A.Corresponding editor Paul Fisette replies: The joists are curved downward either in response to a load that has been placed upon them or because bowed lumber was originally installed. If the joists are responding to load, and they have been deformed for a long period of time, a good portion of the deflection is permanent deformation known as "creep." The bowed joists will not return to the original straight shape, even if you remove all the load. If you jack it up, it will not automatically straighten. If you jack it up and then support the joist with a wall or post, you should be able to keep the joists relatively straight, providing the span from post-to-post matches the design potential of the double joist.

According to most codes, the maximum allowable deflection for floors is L/360, with L equal to the joist’s span in inches (see illustration, below). This means the maximum deflection allowed under full load (usually 40 psf live load) is the joist’s clear-span distance divided by 360. In your case, the clear span of the joist must be greater than 225 inches (18 feet 9 inches) for 5/8-inch deflection to be acceptable. The fact is, 2x8s simply cannot span 18 feet 9 inches safely. I am concerned that you may have overloaded your joists to have this much deflection, so check your spans and loading carefully.

Calculating Deflection