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How far apart should deck supports be for an outdoor spa hot tub?

2025-06-27 15:35

When installing an outdoor spa hot tub, deck design and construction are key. The weight of the spa hot tub plus the water and occupants places tremendous demands on the deck’s load-bearing capacity, and the spacing of support beams and joists directly impacts the deck’s load-bearing capacity. This article will explore the importance of deck support spacing and provide guidance on how to properly design a deck to support a spa hot tub.

outdoor spa hot tub

What are the weight requirements for deck supports versus spa hot tubs?

An outdoor spa hot tub can weigh thousands of pounds (kg), which includes not only the tub itself, but also the weight of the water and occupants. Given these weights, the deck’s support structure must be designed to be strong enough to ensure safe and long-lasting use.


1. Weight composition of an outdoor spa hot tub

Before deciding how far apart to space deck supports, it is necessary to understand the weight composition of a spa hot tub. Generally speaking, the weight of a spa hot tub is divided into the following parts:


● Empty weight of the spa hot tub: A typical 6-person outdoor spa hot tub weighs between 500 and 1,000 pounds (about 225 to 450 kilograms) when empty.

● Water weight: Each gallon of water weighs about 8.34 pounds (about 3.78 kilograms). A 300-gallon (about 1,136-liter) spa hot tub weighs about 2,502 pounds (about 1,135 kilograms) of water.

● User weight: The average adult weighs 150 pounds (about 68 kilograms). If the spa hot tub can accommodate 6 people, the total weight of the users is about 900 pounds (about 408 kilograms).


Taking all of the above into account, the total weight of a 6-person spa hot tub may be close to 4,202 pounds (about 1,905 kilograms). The deck must be designed to safely carry this weight, and the design of the support structure is key.


2. The Importance of Support

The support system of a deck includes joists, posts, and other structural components that work together to share and support the weight of the outdoor spa hot tub. The spacing of the joists, the spacing and number of posts, determine how much weight the deck can withstand. A properly designed support structure can prevent the deck from buckling or collapsing, and ensure the long-term stability of the deck.

spa hot tub

How far apart should the deck supports be for an outdoor spa hot tub?

The spacing of deck support beams is usually determined by the type of joists, the weight of the outdoor spa hot tub, and the design of the deck. Generally, the joists of a wood deck are spaced 12 inches (about 30 cm) to 24 inches (about 60 cm), but when carrying heavy objects, the joists may need to be spaced closer.


1. Joist Spacing for Standard Deck

For typical residential decks, excluding heavy items such as outdoor spa hot tubs, the standard spacing of joists is usually 16 inches (about 40 cm). This is the most common joist spacing because it provides adequate support under normal use and is relatively cost-effective. The larger the joist spacing, the fewer joists are required, the lower the cost, but the load-bearing capacity will also decrease.


However, when it comes to spa hot tubs, 16 inches of joist spacing may not provide enough load-bearing capacity. In this case, reducing the spacing of the joists or increasing the number of joists is an effective way to increase the load-bearing capacity.


2. Reduce the Joist Spacing

In order to increase the load-bearing capacity of a deck, it is generally recommended to reduce the spacing of the joists to 12 inches (about 30 cm). This dense joist arrangement can increase the rigidity and strength of the deck, ensuring that it can bear the weight of an outdoor spa hot tub without deforming or sinking.


Another benefit of reducing the joist spacing is that it can make the weight more evenly distributed, thereby reducing excessive local pressure on one area of the deck. Especially for heavy objects such as outdoor spa hot tubs, dense joist arrangements can help extend the life of the deck.


3. Increase the number of joists

If the deck area is large, in addition to reducing the distance between joists, you can also increase the support strength by increasing the number of joists. For example, if you need to bear a very heavy weight, you can double or even increase the number of joists based on the original design. Although this design is more expensive, it can significantly improve the load-bearing capacity and ensure the safety of the deck.

outdoor spa

What is the spacing of the pillars and the load-bearing capacity of the joists?

The pillars are another key part of the deck's load-bearing structure. The main function of the pillars is to transfer the weight borne by the joists to the foundation, so the number and spacing of the pillars directly affect the load-bearing capacity of the deck.


1. Spacing of pillars

The spacing of the pillars depends on the span of the joists and the design of the deck. Generally, the spacing of the pillars is about 6 feet (about 1.8 meters), that is, there is a pillar every 6 feet. This design can provide stable support for the deck.


However, for heavy objects such as outdoor spa hot tubs, the 6-foot spacing of the pillars may not be enough. To increase the load-bearing capacity of a deck, it is usually recommended to shorten the spacing of posts to 4 feet (about 1.2 meters) or increase the number of posts. In some extreme cases, the spacing of posts can be even smaller to ensure the stability of the structure.


2. Post material and thickness

The material and thickness of the posts are also important factors that affect the load-bearing capacity of a deck. Common post materials include pressure-treated wood, steel, and concrete posts. Pressure-treated wood is usually used for general residential decks, while steel and concrete posts are used for structures with higher load-bearing requirements, such as spa hot tub decks.


If using wooden posts, it is usually recommended to choose 6x6-inch (about 15x15 cm) wooden posts instead of 4x4-inch (about 10x10 cm) wooden posts. Thicker posts provide stronger support and better share the weight.

outdoor spa hot tub

How to calculate the total load-bearing capacity of a deck?

To ensure that the deck can safely support a spa hot tub, its total load-bearing capacity needs to be calculated based on the deck's supporting structure, joist spacing, and post spacing. Here are the steps to calculate the load-bearing capacity of a deck:


1. Determine the Area of the Deck

First, you need to determine the area of the deck. If the deck is 10x10 feet, then its total area is 100 square feet.


2. Calculate the Load-Bearing Capacity of the Joists

Calculate the maximum weight each joist can support, based on the material, spacing, and support methods of the joists. For example, a standard wood joist has a load-bearing capacity of 50 pounds per square foot. If the joists are spaced 12 inches apart, the load-bearing capacity may increase to 70 pounds per square foot.


3. Determine the Number of Posts and Load-Bearing Capacity

Calculate the weight each post can support, based on the number and spacing of the posts. Assuming the posts are spaced 4 feet apart and each post has a load-bearing capacity of 1,000 pounds, the total number of posts is 6, and the total load-bearing capacity is 6,000 pounds.


4. Verify that the deck can support the spa hot tub

Based on the calculations above, the total weight of the spa hot tub is likely to be close to 4,202 pounds (about 1,905 kg). By calculating the load-bearing capacity of the joists and pillars, ensure that their total load-bearing capacity is at least 1.5 times the total weight of the spa hot tub. This can provide an additional safety margin for the deck to avoid structural damage caused by long-term use.


What certifications do your spa products have?

All of our Lovia Spa products are rigorously certified to ensure international buyer confidence and compliance. We hold CE (EMC & LVD), ETL, and SAA safety certifications, along with environmental and material standards such as RoHS and REACH.

Our comprehensive ISO9001 quality management system further guarantees consistency and excellence. These certifications support bulk purchasing, wholesale opportunities, low price quotes, and discount sales across various regions, especially in Europe, North America, and Australia.

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