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.

Roof Trusses

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.



Structural insulated panels (SIPs) can provide roof structure, roof 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. , and insulation in a single package.

Attics can be living space or not. How you use it can effect how the structure is framed and insulated. Trusses can be manufactured to provide living space.


Roof-section Details


LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -H I-joists and roof trusses do not earn credit under MR1 (Materials & Resources) because they are conventionally used. Note that floor trusses do earn credit because they are not conventionally used.

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Chapter 6, "Resource Efficiency": 3 points for use of roof trusses (607.1); 4 points for use of precut I-joists (601.5); 4 points for SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. roof (601.5). Under Chapter 7, "Energy Efficiency": Roofs that contribute to superior thermal performance can contribute to points under the prescriptive path (701.1.1) or the performance path (701.1.2).


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.


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.

Image Credits:

  1. Vince Babak/Fine Homebuilding #153
  2. Donald Blum/Fine Homebuilding Issue 168
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Aug 6, 2009 3:12 PM ET

It doesn't matter much
by Martin Holladay, GBA Advisor

Don't sweat this decision too much. From a green perspective, the difference between choosing engineered lumber rafters, sawn rafters, or trusses is extremely minor compared to more important issues like insulation levels and building envelope airtightness.

For most homes, the most important factors behind this decision will not be environmental; instead, they will be economic and structural.

Aug 6, 2009 2:48 PM ET

Yes, these roofs use less
by George

Yes, these roofs use less wood but the difference is not that obvious, it can only be obvious from an industrial perspective. Are there any other advantages for this kind of roof? I am considering my options now, still haven't come with a plan and I am hoping Andover roofing will help me decide for the best roof option.

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