 12" drip edge, a 2% slope for drainage and 2x4 pt sleepers at 12" oc for my composite to sit on. Should last 100 years even if my torch on starts to fail in 35 years.

My supports were concrete walls. For yours you'll want to put at least 16" of crushed rock under your pier blocks to prevent any frost heave and maybe deeper if you don't get below the organic soil.

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    Thanks, those wood span tables from the AWC really helped, at cms7files.revize.com/watertownct/Departments/… The two 18' beams would need five pier blocks each, for a total of 10. The 2x10 joists can go > 13', so a 12' 2x10 as a joist would be okay between the two beams. Thanks for the advice!
    – Uppsilon
    Apr 6, 2023 at 18:14
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Usually for 18 x 12 deck with 2x10 joists spaced 16 inches on center, you might need around 9 pier blocks placed at corners & midpoints of the longer sides of deck. This is just a rough estimate though.

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