Here is a great picture of clear-spanning roof frame trusses. What clear-spanning means is that the trusses span from one side of the structure to the other. There are no support posts to get in the way anywhere within the room the way that there would have been if we had gone with a beam to support ceiling joists.

Trusses are great for creating a space that is wide open with no visible support posts. They also don’t typically cost more than a traditional ceiling-joist/roof-rafter style framing technique. The reason is that 1 truss replaces 2 roof rafters and at least 1 ceiling joist. Also when using trusses you avoid the need for a ridge-beam and a support beam(s) and post(s) for the ceiling joists.

Because of the typically large nature of trusses sometimes a crane is required to install them, however this cost is still usually offset by the quick installation of the entire roof system.

With many styles of roof trusses you will lose some or all of the attic space that you would of otherwise had with traditional framing techniques. Most truss manufactures can, however, offer solutions that will still give you a usable attic. These truss systems may cost a bit more but usually not prohibitively.