I need to raise a hot tub out of the deck in our back yard to address a critter problem (rodents have been living underneath so I need to clean it and critter-proof it.) The deck seems to have been built around the hot tub and it doesn't seem possible to disassemble the deck in a non-destructive way, so I'd like to hoist the hot tub directly upwards. Currently there is 23" of hot tub above the deck and 20" below the deck, resting on concrete. There is maybe 2" of clearance between the sides of the hot tub and the deck.
What I have in mind is to build a gantry crane similar to this gentleman's, rent two half-ton chain hoists to hang from the crane on either side of the hot tub, then raise it up and support it with wood braces or cinder blocks while I work on it, or possibly flip it on its side on top of the deck. Afterwards I would use the hoists to lower it back into place.
Because I'm hoping not to have to do this again, I'd like to avoid investing too much in disposable lumber and hardware. I was thinking of using 4x4s for the vertical posts, 2x6s for the horizontal support at the top, and 2x4s for the diagonal supports. To secure these pieces together I would use lag bolts with washers and nuts.
Does this plan make sense? I have fairly limited woodworking experience. What size of bolts would be sufficient? I was thinking 1/2" but they are somewhat expensive and I expect may be overkill. Here are a couple drawings of what I have in mind (front and side view):
Here is a photo of the hot tub with some of the surrounding deck boards removed:
I am guessing the hot tub weighs something like 800 lbs, plus or minus 200.
The double 2x6 support resting on top of the tub in the picture above was slid out of the hangers built in to the deck; I imagine it was built that way to allow for easy access to the motor on that side. The other joists surrounding the tub are not easily removable. If I'm unable to lift the tub out I think I would need to disassemble and rebuild that entire portion of the deck which is about 14x14' including several steps.
To connect the tub to the hoist I was thinking of using screw eyes in the wood at the base of the tub. I haven't really figured that part out yet, so my drawing is a bit misleading.
Another consideration: I don't know if I will be successful at critter-proofing the thing on my first attempt, which is partly why I'd like to build a reusable crane in case I need to do this again 6 months from now (as opposed to destroying and rebuilding the deck multiple times)
Some more photos: