How to Add a Hot Tub to a Sundeck

Although they may increase property value, the main reason to build a deck is leisure; sundecks are about creating a space for family and friends to make the most of leisure time in the great outdoors. And one of the best ways to maximise this leisure potential is to include a hot tub in your deck plans. In my opinion, there is no experience so grand as the opportunity to soak in a pool of warm water while staring up at the night sky, sipping a glass of wine, enjoying good company.

There are practical considerations about decks with hot tubs that are not quite as exciting as using the tub itself, but need to be considered, nevertheless. The first question I ask homeowners is the height they envision of the tub’s top relative to the deck’s level. I prefer the tub to appear sunken in the deck partly because they simply look slicker sunken. This means the tub’s top will be mere inches above deck level. Installed this way, the hot tub may not be as easy to enter and exit for some as the user will need to be at deck level to get in and out. Alternatively, if the tub’s base is at deck level, one will need to climb in and out, which is not as safe in my opinion in the innately slippery environment hot tubs present. This point of consideration is up to users, but is certainly well worth thinking about at the design stage. Fortunately, most modern hot tubs can be positioned so their bases are either on or below deck level. If sunken, the builder needs to ensure under-deck access to the tub’s wiring port.

One of the most important elements of hot tub installation is how its weight will be brought to ground. Naturally, if done improperly, results from inconvenience to calamity are possible. We would never, for example, place a tub on anything but a concrete foundation. Under no circumstances should a tub be placed on deck joists, then filled and used. Only the most skookum of wood-frame supports would suffice as a hot tub base, and most deck joist systems are not designed to support much more than people and BBQs. Further – and this is an important consideration in colder climes – the foundation should not be subject to frost heave: a situation in which water beneath a foundation can freeze, expand, then distort the structure it supports.

Ensuring good support or “bearing” for the tub is a relatively simple calculation. First, the weight involved is, in engineering terms, considered a live load, and being less easily predicted, should be considered all the more carefully. Unlike a roof load that will remain relatively constant, and can be designed to allow for snow, for example, a hot tub load will change depending on the volume of the tub and number of users it allows.

Let’s take a moment with a pencil. The tub weighs, say, 300 pounds (136 kg) out of the factory. Let’s then assume this smaller of tubs will hold 500 gallons of water at 8 pounds (3.62 kg) per gallon. That’s 4000 pounds or approximately 2000 kilos in water weight alone. Next we’ll add the load of the number of users. We consider that each user weighs 250 pounds (113 kg). Four hot-tubbers will then add an additional 1000 pounds (453 kg) to the load. Adding it up, when full of water and people, the weight load on this tub’s foundation will be around 5,300 pounds or approximately 2500 kilos. I mentioned “live” loads. Four people – or perhaps eight good friends – frolicking in the tub and shifting that water around is roughly equivalent to having a juvenile elephant dancing on your deck. Clearly, with this Dumbo-like weight considered, it is crucial to determine how your hot tub will be supported to ensure the overall load gets to ground safely.

As a general guideline on your tub’s foundation, determine that the base of the tub will be at least eight inches (20 cm) above ground level to protect its components from decay, and that the foundation base itself will be below your area’s frost line. Where we build in Southern British Columbia, Canada, the frost line ranges between 18 and 24 inches (46 – 60 cm). Further, we typically reinforce the concrete foundation using steel rods as we would with almost any concrete foundation, this on the idea of building it right once. In my opinion, a minimal concrete foundation for a hot tub will be no less than 8 inches (20 cm) of reinforced concrete placed on a compacted substrate as a thickened-edge slab, but we supersede this to minimise the frost heave and decay risk our local climate presents.

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A last important consideration for hot tub installation is electrical. As with any heater system, getting your tub to a comfortable temperature will put quite an electrical demand on your power system. And as is the case with any electrical installation, it is better to be safe than sorry. As we all know, electricity and water together can be dangerous, so when hooking up your electrics, be sure to take no chances with the well-being of you and your friends and family – consult and expert. It is crucial that your tub is properly grounded and that safeguards such as ground-fault interruption are in place.

Now that your tub is safely nestled in your deck, here’s a finishing tip you might find useful. Because tubs come in a variety of shapes, it can be difficult to create a nice finish at the junction of deck and tub. We often use a hefty hemp rope for this purpose as it is flexible, looks good, and can be made to conform to the outline of your tub, however irregular its shape might be. This method makes any scribing and cutting needed much less critical. Simply shape the rope to the tub, and fasten it to the deck surface (not the tub) using a minimum of clear, flexible caulking dollops every few inches or so. This rope “moulding” will need to be replaced every few years, so don’t use to much adhesive.

Article © K. Hunter/Hunter Construction – Reprint/Copy by Written Consent

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