How do I reinforce the attic floor?
In Massachusetts, attic floors must support at least 30 lbs/ft² live load (the same as bedroom floors) if the attic is accessed by means of a fixed stairway (780 CMR 5502.3.1).  Use a span table to determine the necessary joist dimensions, given their span length, spacing, wood species, and grade.  For example, if your floor joists are 2x6 spaced 16 inches o.c., they can then be between 8'5" and 10'11" depending on the wood species and grade (780 CMR Table 5502.3.1(1)).

Uninhabitable attics with limited storage need only support a live load of 20 lbs/ft² (780 CMR Table 5802.4(2)).