Joist Size for Hot Tub Deck

by John Stuart
(Calgary, AB)

I'm struggling to find the appropriate materials for the lower portion of a 2 tier deck that will be supporting a hot tub.

The lower deck structure is 10'x10' ledger to beam construction. So far I have designed the beam running 9' parallel with the house with 3 sonotube footings, both outside footings are 1' in from the ends of the beam, with the middle footing directly supporting middle section of beam.

I have calculated the biggest load area in the middle as 22ft squared (4'x5.5'). Based on articles posted at this website I went with a deck load of 100psf to hold the hot tub. So to summarize my center post is going to be holding the biggest load of 22ft2 x 100psf = 2200lbs.

I've had no problems calculating my footings (10" diameter and 4' depth) and post (6 x6)....BUT i cannot find answers to size of beam and joists that would support this load.

I'm using prem pt lumber thats made from spruce... HOW CAN I FIND ANSWERS TO THESE SPECIFIC ENGINEERING QUESTIONS?

your help is greatly appreciated

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Apr 08, 2011
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Thanks
by: Anonymous

Thanks so much...this is such a valuable forum....really appreciate you taking the time to provide QUALITY answers.

Apr 03, 2011
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Here are my suggestions
by: Editor - Rich Bergman

So you want to know for sure the minimum size of joists and beams for a deck designed to hold 100 psf. Well I am glad you found the deck footing load calcuator helpful.

Deck Footing Calculator

That is your first step. Next you could try the joist and beam calculators to give you another good starting point.

Joist Calculator

Beam Calculator

All the span and species data is based on publicly available data from the American Wood Council. So that is a very good start.

But you want to know the real details and engineering behind those numbers and have it apply directly to your specific application. Here are two possible solutions.

1. Just over engineer. This isn't scientific so do so at your own risk but by using 2x10 double or triple laminated beams and using 2x10 joists spaced at 12" you are going to drastically increase the stiffness of the joist structure. So long as your soil loads are in check with the footings you should be fine.

2. The only final solution is to call a structural engineer. Give him the plan I just suggested. Use my calculators and the print page function and take it down to him. A structural engineer has all the source reference data for determining exactly the flex over distances based on the lumber species used.

This would cost at most $250. At least the structural engineer I use whem I build homes does quick checks like thsi for me. It is totally worth it. And if this is a high value project for you then you must do this.

3. The only other option is for you to find the same source reference engineering manuals and learn the math they use to make these calculations that groups like the AWC use to create their guideline documents.

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