Engineered roof trusses are carefully designed so they maximize strength and minimize the amount of material used. The design of a common truss, although strong and lightweight, has the disadvantage of eliminating functional attic space. Converting a common truss to a truss that allows space for an attic room is possible but it's not as simple as cutting out the framing that's in the way.
In a common truss, two framing timbers called top chords slope up from the eaves and meet at the peak of the roof. A horizontal timber called a bottom chord stretches between the eaves and ties the bottom ends of the top chords together. A vertical timber called a king post spans the distance between the bottom and top chords and supports the peak of the roof. A web of support timbers, the pattern of which varies among different designs of common trusses, connects the top and bottom chords and transmits the weight of the roof downward and outward to the walls of the building. Because the space within the truss is filled with the support members and the king post, the design of a common truss does not allow for an open attic space under the roof.
An attic truss, sometimes called a room-in-attic truss, provides an open attic space by eliminating timbers in the center of the truss. A shorter king post extends from the peak of the truss to a horizontal member that functions as the ceiling joist of the attic room, and vertical members near the eaves form the walls of the room. The bottom chord of the truss serves as a floor joist.
Because trusses are designed to form a solid structure with a minimum amount of framing, removing any part of the truss will compromise its strength. A common truss cannot be converted to an attic truss by just removing the timbers in the center of the truss; a roof converted in this way would be unstable and in danger of collapse. Conversions are possible if the lost timbers are replaced by members that add sufficient structural strength, such as horizontal steel beams that tie the truss together and carry the load that would otherwise be carried by the king post and support timbers.
For an attic room intended to function as living space to have adequate head room, a truss must have about 7 feet of space between the bottom chord and the peak, making some trusses with a shallow slope unsuitable for conversion. Sometimes additional space is created by building dormers that extend outward from the existing roof, but such alterations may be in violation of building codes in some areas. Inspections and permits may be required as well.