How to pour concrete for a hot tub foundation

Written by tom fritchman
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How to pour concrete for a hot tub foundation
A filled hot tub can weigh a lot and needs a firm foundation to support it. (pool spa 1 image by Aaron Kohr from Fotolia.com)

Hot tubs are a great source or relaxation. However, a hot tub can weigh more than 318 Kilogram empty. Once you fill the hot tub with water and then climb inside, the hot tub will become much heavier. A firm foundation is needed to support this weight. A reinforced concrete slab makes an ideal foundation.

Skill level:
Easy

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Things you need

  • Shovel
  • Moisture membrane
  • 1x6 lumber planks
  • Cement
  • Gravel
  • Sand
  • Trowel
  • Rebar
  • Yard stick

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Instructions

  1. 1

    Level the intended site for your hot tub. You will need to remove or add dirt as necessary to make a flat surface. Remove any large rocks as well.

  2. 2

    Lay a thin layer of gravel over the intended site. Cover this layer with a moisture membrane to stop moisture from getting to the underside of the concrete.

  3. 3

    Assemble a wooden formwork along the borders of your intended foundation. Secure the formwork into place by screwing perpendicular planks onto the outside edges of the formwork. Mark the finished formwork every eight inches as a guide.

  4. 4

    Mix the concrete in your wheelbarrow by combining one part cement for every two parts gravel and three parts sand. Add water slowly until the material mixes.

  5. 5

    Pour your mixed wet concrete inside the borders of your formwork until you have a thickness of approximately one inch. It is not necessary that it be precisely one inch.

  6. 6

    Lay rebar into the wet concrete every eight inches using guidelines you marked earlier. This will add structural support to your concrete slab.

  7. 7

    Pour concrete over the rebar until your wet concrete has a total depth of at least four inches. Check the depth with a yard stick, though the yard stick will have to be discarded afterward.

  8. 8

    Smooth your concrete surface with a trowel by pressing the fat of the blade against the top of your wet concrete and rubbing in wide arcs.

  9. 9

    Wrap plastic around the top edges of the formwork and stretch the plastic sheet across the top of the concrete slab. This will protect your concrete from the weather as it cures.

  10. 10

    Allow your concrete slab to harden for at least three days before removing the plastic sheet and the formwork. Use a hammer and pry bar to remove the wooden planks if they do not come off easily.

Tips and warnings

  • A modern 1x4 is actually less than four inches wide, so use 1x6 to build your formwork.
  • A large concrete slab may take as many as 28 days to fully cure before you can install a hot tub on top of it.

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