Having a proper base for your hot tub to sit on is essential for the long-term durability of your hot tub. A proper base needs three things: strength, levelness, and flatness.
Strength
A base for your hot tub must be strong enough to support the weight of the hot tub, the water that is in the hot tub, and the users of the hot tub. This can be upwards of 8,000 pounds. Your best base for a hot tub is a concrete slab that is at least 4” thick.
If you choose to put your spa on a deck, which can come with additional cost for delivery, you will want to make sure that a licensed engineer certifies that your deck can handle the weight of the spa. If your deck was not built to handle the weight of a hot tub this often means that additional supports, beams, and posts will have to be added to an existing deck.
A base of properly installed pavers can also be a good option to put your a hot tub on. Properly supporting pavers for a hot tub requires you to put down a compacted base of crushed rock. For the needed thickness of the gravel base please consult with your material supplier. On top of the layer of gravel, you need to put down a compacted and leveled base of masonry sand. Your pavers are then put on top of this sand.
Levelness
Your base that a hot tub sits on is often sloped to allow for drainage. For concrete and pavers this is needed so that water drains off the slab and away from your house. But too much of a slope can cause a problem with a hot tub. Jets on the high end of your hot tub end up above the waterline and spray water or if the filter is on the high end, it sucks air into the pumps. For this reason, you want the slope from one side of the hot tub to the other not to be more than 1”. If the pad you want to put your hot tub on has a slope of more than 1” over the of the length of the hot tub, consult with your local hot tub professional for how to level the space for your hot tub. Different hot tub manufacturers have different recommendations for the best way to do this for their hot tubs.
Flatness
In addition to being strong enough to support the tub and level enough for the hot tub to function properly, your hot tub base must also be flat enough to protect the hot tub from damage due to sagging. The base must not have any dips or low spots. If there are any low spots, this will cause your hot tub to flex and sag in this area which can cause the shell to crack. If this happens due to a base that was not fully supporting your spa, the hot tub manufacturer may not cover it under warranty.
If you have any questions if your pad is a good place to put a hot tub, consult with your local hot tub professional.