How to work out if a deck can support a hot tub

Hot tubs and decking go hand in hand. But you need to make sure your decking can support the spa you’ve got your eye on before you install it there, or things could end very badly. 

Here’s everything you need to know to be able to work out if your deck can support the hot tub you’re thinking of buying.

Can a hot tub go on decking?

You can install a hot tub on a deck as long as it ticks a few boxes:

• It needs to be big enough to fit the hot tub you’ve got your eye on (you can use the typical hot tub dimensions to ballpark that if you haven’t decided on a particular model yet).
• It needs to be perfectly flat, or the water on your hot tub won’t be level.

• It needs to be anti-slip to prevent any nasty accidents.
• It needs to be near a water supply, an electric supply, and drainage.

If your decking fits this bill, it’s time to work out if it can support the hot tub you’re after. First, you’ll need to figure out how much that spa weighs.

How to work out how much your hot tub weighs 

To work out how much a hot tub weighs, simply grab the manufacturer’s handbook (or ask the dealership to check for you). This will include the spa’s dry weight, which is how much it weighs without any water in.

A hot tub’s manual will also include its water capacity. Water weighs almost exactly a kilogram per litre, so add its max water capacity to your running total in kilos.

Then you’ll need to work out the total weight of the maximum number of people who can fit in your hot tub. The average British man weighs 13.16 stone (83.6kg) and the average woman 11 stone (70.2kg) if you need to make a rough estimate. It’s important to make this calculation based on the maximum number of people who could fit in your spa, even if you’re not planning on ever having that many people in at once (you never know!).

Add all those numbers together and you’ve got a spa’s weight. Let’s look at a six-person hot tub as an example:

• Its dry weight is 370kg
• Its water capacity is 1,400 litres (or 1,400kg in weight)
• Six adults weigh an average of 462kg

Which means when this hot tub is full of water and people it will weigh 2,232kg in total.

How to work out whether your deck can support your hot tub

You can expect a small hot tub to weigh about 1,400kg all in and the very largest spas to come in at around 3,000kg in total. If the decking you sit your spa on can’t support this kind of weight it will eventually collapse, damaging your spa and potentially injuring you and your nearest and dearest. 

So it’s crucial you know that your decking can definitely support the spa you’ve got your eye on before you think about having it installed there.

As a general rule of thumb, your decking will be able to support 45kg per square foot. However, there are lots of factors that influence how much weight your decking can actually hold, including:

• How high your deck is off the ground (the higher it is off the ground, the less weight it can support)
• What material your decking is made from (since some are sturdier than others)
• The size of your decking’s joists and beams (they need to be at least 4” x 2” if they’ll be supporting a spa)
• Your deck’s tributary areas (the cross-members supporting your deck should be spaced no more than 18” apart to distribute the weight evenly)
• Whether your deck is free-standing or attached to your house
• How sturdy the foundation your deck has been built on is

And remember, your decking doesn’t just need to withstand the weight of your full hot tub. It also needs to be able to support any outdoor furniture or other people that will also be on the deck at the same time.

How to work out whether an existing deck can support a hot tub

Working out whether your existing decking can support a spa you’ve got your eye on is difficult – especially if your deck is too low for you to get a proper look at its supports.

A standard deck should be able to support 45kg per square foot, but that’s not a guarantee – especially if yours is a few years old, as its components might have degraded and not be able to withstand as much weight. 

We therefore strongly recommend you hire a contractor to take a look at your decking to work out its maximum load capacity. A professional will also be able to tell you whether your decking will need any support beams adding where you’re planning on sitting your hot tub (where your deck will need more support).

How to build a new deck that can support a hot tub

We also strongly recommend you hire a contractor to build a deck you’re planning on using as a base for a hot tub rather than going the DIY route. There’s a lot at stake when it comes to putting a hot tub on decking, and accommodating for the extra weight isn’t as simple as just adding more support beams (this could actually end with your deck sinking into the ground if you get things wrong).

So, ask your local dealership if they have any recommendations for a contractor you can work with to plan the decking for your spa to sit on. This will guarantee your decking will support your hot tub with plenty of margin for error. 

The final word

Follow the advice we’ve laid out in this guide – and the guidance given to you by a professional contractor – to make sure your hot tub isn’t too heavy for your decking.

And be sure to pick up your free copy of WhatSpa? magazine for help picking the right hot tub for you and for the latest hot tub reviews – whether you choose to place yours on decking or not.

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