I recently heard from a reader who was curious about storing items in his attic, which is framed with 2x4 wood trusses. Unlike conventional roof framing, in which rafters are cut and installed on site, trusses are built in factories and shipped to the site. Trusses cut down on time and expense in home construction, but they also can cut down on the use of attics for living and storage purposes.
Which isn't to say that you can't store things in an attic with trusses. It really depends on the type of truss. Many styles of roof truss have diagonal ("W"-shaped) members that leave little room for storage. But, as this article on attic storage points out, there are specific types of trusses that leave a big open space in the middle specifically for the purpose of creating storage. These are typically called "storage trusses" or, sometimes, "attic trusses."
If you have this type of truss, you can use the space for storage. Install some plywood on top of the bottom members of the truss, and you can safely place light items on top. But I wouldn't want to store heavy items on the trusses, especially if the bottoms were 2x4s spaced 24 inches apart. That's not a strong floor.
For more specific suggestions on how much weight can be placed on top of trusses, you should consult an experienced builder or engineer, who might have to take a closer look at the attic.
Photo courtesy crd! @ flickr/Creative Commons