By definition, is the amount of component movement when under load. Excessive deflection can be cosmetically unpleasing, such as roof sagging, or it can impede building use making it difficult to open doors and windows. “Excessive deflection generally isn’t a concern as long as proper wind and snow loads have been prescribed,” says Dave Bohnhoff, ag engineer at University of Wisconsin. The major exception being when exterior walls and rafters are to be covered with drywall, because excessive deflection can cause seams to open up. Normally, engineers try to limit deflection to the unsupported length of a member divided by 180. This means, for example, that the deflection of a 90-in. long member would be limited to ½ inch movement. For all walls and ceilings with attached drywall or plaster, and for all floors, limit component deflection to the unsupported length of the member divided by 360. (Source: May/June Issue of FIN)