Composite Decking That’s Strong Enough for a Hot Tub

What could be a better combination than a hot tub out on your deck to enjoy the great outdoors? Composite decking with its low maintenance requirement allowing you to enjoy more time in the hot tub than working on the deck around the hot tub. Plus, composite decking is better with water than wood decking, allowing you to enjoy the hot tub without worrying about getting water on the deck.

But composite decking is made out of plastic. Is composite decking strong enough to hold a hot tub? Will the weight of the hot tub full of water crush the decking under it? Creating a disaster with the composite decking collapsing under the weight of the hot tub.

A hot tub or spa full of water can exert as much as 112 lbs per square foot of decking. A fraction of the 500lbs or more of concentrated load most composite decking can support. Making composite decking strong enough for under and around a hot tub.

How much compression or weight composite decking can support varies by brand. Just as the actual weight of a hot tub depends on the size and brand.

Meaning it not as easy as to say that you can put a hot tub on the deck. The impact of the weight of the hot tub on the decking will vary by brand.  Some composite decking can support twice the weight of other composite decking.

More importantly, whether or not the composite decking can support the hot tub’s weight is the question can the deck’s substructure support the load.

An overloaded deck can collapse under the weight of a hot tub. So, even if the decking is strong enough, the joists and footings supporting the decking need to be strong enough to hold the hot tub. Composite decking is only as strong as the joists below, supporting it.

So, to answer, is a composite deck strong enough for a hot tub. You need to answer a few questions first.

1. How much does a hot tub weigh?
2. Will Composite Decking Break Under a Hot Tub?
3. Will a Hot Tub Indent Composite Decking?
4. Can the Deck’s substructure support a hot Tub?

How much does a hot tub weigh?

To tell if the composite decking is strong enough for a hot tub, we will need to know how much the hot tub weighs. Full of water and people. Then we can figure out how much pressure the decking under the hot tub will be under. Telling us if the composite decking will break or be indented by the hot tub.

I am not a hot tub expert, I build decks, but Jeff Campbell from HQ Hot Tub Owner has done a lot of research and has owned multiple hot tubs. So, I will borrow information from him, but feel free to check out his site for more information about hot tubs.

But here, we will focus on decks and decking. 

Jeff created this great chart showing the average weight of different hot tub size. Of course, each model will vary, so you will need to check your specific model for exact numbers, but this gives us a good idea when thinking about composite decking under a hot tub.

As we see, hot tubs are heavy. The bigger the tub, the heavier they get.

But that only tells us half the information for setting a hot tub on composite decking. We need to figure out how much weight per square foot. To know how strong the decking needs to be under the hot tub.

For that, we turn to Blue Lagoon Spas. Not that you need to buy a hot tub from them, but they give us some good averages of hot tubs footprint to calculate how the weight is distributed. Combine their information and Hot Tub Owner HQ.

The bigger the hot tub, the bigger the range. This makes sense. The bigger the hot tub, the more options you have in design and layout. Going from a hot tub of water to a retreat for friends in your backyard. But you probably already knew that.

For us, the number 112 lbs per sqft is most important. Yes, we can use the lower weights for a smaller tub, but I think it is better to be safe than sorry. Better to have decking too strong and not be damaged by the hot tub than breaking under the tub. Besides, all the top weight for tubs is very close.

As you compare composite decking, keep the 112 lbs per square foot in mind. An easy refers to tell you if the decking is strong enough for a hot tub.

Will Composite Decking Break Under a Hot Tub?

The worse thing that can happen is the composite decking breaking under the hot tub. The tub, water and people falling through the deck to the ground.

The good news is the composite decking is multiple times stronger than is required to support the hot tub. The weight of the hot tub will not break the composite decking when the weight is evenly distributed.

The breaking point of composite decking varies. Some can support ten times the weight of a hot tub, others only five. But the good news is they can all support the weight of a hot tub.

Here are a few to see the range between composite decking. Helping you pick the best one for under the hot tub.

But I bet, in the end, it will be the appearance of the decking which will guide your choice. There is some good looking composite decking out there. Anyways back to composite decking strength.

Composite Decking	Kn rating	Lbs per sqft
Armadillo decking	2.35	538
Azek PVC	2.59	582
MoistureShield	3.02	679
Trex Transcend	3.88	872
DecKorators® Vault, Voyage, Frontier, SLS and Ultra	4.78	1075
Treated Pine	31.7	7 130 lb*
Cedar	20.28	4 560 lb*

These are tested for concentrated load at 50°c. A scorching day, one that I doubt you will be in the hot tub in, but you’re not going to move the hot tub off the deck just because it’s hot outside.

Plus, composite decking increases in strength as it cools. On cooler days, the decking will be even stronger. Supporting the hot tub even better.

Also, keep in mind this is a failure weight. How heavy the tub can be before destroying the decking.

This does not mean that it will not compress the decking at a lighter weight.

The decking may not break under the hot tub, but it still could indent the decking leaving a mark. The stronger the decking the less indent and risk.

The best composite decking on our list for concentrated load is Deckorators. Both their mineral and wood base decking have a rating of over 1075 lbs per square foot. That is almost 10 times the weight of a hot tub on the decking.

Giving plenty of room for error. Reducing the chances that the decking will break under the hot tub. Such a high concentrated load rating also minimizes the possibility of indenting the decking and leaving a mark on the decking.

I was very surprised to see the crushing strength of both Cedar and Treated Pine decking. Slightly different tests, as one is decking failure under load, the other is crushing of the wood fibres. But both are considerably stronger than composite decking, supporting the weight of a hot tub. There are other advantages of composite decking under and around a hot tub, but wood decking has a superior compression strength.

Not sure what to think of it, not on their CCMC report, but Fortress decking has an even lower live load and concentrated load strength. Fortress decking is rated for 100 lbs/sqft of live load and 300 lbs/sqft concentrated load. Which is low compared to other composite decking.

This is a weight rating, not a failure rate. Meaning, the decking can support a hot tub of up to 300 lbs per square foot. Where the other ones are a comparison of what weight will break the composite decking. Fortress may be able to be loaded many times this weight before breaking, but they are only recommending 300 lbs or less. Enough for a hot tub but less than the other composite decking.

Fortress also qualifies its strength according to the spacing of the joists supporting the decking. As we will soon see, this is a much bigger limiter of your deck than the composite decking.  

Will a Hot Tub Indent Composite Decking?

One thing to consider when placing your hot tub on your deck is will the weight of the tub leave a mark?

To answer this question, we turn to the Janka Hardness test results for composite decking.

What is the Janka Hardness Test, and why is it relevant for placing a hot tub on a composite deck?

Janka Harness test is a simple test in which a small steel ball 0.444″ (11.28 mm) indiameter, is pushed into the decking. To determine how much force is required to indent the decking. The higher the number, the heavier the hot tub can be before indenting the decking.  

To determine if the decking can hold the tub, the Concentrated Load number is better. But the hardness test can tell us if the hot tub will leave a mark, indenting the composite decking.

Composite Decking	Kn rating	Lbs per sqft
Azek PVC	2.93?	659 ?
DecKorators® Vault, Voyage, Frontier, SLS and Ultra	3.1	697
Trex Transcend	3.8	854
Fiberon, Horizon, Horizon® Symmetry™, Sanctuary, and Protect Advantage	8.0	1798
Armadillo decking	8.1	1821
Treated Pine	3.06	690

Armadillo’s hardness rating is simply outstanding, at 1 821 lbs per square foot. A multiple many times greater than the pressure a hot tub full of water will exert on the decking.

Trex, which generally comes in as a very durable decking board, comes in at 854 lbs per square foot. Considerably less than Armadillo decking, but looking down to Treated Pine, the go-to decking, and we see that Trex decking is less likely to be indented from a hot tub than pine. That’s a positive sign.

In fact, all the composite decking except Azek PVC boards are less likely to be indented than pine. Making composite decking a good choice for under a hot tub.

Of course, don’t take my word for it, always check your specific decking technical information before setting your hot tub on your deck.

Can the Deck’s substructure support a hot Tub?

As we have seen, composite decking is amply strong enough for under a hot tub. Vary in strength, but all brands we looked at can support the weight of a hot tub.

But the deck’s substructure. The joists, beams and footings under the decking may not be.

Most decks are built to the International Residential Deck Building Code standard, which gives us a benchmark to compare to. The International Building Code prescribes that a deck must be able to support 40 lbs live weight and 10 lbs dead weight per square foot . Dead weight is the deck’s actual weight and doesn’t directly impact putting a hot tub on the deck. Live weight does. Live weight is how much the deck can hold. How much furniture and people can be placed on the deck. For us, how heavy of hot tub.

And here we have a problem. It does not matter what sizes of hot tub you have. All hot tubs are too heavy for the deck without additional support. Even the lightest hot tub at 85 lbs per square foot is too heavy for the deck.

Regardless, what type of decking you have, composite or wood. To support a hot tub on your deck, your deck’s substructure will need to be reinforced.

Without involving an engineer, which you probably should before destroying your deck with a 3000 lbs tub of water. We can do some quick math of how much the deck needs to be reinforced to support a hot tub.

If a deck is designed for 40 lbs, then bare minimum, we need to reinforce the deck three times. For the deck can support 120 lbs per square foot. Enough to hold the hot tub, water and the people in it.

There are several ways to reinforce a deck for a hot tub.

But ultimately, the additional weight of the hot tub will be transferred to the deck’s footings.

So, we need to start there. Increasing the deck’s footings to support the hot tub, then working our way up. Reinforcing the beam and joists to support the hot tub.

Additional Deck Footings Need to Support a Hot Tub

The assumed bearing capacity of clay is 1500 pounds per square foot. Your yard may differ, so check your local soil bearing capacity or get that engineer involved. But for now, let’s work off this number.

In Calgary, where I build decks, we use 10″ (254mm) concrete columns for deck footings, with a 0.5 sqft base. Every footing can support 750 lbs. Divide the hot tub’s weight by 750, and we know how many additional footings are required.

Placing these footing directly below the hot tub will reduce the deflection of the joists. Minimizing how much the deck will sag under the tub.

Reinforcing a Deck’s Beam, for Under a Hot Tub

To support the weight of the hot tub, the deck needs to be three times stronger. Practically speaking, you can not triple up a beam. If the deck currently has a two-ply beam, most are three. The beam will need to be increased to 6 plies. At a 1 ½” (38mm) per ply, the beam would be 9″ (228mm) wide, almost as wide as the footing. Wider than the 6×6 post supporting the beam. 

You could install the beam directly on the concrete footing, but the saddle for holding the beam will still be too small.

You are better off reducing the beam span between footings. Which, fortunately, you need to add additional footings to support the hot tub anyways. It is just a matter of strategically placing them to reduce the beam span by a third. Or a combination of additional beam ply’s while decreasing the span between the beam.

Increasing Deck Joists to Support a Hot Tub On Composite Decking

Like the beam, the best way to increase the deck’s strength to support the hot tub will be to increase the number of joists while decreasing the span.

Composite decking can span 16″ (400mm), so you can reduce this to 8″ (200mm) or sistering two joists together every 16″ (400mm) will double the strength of the joists. Increasing the joist’s strength by about a third.

Double joists fasten together will increase the deck’s strength more than halving the spacing. Joist membrane should be used to prevent water from being trap between the joists, causing rot.

Third, span joist’s blocking will further increase strength and reduce joist deflection under the hot tub.

But the hot tub needs the deck to be able to support three times the standard weight.

Therefore, the span of the joist should also be reduced. Reducing the span in half, should roughly double the amount of weight the joist can support. Compound this with the increased strength of doubling the number of joists and blocking should make the deck strong enough to support the hot tub.

But before filling your hot tub, have an expert check your deck design and specific tub weight.

Low Deck Alternative to placing a hot tub on the deck.

The extra engineering and construction to support a hot tub on a deck much more common is building the deck around the hot tub. Which for a low deck less than 2′ (0.6m) off the ground is a practical solution and increases access convenience. Not requiring stairs to climb into the hot tub.

This is my recommended solution for a hot tub and composite deck. Yes, the composite decking is strong enough to support the weight of the hot tub. But the hot tub can be better supported directly on the ground with the low maintenance, water-resistant decking surrounding it.

It will take a little more work and planning to build the deck around the hot tub. But the reduced risk is well worth it.

You will need to provide access panels to service the hot tub below the deck level. This can be done by designing decking boards that can be raised. Or moveable composite steps by the hot tub that can be moved to access the hot tub.