I am moving a 9ft x 9ft hot tub that was sitting on a cement slab at our old house to a new one. Would like to place it on the existing deck & would be willing to “beef it up” if necessary to support the weight of the hot tub, water & folks who will be using it.

Here are details for existing deck. 2x8 pressure treated Southern pine stringers & joists spaced 16” O.C. Current deck extends 9’6” out from 2x8 stringer attached to side of house. Deck is 9 ft above grade. Galvanized joists hangers used on both ends. Support beam underneath joists is a double 2x12 - also pressure treated Southern pine. 6x6 pressure treated Southern pine posts placed on 12” diameter poured concrete piers sunk into clay soil below frost line (MN). Double 2x12 beam sits on 3” deep “ledge” cut into top of 6x6 posts & fastened with appropriate engineered fasteners.

Center of cement piers are 9ft from bottom of home’s frost footings (walkout basement). Bottoms of 6x6 posts are fastened to cement piers with appropriate steel fastening plates & threaded rods in poured concrete piers. 6x6 posts fastened to cement piers are 11ft apart. First one is just inside outside corner.

I would like to extend the deck from 9’6” to 12’ so there will be more space on two sides of the 9’ x 9’ hot tub & the casement window on the one side will be able to open fully. I could easily remove the composite decking boards & double up the 2x8 joists & add an additional 2x12 to the beam if needed. Could also replace 6x6 posts with bigger ones.

I am hoping the 12” diameter cement piers don’t need to be bigger. I could add a couple more if needed (& posts of course). I could also add another beam with posts & cement piers somewhere between the house & the existing beam.

Original deck was inspected by city building inspectors & is well built.

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    Commented May 9 at 14:00

1 Answer 1



2x8 joists are barely adequate for that span anyway, and they certainly aren't for that massive load (which will be somewhere in the 4-6 ton range). You need heavier joists or a shorter span, which means either more posts or more beams (or both).

I'm not sure I completely follow your expansion plan, but you seem to be suggesting a lengthened cantilever. That's not going to fly. Cantilevers are strictly limited these days as they've been a common failure point.

6x6 posts are large enough, but the footings will almost certainly need enlargement. 12" is minimal even for a deck unladen with water. My guess is that you'll need bases of 20" or more (which can be just footing pads 10" thick or more, with smaller-diameter piers).

Unfortunately, you're more or less looking at a rebuild. Hire someone local to do a revision plan. Run it by the city inspection office before you begin--they're usually happy to help you avoid mistakes.

Or put in a concrete patio or a second level below the existing deck.


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