Deck Weight Support

[tri4sale]

Working with an existing deck, built with 2x10 lumber, trying to calculate the load it can support for a small blow up portable hot tub, around 180 gallons water (so about 1440 pounds of water).

Joists are 2x10's that are 12 inch on center. How do you calculate the weight limit for this?

 

[Wapitiwrangler]

Working with an existing deck, built with 2x10 lumber, trying to calculate the load it can support for a small blow up portable hot tub, around 180 gallons water (so about 1440 pounds of water).

Joists are 2x10's that are 12 inch on center. How do you calculate the weight limit for this?

What is the joist span? That will determine the live load. How many people + tub + water weight. Also how far above grade? Is deck just attached to house via lag bolts & ledger board. Check IRC for new deck mounting code requirements. Good article in fine Homebuilding last year.

 

[JimD]

I would probably look at span tables for 2x10s on 12 inch centers and see what kind of lbs per square feet they can support. You need to consider not just the joist size but also the spacing between beams supporting the joists. You need to calculate the load of the hot tub, with occupants in lbs/ft2 and compare. The weight seems significant but if it is spread over enough area, it might be OK but It seems to be pretty high. Normal house floors are designed for 40 lbs/ft2. But not everything is built to minimum. I wouldn't exceed recommended loadings but you may also want to know that the spans are based on deflection of 1 inch in 120 inches not failure of the joists.

 

[RickR]

Retired engineer here... I don't think it is possible to evaluate the load capability without a lot more information. Even then it would be speculative. I agree with JimD that it seems like a lot of weight, especially once occupants are added to the equation. Moreover, I'm guessing a 180 gal portable hot tub is about 4 ft or so interior diameter so all of the weight will be concentrated on 3-4 of the joists. Unless both ends of the joist are resting atop a girder beam, I'd be worried more about the connection of the joist to its supporting beam. It is usually unadvisable to locate large concentrated loads on ANY structure unless it was specifically designed for such. As an aside, I was a civil engineering undergrad at UCONN when the Hartford Civic Center (CT) collapsed so we were forced to pay attention. The truss members all had adequate strength for the load but the joints at the ends were inadequate and ultimately the point of failure.

 

[tri4sale]

Joists are 12 foot spans, and connected with joist hangers. Deck is free standing, it does not connect to the house. One side is a few inches above grade, and other side is probably around 18 inches above grade (from ground to bottom of joist). Support posts are about 6 feet apart (4x4 posts).

 

[JimD]

That is enough to say that the existing structure is overbuilt which is fortunate. According to the table I just referenced 2x10s of southern yellow pine can span just over 16 feet on a deck when they are 12 inches apart.

To check deflection I think you need an actual calculation which would require a free body diagram (basically a sketch with parts labeled and dimensioned) and some time with the deflection calculation and a calculator. That is not terribly difficult and the equations are available over the internet but if you have not done it before it would be best to have a structural engineer look at it. If you bring them the diagram (good labeled and dimensioned sketch) and some pictures so all they have to do is sit down and calculate it should not be expensive. If you need a permit for the work you will need their stamp anyway.

A simpler thing to do that is not wholly without risk is to assume it needs beefed up a bit and put a beam or two spanning the area of the hot tub under the deck supported by a concrete foundation and posts. It sounds like you are pretty close to the ground which helps in some ways but may mean you'd have to pull off some deck boards for access. If you get the calculations done it will tell you were and how big the beam has to be to give you the extra strength.

I have never been responsible for structural calculations but I am also a retired engineer (mechanical). I know what needs done but am not qualified to sign off on the work.

 

[wooduser]

Another retired engineer with some more advice.
What I am reading in all this is the following. I would be concerned about the joist hangers, what is their original load capacity, how are they installed and a visual inspection of them now for corrosion and dampness Is very much in order. With the closeness of the ground, water could cause structural degradation to the metal and also the lumber used to construct the deck.

Lecil

 

[tvrgeek]

With Jim on the "obviously big enough" design. I might go pour 4 pads and put in two 6 x 6 under the joists.

 

[chris_goris]

Your calculation should be based on pounds per sq ft. And based on live loads as someone mentioned. Looking at one span chart I found, a 2x10 @ 16" OC SYP #2 with a live load of 40#/ft can be as much as 15'-8" span. Youre at roughly 1/3 that length, and the numbers are linear, so you can easily support 100# sf. So calculate the Sq footage area multiply by 100. The rest of the structure assuming built correctly, should sufficiently support this loading.

 

[tri4sale]

Another retired engineer with some more advice.
What I am reading in all this is the following. I would be concerned about the joist hangers, what is their original load capacity, how are they installed and a visual inspection of them now for corrosion and dampness Is very much in order. With the closeness of the ground, water could cause structural degradation to the metal and also the lumber used to construct the deck.

From what I can see, and that's not much since so low to ground, there is no corrosion on them. No clue on original load capacity of them. Everything is damp right now with all the dang rain we've had

Your calculation should be based on pounds per sq ft. And based on live loads as someone mentioned. Looking at one span chart I found, a 2x10 @ 16" OC SYP #2 with a live load of 40#/ft can be as much as 15'-8" span. Youre at roughly 1/3 that length, and the numbers are linear, so you can easily support 100# sf. So calculate the Sq footage area multiply by 100. The rest of the structure assuming built correctly, should sufficiently support this loading.

I've looked at this, and it's a 72" radius, but I based the Sqft area on a 6x6 area, so 36 sqft. Basing weight on 1500 pounds of water, 100 pounds for hot tub (it's a small inflatable/portable unit), and 400 pounds for 2 people in it, to get to 2000 pounds. That comes to 55 pounds/sqft.

Honestly leaning towards just building a small pad on the ground, found some inexpensive pressure treated ground contact 2x10x8's and that's probably the safest route to go.

 

[chris_goris]

I look at this way, back in the day I had a california king water bed in my apartment, about 42 cu ft of water /.13 = 320 gals x 8.3 = 2656# of water on a 42 sq ft footprint. Im pretty sure your deck will hold it.