We have all seen pictures and movies where people walk out from the back of their home and there on their raised deck is a hot tub or spa. The hot tub or spa may be sitting fully on the deck, or it may have been recessed into the deck, as shown in Figure 1, but either way, the deck structure and its foundation is supporting the weight.
Figure 1 - Hot tub on raised deck
Most raised decks are constructed to hold a family, maybe a few friends, a barbecue and some outdoor furniture. All of these items are pretty light weight things when compared to a hot tub or spa that is filled with water.
Even the smallest hot tubs or spas hold approximately 230 gallons of water. If you combine that with the weight of 2 people, plus the weight of the hot tub or spa and you will have over 2,000 lbs of weight in a relatively small footprint.
Larger spas and hot tubs filled with water and people moving around can easily exceed 10,000 lbs, and while the footprint may be larger, the weight is still transferred to the support posts or piers through the footings to the ground.
Can a raised deck that was not originally constructed to hold a hot tub or spa take the added weight? It is highly unlikely that the construction of a standard raised deck is adequate to hold a hot tub or spa.
Note: The supporting structure of the deck can most likely be modified to take the additional weight of the hot tub or spa. It is usually only necessary to add extra structural support to the area under the hot tub or spa, not the entire deck.
Note: Most jurisdictions will require a building permit for the addition of a hot tub or spa on a raised deck.
Most raised decks are constructed with piers or posts every 6 to 8 feet along the perimeter and if the deck is wider than 6 feet, there are usually piers or posts in the middle. A standard wood raised deck 10 feet deep by 20 feet long would have piers or posts in similar locations to those shown in Figure 2.
Place additional piers or posts, including foundations that are at a minimum 1 foot below the frost line at distances of less than 30 inches from the center of one post or pier to the center of the next post or pier below the foot print of the spa or hot tub to be added. The additional piers or posts would be located in similar locations to those shown in Figure 3.
Figure 3 - Typical raised deck pier positions for hot tub or spa
Note: It is important that all piers and posts have proper foundations that are at a minimum 1 foot below the frost line.
Note: If rim joists are connected to the house, as they would be in Figures 2 and 3 they should have additional lag bolts installed in line with the hot tub or spa footprint.
If the intent is to recess the hot tub or spa into the deck, you can construct a separate support system for the hot tub or spa with posts or piers laid out in a grid of no more than 30 inches from center to center where the hot tub or spa will be supported.
Note: An inspection should be made of all decking and joist lumber to ensure that all the wood is sound and there is no rot or other damage.
Warning: If the deck is at a height where it is possible for someone to actually crawl or stand under the hot tub or spa footprint that area should be fenced off. At no time should anyone be under the hot tub or spa when it is filled with water.
When considering placing a hot tub or spa on a raised deck consideration should be given to the availability of electrical power.
Most hot tub and spa manufacturers will be pleased to assist you with your deck design and provide information about specific weights and construction considerations.
Additional information on: