Install a hot tub after building a secure floor support system.
A screened porch is a good place for a hot tub for several reasons. The screening offers protection from flying insects and significant privacy. As you develop a plan to install a hot tub, focus on creating structural soundness of porch flooring. A hot tub full of water and people weighs anywhere from 4,500 pounds to 10,000 pounds or more. It's better to overcompensate with support posts, for example, than risk having the porch collapse.
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Adequate Floor Space
A hot tub usually requires a footprint that is roughly 8 feet square. Many are 12 or 13 feet long (see Resources). A screen porch that is 15 feet wide by 18 feet long will accommodate many tub sizes; however, consider space needed to remove the hot tub cover and room needed to walk around the tub to get in it. It's practical to have at least 2 feet of walking room on all four sides of the hot tub, if possible.
Generous Ceiling Height
A taller ceiling accommodates hot tub moisture and noise. A vaulted ceiling or 9-foot ceiling in a screen room allows moisture to rise out of the way. You can install a hot tub under a 7-foot ceiling, for example, but the steam may annoy anyone sitting at a table or chair nearby. Don't forget that steam can penetrate the seams of a low ceiling, sending moisture into porch rafter space. A higher ceiling allows room for the noise of hot tub jets to dissipate, too.
Strong Support Flooring
Decking and its framework must be designed to support the screen room and the hot tub. Use boards at least 2-by-6-inches for the floor decking. Floor joists under the floorboards should measure at least 2-by-10-inches or 2-by-12-inches (see References). Vertical support posts under the hot tub space, bracing up the weight of the tub, should rest on concrete pads at ground level. Envision four wooden vertical posts that are at least 4 inches square placed like table legs beneath the screen room decking. Each vertical support post will sit on its own individual concrete footing, which is approximately 8 inches deep and 6-inch square at ground level. The porch foundation, made of concrete blocks or concrete columns, is totally independent of vertical hot tub support posts.
Added Privacy Measures
Extra screening materials to block the neighbors' view are often a good idea. If your screen porch is close to neighbors, it helps to install wooden louvered panels or lattice sections to increase privacy around the hot tub. After dark, screen material by itself is often not enough of a privacy barrier. Lights inside the porch will show up bathers even more. You want family and guests to move from the tub to other porch areas without worrying about onlookers.
Compatibility with House Siding
Talk with experts before assuming your porch is ready for a hot tub. Your porch might work in terms of structural strength and other issues. However, you need to examine issues such as steam rising into nearby house materials. Moisture from the tub might find its way into siding seams and house insulation over time. Placing the tub on the outer edge of the porch might work better than installing it right next to house siding.