Expert: Dan Griffin - 3/6/2009

Question
QUESTION: hi there,am buliding a garage which is 18ft wide by 32ft
i want to put a room upstairs,would 9x2 at 16inch centres be ok
thanks
rich

ANSWER: Rich, you're not even in the ball park.  Go to this site to play around with joist possibilities:
http://www.awc.org/calculators/span/reversecalc/reversecalc.asp

There is no such thing as 9x2's.  There are 2x4's, 6's, 8's, 10's, and 12's.  Looking at 2x10 on 16" centers, the most they can span is 16'-1 with 40 LB loading.  IF you reduce the loading to 30 LB, 2x10/#2/Southern Yellow Pine can span 18 feet.

I hope this answered your questions.

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

QUESTION: well on the house we just done,we used 225mmx50mm=9x2,so dont understand when you say they dont make 9x2

Answer
Rich, I'm sorry I am too ignorant to know that you were located in a metric part of the world.  Standard terminology in the Unitied States is to reference "dimension" lumber.  The actual after milling dimensions of a 2x10 is 1 1/2" by 9 1/2".  I am not familiar with metric measurements (yes, I can use formulae to convert them), nor am I familiar with normally available metric framing lumber.  The standard American terminology for windows, door, lumber, etc is to use the smaller dimension first, thus 2 by 10.

The basic problem and solution doesn't change.  I don't have a span calculator for metric lumber.  
You need to decide what loading you intend to design for.  
Very light storage - 20#per SF
Code compliant residential - 40#per SF
If you use the term storage on your permit appplication, be aware it will typically require 100# per SF.
You may want to check with your local building inspector.

Next, decide on what deflection you can live with.  Normal is L/360, so a 2x10 will sag .6 of an inch ( a bit over 1/2").

Find out what species and quality grade of lumber you have available that can meet your requirements.