Any time you construct a building -- whether it’s a large office structure or a small outdoor spa -- a firm foundation is always essential. Therefore, much thought should always go into properly preparing the foundation for your outdoor spa. If your foundation is not suitable, there’s always the chance your spa will shift or settle after it's put in place, which can cause enough stress to alter the hot tub’s seal.
Step 1 - Mixing the Concrete for Concrete Slab
You’ll want to put your hot tub on an elevated foundation, which should be a 3” concrete slab. (However, if you choose to construct your spa on a raised wood deck or other structure, instead, it is strongly suggested you check with a trained structural engineer or contractor, first.) This concrete is to be comprised of one part Portland cement, sand, and crushed stone, to 24 parts water. You’ll know it’s right if it has the texture of peanut butter.
Step 2 - Preparing the Site
Correct soil preparation is a necessity. Begin by exchanging crushed stone for the soil immediately underneath where the concrete will be laid. Doing so will make the drainage better, as well as prevent any frost heaving hazards. Soil preparation should take place approximately six to twelve inches below where the concrete will go. This task is intended to take out the most active soil. Soil freezes and frozen soil can cause your concrete to flake.
Step 3 - Compact the Stone
Compact the stone with a gas-powered compactor, which will ward off any settling after you pour the concrete. If you skip this step, the concrete could crack when the rocks settle beneath the foundation.
Step 4 - Build Concrete Forms
Determine the total distance for the circumference of the slab, which will tell you how much wood and concrete you’ll need. Clear the space of any plants, and take out four to six inches of the soil. Spread a gravel layer where you’re going to pour the concrete. Place the 2x4s around the slab location, and pound in the stakes on the 2x4s’ sides with a hammer.
Step 5 - Pour the Concrete
Pour the concrete into your form, moving it out to the corners with a regular garden rake, as well as halfway up on each side of the form. Put down some mesh, and after the mesh has been placed, add more concrete on top of it. Once you’ve allowed it to cure a while, remove the form and wait for the concrete to completely dry.
After you’ve followed these five basic steps, you will have the concrete foundation you’ll need for your hot tub.