When you add a spa or hot tub to a deck you are adding a lot of weight. This "How to Guide" is to show one possibility of reinforcing your deck so that it can properly support the added weight from your spa or hot tub. Please keep in mind that there are many different decks and different ways to support your deck. This is one way that my engineer provided to reinforce my deck for my spa. Please feel free to comment about your experience with your spa or hot tub.
The first step is to talk to an engineer. After adding a spa, plus water, plus people in the hot tub you can add around 6000 lbs to your deck. My spa started off at 600 lbs. The spa filled has 320 gallons or 2672 lbs. (8.35 lbs / gallon). If 7 people sit in my hot tub that is an additional 1050 lbs. (average 150 lbs). That is a total of 4,322 lbs. My engineer informed me that my current deck needed more support even before adding a spa or hot tub. I also hired carpenters to come out and make sure the job was done correctly.
My engineer recommended to add an additional joist to each of the existing joist on the deck where the spa would be sitting. He also recommended to add additional Ledger board fasteners to the ledger. Our first step was to knock out the nailed in blocks so we could add the additional beams to the deck.
We then added ledger board fasteners to the existing deck ledger. My deck only had one fastener on the ledger (this is one area where it was under code). We added an additional 4 fasteners to each area between the deck joists. Each fastener was 3 - 5/8" long. This was to help support the weight for the new spa.