What's The Proper Beam Size For a Hot Tub Deck?

If you've just joined us, we were discussing how beam size is crucial especially when you have to support a hot tub with 8,000 lbs of hot water and bathers.

In this example we used the ConstructionCalc Wood Beam Calculator to make the calculations for us so our deck doesn't collapse.

Tim Garrison, the mind behind all the ConstructionCalc software was kind enough to provide the following explanation how to design the beam size for this hot tub deck.

This is step 5 from Tim's Deck Design Explanation. Read the full pfd version here.

Wood Beam Calculator

Here are the top and side views of the hot tub deck design.

Hot tub drawing 1
Hot tub drawing 2

3) Beam Design.

Beam Design (This example will go a little faster because I won't repeat the basic stuff learned in the joist design above.)

Click here to get a pdf version of the beam design report.

  • Let's input the required data into the Wood Beam Calculator:

  1. General: Change Beam ID from Joist to Beam (optional).
  2. Span: Inside of post to inside of post is 8' (pure coincidence that this turns out the same as the joists span; it usually doesn't work this way).
  3. Deflection: Use L/360 and L/240 defaults
  4. Load Duration: Ten years live
  5. Add Self Weight? Yes we want to add the beam's self weight to the design because the 15 psf dead load we'll use in a minute does not include the beam's self-weight (though it does include the weight of the joists and decking).
  6. Loads Other Than Uniform? No, because the loads applied to this beam comes from uniformally spaced joists of equal length and loading. In other words, the beam's load is uniform across the beam's length - there's no point loads or other weird loads.
  7. Uniform Loads Over Full Length of Member: Let's use the same Floor. Load row as before:
  1. Live: 125, same as before
  2. Dead: 15, same as before
  3. Tributary Width: This is the only loading criteria that changes.

    This beam gets its load from the joists, and in particular half the span of the joists goes to this beam. The other half of the joist's load goes to the beam on the other side of the deck. So the tributary width for this beam is half of the joist's span: 8/2 = 4'.

    Done, now let's check results.

  • Allowable Solutions.

We need Hem Fir, pressure treated again, so let's give ourselves a couple options:

  1. Tripled 2x. In the 4x And Smaller section; first, this beam gets no help from another member, i.e. it acts alone so we select No to Repetitive Member Use? Make sure Hem Fir, No.2 is selected in the drop-downs and you can see that (3) 2xlO's work.
  2. Single Beam. In the 5x and Larger section, select Hem Fir, No.2 and a 6x12 or 8xlO are all that will work.
  3. But Wait. Back in the 4x and Smaller section we see that a 4x12 also works. This may come in handy if we want to use 4x posts.
  • Final Member
  1. I like the 4x12 because I want to use 4x posts and want my beam to match up well. Selecting it shows it makes it by 14.8% - okay, even for a pressure treated member.
  2. Minimum Bearing Length is 1.58" - okay. This beam will sit on posts of at least 4x4 dimension, so there will be plenty of space to accommodate 1.58" bearing length.
  3. Print the results if you wish. You've got your deck beam size!
  4. Check your work. If you're worried you may have goofed something on the input and want a rough check, first, look at the Load and Span Diagram in the upper right of the sheet. The span is shown in feet on the horizontal axis and the loads on the vertical. A uniform load will look like a rectangular box over the entire beam.
    Now take a look at the Reactions at the bottom of the sheet. We know the hot tub and occupants weighs 8,000 lbs; plus the deck itself (dead weight) will be a couple or few hundred pounds.

So the beam size we just designed for this hot tub deck should carry half of this total weight or about 4,200 lbs give or take (the beam on the other side of the deck carries the other half).

Because our beam is loaded uniformly, we'd expect each end to have the same reaction, which should be half of our 4,200 lbs = 2,100 lbs, give or take. The printout shows reactions at each end of our beam of 2,240 lbs. Close enough; we're good to go.

Let's go to the next page and learn how to design and calculate the post size to support this hot tub.

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