Loft Truss Construction

Roof Truss Construction

• A common truss consists of two sloping top chords that form the peak of the roof, a horizontal bottom chord that forms the ceiling of the building's interior space, a vertical king post that supports the roof ridge, and a web of support members that transmit the weight of the roof from the top chords to the bottom chord and, ultimately, to the building's walls. The support web fills the space under the peak of the roof, so to allow for an attic or loft space, a truss design other than the common truss must be used.

Loft Truss Construction

• Trusses designed specifically to provide a loft or attic space, called attic trusses or room-in-attic trusses, are an alternative to common trusses when a room under the peak of the roof is desired. These trusses are designed so that the center of the truss is free of support members. The king post is either shortened so it extends downward only to the level of a chord that forms the ceiling of the loft space, or the king post is eliminated entirely. Vertical members near the eaves form the walls of the loft space, and the bottom chord of the truss forms the loft's floor.

Loft Conversion

• Although not an ideal option, it is possible to convert a common truss roof so that it will provide a loft space. Simply removing support timbers within the truss in order to create space is not acceptable because such an alteration would compromise the strength of the truss and the entire roof structure. New frame members are required to functionally replace the truss timbers that will be removed, and the new structure must be designed so that the weight of the roof is effectively transferred to the new frame elements.

Advantages and Disadvantages

• Truss construction saves time, labor and material cost compared to conventional timber framing, and a loft truss design is cheaper to build than a loft made with individual roof rafters and floor joists. Rafter-and-joist construction, however, is more flexible in terms of design, and designs utilizing rafters and joists typically are able to offer more usable loft space than truss designs.