I am pouring a 5 inch concrete pad for a hot tub. I am planning on putting down 4 inches of compacted gravel under the concrete with rebar mesh in the center. My concrete pad will be 9 feet by 9 feet. Should I clear an area a few inches larger than 9 feet and compact the stone and place the form on top of that for my concrete (5 inch tall form) or should I use a 9 inch tall form and compact the stone inside the form and pour the concrete on top? I have seen a few videos of it done both ways and I was wondering which was better. Thanks.

  • Your slab should be placed after the walls and the adjoining slab were built, then you shouldn't need form works.
    – r13
    Commented Mar 24, 2021 at 2:19
  • There are no walls or adjoining slabs. The slab is going on the ground just past an existing deck. Commented Mar 24, 2021 at 3:08
  • You should dig 8" wide trench around perimeter of the slab. The trench should be a few inches deeper than the bottom of the interior gravel backfill; chamfer the inner cut edges for smooth transition (from beam to slab). The form should be erected on the sides of the finished slab, place the gravel, rebars, then pour the concrete to form a slab with turn-downs (thicker edge).
    – r13
    Commented Mar 24, 2021 at 4:19
  • Correction: After excavation, place and compact the gravel backfill, then forming the edge. BTW, is this outdoor construction? If so, watch out local frost penetration depth. In such case, it is advisable to extend the turndown (edge beam) to just below the maximum frost depth. Also, thicker layer of well graded gravel backfill is required to prevent frost heave. I suggest to seek advice from a geotechnical engineer for the best practice.
    – r13
    Commented Mar 24, 2021 at 5:11

1 Answer 1


The problem with just pouring a flat slab is the rock can migrate out if above grade. If this pad base will be at grade or below so the rock can not migrate it will be fine to leave it flat. If you are adding the rock on top of grade make it a monolith pour so the outside edge goes down to grade this also creates the footing I usually add.

If I think a Gazebo or a roof may be added I will create a small footing trench (an additional 4x4” deep/wide ) so the pad edge could support a small structure on the perimeter.

As a patio you are going well beyond what would be needed at 5” thick reenforced but I would still add a thicker perimeter edge just in case.

You don’t have to make the pad wider to do this just don’t fill the gravel all the way to the form to make the footing.

  • My plan was to dig down 4 inches below grade, add gravel, and compact it. After that place the form on top of the gravel. A hot tub is going on it. That is the reason for the thickness. I am planning to extend a nearby deck around it. Commented Mar 24, 2021 at 14:45
  • Brian I have installed a few hot tubs with a good base (maybe a little less) as your compacted gravel I have put 3” fiberglass reenforced pads with the trench to support a 14’ gazebo with a sliding roof it held up fine. Others not quite as large I used the same form material a simple 2x4, topsoil removed, rock compacted form set slightly below grade to help prevent leaks ends up being 3” plenty strong for a hot tub the load is evenly distributed over a large area.
    – Ed Beal
    Commented Mar 24, 2021 at 14:57
  • The manufacturer recommended a 5 inch slab. I shouldn't need a trench if I am not adding a structure to it do I? Commented Mar 24, 2021 at 15:11
  • No you don’t but I have seen folks decide to add later and crack there pad. Believe it or not even metal roof structures have a higher point load than most hot tubs. An 8X8 hot tub at 500 gallons is only 63 lbs per square foot where a 2” leg of a metal roof support may only have 6-8 square inches of support for well over 100 and up to 200 lbs and is set on the edge. I put them in because with small pads it is a short load anyway and with hand pours the extra perimeter thickness is insurance.
    – Ed Beal
    Commented Mar 24, 2021 at 15:42
  • 2
    I second @Ed Beal's suggestion to add a perimeter trench around the slab. Note it adds stiffness to the slab, and most importantly, it restricts the migration of the gravel outward due to losing soil around the perimeter. The width of the trench shall be no less than the thickness of the gravel. I usually suggest 6" as the minimum.
    – r13
    Commented Aug 21, 2021 at 16:26

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