Things You'll Need
Gravel and concrete (optional)
Hot tubs are typically heavy: Their internal sidewalls must be thick enough to support the outward pressure of hundreds of gallons of water when full, and the frames that buttress the inside of the case have to hold the entire tub structure off the ground. When filled with water and two humans, the average 8-foot-by-8-foot hot tub weighs approximately 5,000 lbs. A deck built to support a hot tub must, therefore, be sturdy and well-braced.
Consult the manufacturer's documentation that came with the hot tub, or consult the supplier if you're going to build the deck before delivery, to learn the hot tub's maximum in-use weight. If the unit is used, and thus a true weight is not handily available, most hot tub manufacturers' websites give comprehensive specifications for each model they make. Home improvement warehouses usually have free plan sheets in their deck aisles giving very specific directions for the sizes of joists, deck boards and hardware required for hot tubs of various weights; obtain such a plan and use it to work up a shopping list of all the materials you will need. Alternatively, software packages are available that calculate all loads for the user and provide a specific list of requirements to support any given size of hot tub.
Call your local code enforcement office to confirm that your plans comply with any directions and load limits your municipality's regulations may mandate. Code enforcement officers are unlikely to inspect a private installation; but, when it comes time to sell the property, the prospective purchaser's home inspector may note an off-code installation.
Measure and mark the area you wish to give over to the deck, using a proprietary inverted surveyor's spray paint for the outside lines; these are available from most home improvement warehouses. The deck need only be the size of the hot tub itself, but it is advantageous to have an oversize deck that works as a landing or a toweling-off area.
Pound stakes into the ground at the corners of your marked area and draw strings tightly between each opposing stake to make a diagonal cross over the enclosed area. Where the strings meet is the center; mark the corners and mark the center.
Measure each string from the corner to the center, and make four more marks at the half-way point of each string. Remove the stakes and strings; nine marks should be left. These are the locations for your foundation piers.
Dig out 1-foot-deep holes sized to the bases of the foundation piers you are using at each mark; most foundation piers are slightly pyramidal in shape, the bases being somewhat larger than the tops. Put exactly the same amount of builder's sand in each hole and level it off; 2 inches of sand is usual.
Install the piers in the holes. The tops of piers have ridges cast into them to stop the joists they support from moving from side to side; ensure that the ridges are aligned in the direction you intend to install the joists that will support the deck. Use a laser level to ensure that the tops are all at exactly the same height above ground level. Tamp soil or builder's sand around the piers to lock them in position and prevent water from getting into the holes in the future.
Contract with a concrete supplier if the hot tub is being ground-mounted, and discuss pouring a concrete slab rather than installing foundation piers; for large projects such as this, having concrete delivered is almost always cheaper than buying bags of mix and making it yourself. Dig down below the sod if you're building the deck in an area that was previously lawn, getting down to undisturbed soil. Fill the excavation completely with gravel, from side to side and into every corner, to a depth recommended by your concrete supplier.
Cut to size and arrange the support joists, then lock them into place by fastening the end pieces -- called "rim joists" in industry speak -- into place; a circular saw is ideal for this part of the project, and for cutting the deck boards. Use all the reinforcing hardware recommended by the plans, securing the joists and rim joists firmly together. If you're using piers, the lattice of joists should be evenly supported by all nine piers; if you're using a poured slab, the joists should rest directly upon the cured concrete.
Measure and cut the deck boards to size, then install them crosswise above the joists. Fix them in place using proprietary deck fasteners; these are designed to resist weathering, corrosion and warping in the lumber.
Because of the massive, tightly focused weight loading created by a hot tub, regular decks are seldom strong enough to support one. If you are adding a hot tub to a regular deck, it is almost always necessary to remove a section of the pre-existing deck and then install a specially constructed new platform. If you do not have a laser level, ensure that the foundations are level by resting a perfectly straight length of lumber from each pier to each pier in turn, with a regular level on the length of lumber.
The piers may need to be larger and dug deeper into the ground if you're installing the deck in a region where the soil freezes in the winter.