Expert: Dan Griffin - 1/19/2008

Question
QUESTION: what size floor joice is needed to support 100psi? span is 20 feet. what would the spacing be?

ANSWER: Brian,

I need to clear a number of assumptions.

Those things are called joists.
I'm sure you mean 100 pounds per square foot, not square inch.  If you do mean psi, these figures will NOT work.

100 PSF is only required for heavy storage and commercial corridors.  Why so heavy?

Southern Yellow Pine #2 on 12" centers will barely make 100 with L/360 deflection  (about 5/8").  This is for 100 PSF uniformly distributed live load and 10 PSF dead load.  If the span is any more or there are point loads involved you will need to go to LVL or truss joist type.

---------- FOLLOW-UP ----------

QUESTION: working with eastern white pine, and rough sawed lumber.  will a 2x14 actual size be sufficent.  Would red elm be better than this pine?  Planning on storing cut lumber 8 feet high.

ANSWER: White pine will NOT work, its maximum span for 100# loading on 12" centers is just over 10 feet.  I have no data for red elm, but I've not ever seen or heard of elm used for structural lumber.

If you are not using standard materials for which span tables exist , I cannot help you.  Seek the services of a structural engineer.

---------- FOLLOW-UP ----------

QUESTION: What would be max span for white pine with 50# load?

Answer
Here is one of many span calculators available for free on the internet.  I suggest this one because it has many species of wood.  It just makes a lot more sense for you to try different scenarios on your own.

<http://www.awc.org/calculators/span/calc/timbercalcstyle.asp>
Click on the little triangles at each item to change the selection.  The grade of lumber will be a hard one to fill out if you are using rough cut lumber as no one graded the lumber.  Grading has to do with number of knots, size of knots, and location of knots in relation to the edges.  I suggest you use the #3 or utility grade unless you know you have some exceptional stuff.

Your eastern white pine at 50 pound live and 10 pound dead spaced at 12" on center allowing L/360 (length in inches divided by 360 as safe deflection, the amount it will sag) has a maximum span limit of 11'-11".  You can allow more deflection or use a higher grade lumber.  If your lumber would make #2 it could span 15'-3".