Size of Concrete slab for hot tub

by Troy

Q. I recently purchased a home in NC (eastern, frost not a concern) that came with a sunken hot tub in the sunroom/deck area. Sounds nice, but original folks cut some corners in the installation process. The tub is partially on the ground under the deck and shored in other areas. What I would like to do is remove the tub and pour a concrete slab to support it, while maintaining the sunken installation. I have worked with concrete in the past, but I want to undertake this project once.

Here are my concerns: Size? How much bigger should the slab be than the hot tub? Slab thickness, 4 or 6 inches? Footers, needed or not? And finally, reinforcement?

A. Those are really good questions. We do slabs for hot tubs all the time. Here is what works for us.

Make the slab 6" larger, all the way around, than the hot tub.

The slab thickness should be 6", use 4000 psi concrete, you do not need footers. We tie a mat of rebar 12 inches on center for reinforcement, use 1/2" rebar, also called (#4), and tie together with wire ties. You can put some bricks under the rebar to make sure it is in the middle of the slab.

The day after the pour you can apply a concrete sealer to help it cure slower or keep the slab moist for 7 days.