Tradeoffs in Roof Structures

Two options in roof framing affect space and cost.

Rafters, Trusses

Photo: home-improvement-and-financing.com

There are two common options for framing a home’s roof. A rafter system that connects to a ridge beam is the most flexible, allowing for design elements like vaulted and cathedral ceilings and wide-open attic spaces. A rafter system, however, must be built piece-by-piece on location. A truss roof can span much farther than a one using beams and rafters, allowing the creation of larger open spaces below. Truss systems are faster to erect, as they arrive on site pre-assembled and ready to install. However, the wide-open space created by a rafter system is often lost when using roof trusses, as the lumber webbing that give the truss its strength often crisscross through the space traditionally left open by rafters.

At Home Again’s Vermont Farmhouse project in Quechee Lakes, Vt., architect Hunter Ulf of UK Architects in Norwich, Vt., took advantage of both the speed and cost effectiveness of truss systems, while still making accommodation for a bonus space above the garage. “The truss we used is setup so there are no webs in the center of the truss allowing you to use the space,” says Ulf. Although not as spacious as a roof system formed by rafters, “it gives you an affordable roof system and useable space,” notes the architect. Bob Vila says, “The prefabricated trusses are delivered to the job site ready for installation. Assembling the roof structure with this system is very fast, and a crew of about a half a dozen workers can raise the truss system and sheath the roof with plywood in a single day.”

Plan ahead carefully when working with your architect or designer when choosing a roof system. Truss systems cannot be modified once installed. Cutting individual lengths of wood in the trusses can significantly weaken the entire roof structure (boring holes to run electrical wires is even discouraged). “The trusses used in our farmhouse project constructed from 2x4s. Ordinarily, 2x4s are too small for use as roof rafters, but the engineered construction of each truss allows the use of smaller dimensional lumber in this application,” Bob notes.