Hot tubs are a popular addition to any home, especially for people who entertain regularly. Filled hot tubs, which usually go outside, typically weigh over 3,000 pounds for a six-person tub plus the weight of the people in it, so it's important to have a sturdy base that can handle the weight — grass or bare dirt just won't do. If you choose to place your hot tub on a concrete slab, you'll have to build something more substantial than a normal 4-inch concrete pad, instead using footings much like the foundation of a house to handle the weight.
Things You'll Need
How to Build a Concrete Slab for a Hot Tub
Step 1: Mark the Area
Stake out the perimeter of the slab you want to pour. Slabs for hot tubs generally are 8 feet by 8 feet. Run twine between the stakes.
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Step 2: Dig Out the Area
Excavate the concrete pad area 6 inches deep to prepare the ground for the concrete. Excavate the perimeter 1 foot deep and 1 foot wide to form a deeper trench around the excavated area.
Step 3: Make Concrete Forms
Build the forms for the footings by placing 2-by-10 lumber around the outside of the trench and securing it with wooden stakes. Secure 2-by-6 lumber on the inside of the 2-by-10 lumber, 6 inches away, forming a 6-inch wide frame for the footings you're about to pour.
Step 4: Add the Gravel
Dump 2 inches of gravel into the trench inside the forms you just built.
Step 5: Prepare the Concrete
Mix the concrete using your wheelbarrow, hose, and shovel according to the manufacturer's directions.
Step 6: Pour the Concrete
Pour the concrete into the forms to create the 10-inch-deep footings that will support the weight of the pad and the hot tub. Flatten the top surface of the footing by running a 2-by-4 along the top edge of the forms.
Step 7: Let the Concrete Dry
Wait two days before removing the wooden forms from the footings. They will cure fully in about a month, but you don't have to wait for that before moving on to the next steps.
Step 8: Prepare the Interior Area
Tamp down the dirt inside the footings, which should be about 6 inches below the level of the footing you just built. Pour an even, 2-inch layer of gravel onto the tamped dirt, then tamp the gravel down as well.
Step 9: Install Rebar
Position a rebar grid or 4-inch steel mesh over the area. Wire it to rebar stakes to keep this support grid a few inches off the ground. This sets the support in the middle of the concrete pad, where it helps to prevent the concrete from cracking under the weight of the hot tub.
Step 10: Prepare the Concrete
Mix more concrete as before and then pour it into the enclosed footing using the footing as your form. Run the 2x4 along the top surface of the footing to make the central pad level with the footing.
Step 11: Let the Concrete Cure
Allow the concrete pad to dry and cure for 21 days before placing the weight of the filled hot tub onto the pad.
Clean the tools, like your shovel, before the concrete has a chance to dry or you're likely to never get it off again.