The last thing you want is to lose sleep at night worrying whether your decking is properly able to hold your new hot tub without causing damage and injury. So what determines a deck’s capacity for supporting a spa?
Decks built to code are able to support a minimum of 50 pounds per square foot. Most decks are able to withstand 100 lbs per square foot, which is enough for inflatable hot tubs and most hard-shell spas. The age, condition, height, and size of the joists, beams, and support posts all determine a deck’s load-bearing capacity.
In this article, I’m going to show you:
- How to assess your deck’s load-bearing capacity
- How to calculate the weight of a hot tub
- What other factors must be taken into consideration
How do I know if my deck can support a hot tub?
Unfortunately, there’s no simple yes or no answer to whether your deck can support a hot tub. Decks come in all shapes and sizes, which is why you need to learn how to assess your own.
A deck’s load-bearing capacity can quickly become confusing, so I’m going to make it as clear and simple as possible for you.
If you already have an existing deck, then it most likely wasn’t built with the intention of holding a hot tub. Most decks are designed to hold people and furniture, not the weight of a hot tub full of water and people.
If your deck was built to code, abiding by structural requirements, then it’ll be able to support a minimum of 50 pounds per square foot. That is the default standard for deck building and the most common.
However, in reality, most decks can withstand 100 pounds per square foot. To properly estimate a deck’s capacity to hold a hot tub, it’s important to understand its design load.
The design load is the total load value that the decking is intended to support. The design load is comprised of 2 measurements: the dead load and the live load.
The dead load is the weight of the actual structure itself (the deck), and the live load is the weight of everything that sits on the deck’s surface.
Of the load-bearing capacity, ⅘ of the total is the live load. For example, decking that is rated 50 lbs per square foot has a 10 lbs per square foot dead load and a 40 lbs per square foot live load.
To determine how much weight your deck can support, you need the following information:
- Total area in square feet (sqft)
- Design load in pounds per square feet (psf)
Begin by calculating the total area of your deck and multiply by the design load. Here are 2 examples:
- A 100 sqft deck with a 50 psf is designed to support 5,000 lbs (100 × 50)
- A 100 sqft deck with a 100 psf is designed to support 10,000 lbs (100 × 100)
Although the total load-bearing capacity might equate to more than the weight of a full hot tub, you need to remember that the weight of a hot tub is concentrated in one area, meaning extra reinforcement may be required.
How much does a hot tub weigh?
Purchasing a hot tub is a large investment of your hard-earned money, which is why it’s best to choose the right one before you make any structural changes to your deck
I’m going to show you how to calculate the weight of a hot tub. To determine the weight, we first need to know the following information:
- Weight of hot tub when empty = depends on model
- Weight of 1 gallon of water = 8.34 lbs
- Weight of average person = 185 lbs
- Square footage of the hot tub = depends on model
To make things clearer, let’s say you’ve just purchased the 4-person Master Spas LSX 700, which is an average-sized hot tub.
- Weight of hot tub when empty: 1,095 lbs
- Weight of water: 2,544 lbs (8.34 lbs × 305 gallons)
- Weight of 4 people: 740 lbs (4 × 185)
- Square footage of the hot tub: 49 (7′ × 7′)
Now it’s time to determine the weight per square foot, which is the following calculation:
Weight of empty hot tub + weight of water + weight of people ÷ square feet = pounds per square foot
(1095 + 2544 + 740) divided by 49 = 89.4 pounds per square foot
When full of water and people, the total weight of the Master Spas LSX 700 is 4,379 lbs.
Divide that figure by the spa’s footprint of 49 sqft, and we can see that our decking needs to be able to support just shy of 90 pounds per square foot to safely hold the hot tub.
Other factors to consider
- Age and condition of the deck
- Type of wood
- Space between the joists
- Size of the joists, beams, and support posts
- Foundation and soil bearing capacity
- How far off the ground
- Additional loads such as snow
For obvious reasons, the age and condition of your deck are some of the most important factors to consider.
The wood on older decking might have become weathered, containing mildew and mold. You must ensure that all footings, joists, and decking are structurally sound, with no signs of rot.
Unless your decking was built recently, it may no longer comply with modern construction guidelines.
Everything from the species of wood to the joist hangers, lag screws, and the size and location of the main beams influence structural integrity.
Support posts need to be a certain distance from one another in order to safely take the weight of a full hot tub.
Ideally, the support posts should be set in poured concrete and run at least a foot below the frost line.
Knowing the bearing capacity of the soil is highly important as it determines how much support is provided for each footing.
It doesn’t make a difference how far off the ground the decking is, whether it’s a couple of feet or a couple of inches, you may still need reinforcement.
If you live in an area with harsh winters, you must also factor in the weight of any snow that falls and stays sat on your decking as it can add a tremendous amount of additional weight.
Consult an expert
In order to determine whether your deck is able to safely support a hot tub, you need to be able to understand the actual physics of the decking.
Working backward like this trying to assess whether or not it can withstand the weight is a massive headache unless you are a structural engineer with specialized data and tables.
Save time by having qualified engineers contact you with the best prices in your area simply by inputting a few details on BuyerZone.
Expert engineers can assess the deck frame and beam design, as well as the load-bearing ability of the soil in the region where you live.
They’ll also use a device that is kind of like a needle that depresses the soil to determine its load-bearing capacity.
An engineer will know right away if your deck meets the minimum threshold, so you can sleep sound at night without worrying about causing damage and injury.