Roof Framing Choices: Engineered Lumber, Trusses, or SIPs?
Framing Roofs with Trusses or I-jJoists Uses Less Wood
Engineered Lumber Rafters
Engineered lumber comes in various forms, including I-joists and laminated veneer lumber (LVLLaminated veneer lumber. Engineered wood product in which wood veneers are glued together in thick sections for use as beams or other structural members. LVL is stronger, straighter, and less prone to warping or shrinkage than conventional lumber and does not require the destruction of mature trees.), both of which can be used for rafters. Because I-joists combine oriented strand board (OSB) webs with 2x3 flanges, they make good use of small trees and wood chips. Like plywood, LVL rafters are assembled from layers of veneer and glue. Because LVLs can be ordered in almost any length, they are particularly useful for buildings that have long spans.
Trusses are manufactured from 2x4s or 2x6s, so they can be made from smaller trees than the ones needed for dimensional rafter stock — usually 2x10s or 2x12s.
ABOUT ROOF FRAMING
Framing a roof with wide dimensional lumber is standard for many builders. Work starts as soon as the lumber has been unloaded, and a skilled framing crew can cut a roof very quickly.
But the 2x10s or 2x12s that are often used for rafters come from big trees and represent more raw material than alternatives such as roof trusses, I-joists, or structural insulated panels (SIPs).
Yet the use of 2x10 or 2x12 dimensional lumber for roof framing may still make sense in regions of the country that have local sawmills. Delivery distances for dimensional lumber from local sawmills are short, saving transportation energy.
MORE ABOUT ROOF FRAMING
Venting, insulating, and air sealing the roof and attic are possibly two of the most difficult details to get right in a house. Vented roofs are designed to cool attics, dry insulation, and prevent ice dams, among other things. There are now a number of successful ways to use spray foam or rigid foam insulation to build a durable, energy-efficient roof without vents.
Complicated roofs are difficult to vent. In some cases, an unvented roof assembly is the only one that will work.
Unvented roofs can perform well as long as they include thick insulation and are properly detailed to limit moisture transfer from the interior. Construction details vary depending on climate, but unvented roofs insulated with closed-cell spray polyurethane foam (specifically allowed by Section R806.4 of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.) can be used anywhere.
One criticism of unvented roofs insulated with spray foam is that roof leaks may be difficult to detect, and could cause extensive roof sheathing rot before the homeowner notices any problems.
Spraying closed-cell polyurethane foam directly on the underside of the roof deck to form an air, vapor, and thermal barrier is a foolproof but expensive way to insulate cathedral ceilings and conditioned attics. Closed-cell foam is denser and heavier than open-cell foams, and also has higher R-values. According to a description of this technique by the Building Science Corp., a minimum of 1 inch of closed-cell foam acts as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., while a minimum of 2 inches creates a vapor retarder.
The foam sticks tenaciously to the roof deck and expands to fill cracks and voids that in a cooler areas might allow warm, humid air to reach the back of the roof deck and condense. In warmer climates, the foam blocks humid air from entering the house, where it could condense on cooler surfaces.
Closed-cell foam can be combined with other types of insulation, such as fiberglass or cellulose, to get the benefits of an air or vapor barrier and to meet local energy guidelines at a lower cost.
The IRC includes requirements for roof ventilation in Section R806.
Structural insulated panels (SIPs) consist of an insulating foam core sandwiched between an inner and outer face, typically made of oriented strand board (OSB). They combine structural framing, insulation, and sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. in a single product for use in roofs, walls, or floors.
- Vince Babak/Fine Homebuilding #153
- Donald Blum/Fine Homebuilding Issue 168
Aug 6, 2009 3:12 PM ET
Aug 6, 2009 2:48 PM ET