I want to build a floating deck with a pergola but I don't want to set posts in concrete.
My yard is mostly shale with about 5 inches of actual topsoil, if that.
My question is, could I put treated 4×4 or 6x6s on the ground and anchor them with rebar, going straight through and into the shale? Or would that be too weak to hold it? So posts on the ground, held down with rebar, and then the vertical posts attached to the horizontal ones.
The highest winds we get in my area would be around 50mph, so I'm trying to figure out if the weight of the deck and pergola, plus the rebar anchors, would be safe enough.
Best Answer
I suppose you could do what you're describing; but, my years of building deck and fence in college tells me it's a bad idea.
Even with rock and shale (and no topsoil) wood resting on the ground picks up water, which accelerates rot, even if the wood is treated. Also, nearly all deck framing assumes strong anchor points, so a wooden base (enforced or not) would have more lateral movement than most anchoring systems.
If you don't like posts in concrete (in my area there's enough water that these are problematic) I'd recommend looking at piers to replace the posts. These are concrete cylinders that the entire deck framing rests upon.
If you get 50 MPH winds, then a gust would easily be a very temporary 70 MPH. Even with low winds of half of that, I can't imaging rebar directly in dirt having enough surface area contact with the surronding earth the hold back an 8 foot lever being pushed on it's top end with 50 MPH winds.
Put the posts at least 6" below the frost line, and minimally two feet deep. The cost of the wood that will go into the deck should be enough of an incentive to do the deck's foundation right.