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I'm looking to install a 30' decking platform (for a yurt), 3' above grade, on a plot of land I own. The site has a mild grade (5-10%), made up of metasedimentary rock, with ~1" of organic matter at the surface (not enough to dig into). What footings or reinforcement strategy would you recommend for a site like this?

2 Answers 2

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1" of organic matter on rock? remove all the organic matter from the locations where the footings go, pour concrete footings or piers on the rock surface directly. Might be a good idea to drill holes in the rock and grout in some rebar at each location to tie the pier or footing to the rock.

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  • Excellent. This was my initial instinct. Someone had alternatively suggested placing pier blocks right on grade, and using tie-down straps anchored into the rock to hold the deck in place. I have a hard time seeing how this would ensure the deck remains level and secure over time, as the only thing holding the pier blocks in place is the pressure applied by the straps. Is my intuition correct? Commented Feb 13, 2023 at 20:17
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    There's no reason to screw around with tie-down straps when you can put rebar connecting the anchor bolts holding your beam brackets or whatever fitting you end up with on top right into holes in the rock, all grouted/concreted into place. Exact sizes, etc would best be designed by our engineer considering wind uplift, seismic, and other appropriate loadings. Can't imagine using pier blocks when wet concrete will meld precisely to the shape of the rock under it and make a much better pier, the exact height you need in this exact location.
    – Ecnerwal
    Commented Feb 13, 2023 at 20:43
  • ^ our engineer^ your engineer
    – Ecnerwal
    Commented Feb 14, 2023 at 0:50
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sedimentary rock can be quite soft / should be easy to drill. fine for compressive strength.

  1. drill 8" deep holes using rotary hammer 3/4" quad carbide bits
  2. blow out the holes / clean holes with hilti wire brush
  3. epoxy (HY150) 5/8" rebar into holes and have that extend up into your footing
  4. 8" diameter sonotube the footings to get yourself to the height you want for your platform
  5. put post anchor saddles into place on the sonotube to receive your beams or posts
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  • Thanks @freshcodemonger. I appreciate the detail here - helps me think through execution. Commented Feb 13, 2023 at 20:19

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