Beam Loading Question
TIM: Lacking a lot more of specific informaito I suggest that you talk to the building department and/ or engineer's office in your town or city. Building code books tell you how to compute the beam loading and the deflection/stress limits. I suspect if the builder spaced them at 9 ft, that is pretty much the limit. 1 inch deflection sounds like a lot over the 15.5 feet. Assuming the 1 inch deflection is correct then for a uniformly distributed load the deflection is proportional to the fourth power of the unsupported length. That means at 1.0" at 15.5 ft span corresponds to 0.11" at the 9 foot span. You may have a problem here.