Expert: Ted Barnhill (Principal, Design45 LLC) - 4/22/2009

Question
I want to replace my existing bathtub with a Jacuzzi whirlpool that requires my floor be able to support 50 lbs per s.f.  My floor is over a crawl space and consists of 2 x 8 douglas fir joists 16" on center with a 12' span?  What is the live load of this floor and do I need to beef it up to support the whirlpool tub?  Thank you for your help

Answer
Marjorie,

According to the International Residential Code, maximum spans for #2 douglas fir joists, 2x8 at 16" on center, are as follows:

40 lb live load + 10 lb dead load: 12'-7"
40 lb live load + 20 lb dead load: 11'-6"

If you believe your joists are #1 douglas fir (few and small knotholes, no cracks/checks), the spans are:

40 lb live load + 10 lb dead load: 13'-1"
40 lb live load + 20 lb dead load: 12'-4"

In short, if there are no other large loads on these joists, and the framing has been done properly, I believe your new tub load will not be a problem.  If in doubt, it is always wise to have an engineer, designer, or knowledgeable contractor look at your home to offer a more informed opinion.  Good luck, and enjoy your Jacuzzi.

Ted Barnhill