Expert: Ted Barnhill (Principal, Design45 LLC) - 3/7/2011

Question
We would like to put bookcases loaded with books against two non-weight bearing walls in an upstairs bedroom.  The home is new construction. This is a response I received from the development inspector: Your home is constructed with web-truss floor joists which are basically trusses built out of 2X4s and the floor plywood is ¾” tongue and grove OSB. I don’t have specific numbers, but a structural engineer certainly would. When these plans are drawn up, I would have to believe that they take into consideration “normal” furniture, entertainment center, and possibly even the weight of a large waterbed,  with the weight spread out, but I don’t know if the architects figure in the possibility of something with that amount of overall weight in bearing down on a specified spot (I am quite familiar with how much books weigh, my parents are book dealers). If there is a wall located directly underneath the area in question that would definitely help, as would the benefit of securing the book cases to the wall, to help with the overall load. There could be an issue if the cases ran parallel and lined up right in between floor joists, if that were the case you would be relying on only the subfloor (plywood) for support."  I am not a builder, and I don't really know what it all means. The bookcases are floor-to-ceiling and will be fully loaded.  Thanks for your help.  Terry

Answer
Terry,

Table 503.2.1(1) of the International Residential Code gives the maximum load of a standard 3/4" subfloor with 24" span as 175 lbs per square foot.  (This assumes that your subloor has a span rating of 48/24, the standard for 3/4" but you should check to be sure.)  If you have, say, floor to ceiling encyclopedias, or reams of paper stacked up 8 feet high, you might easily exceed this limit.  To be 100% confident that you are not overstressing your subfloor or one of your floor trusses (in the scenario that your inspector lays out), you would need to determine the true weight of your books, the length of your bookshelves, and and the exact location of your structural members.

To give some context, library stacks are typically designed for 150 psf (pounds per square foot) or so, compared to residential loads of 40 psf.  A few bookshelves are common in residential use and should not overstress a properly built floor system, but books are heavy, and if you want to design a densely-packed library within your home it may be advisable to talk to a structural engineer about your specific case.

Ted Barnhill
Design 45, LLC