Engineered-Lumber Rafters Are Stronger and Straighter, But They Cost More
Bird's eye view
Small trees can be used for long rafters
Engineered components such as I-joists, 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.), and glulams are generally made from low-value, small-diameter trees, giving these components a significant environmental edge over the dimensional lumber traditionally used in stick-frame construction. Engineered materials have fewer defects, are predictably straight and uniform, and are lighter and often stronger than sawn lumber of the same dimensions. Those advantages have convinced many builders to use engineered materials in floor systems, and they can be just as valuable in framing a roof.
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Use appropriate hangers
Engineered lumberLumber made by gluing together veneers or strands of wood to create very strong framing members; stronger and less prone to warping than standard framing lumber and can be made from smaller-diameter trees, saving old-growth forests. rafters require special hangers and other hardware. United Steel Products sells many hangers designed for I-joists (for example, the TFI Series I-Joist hangers), as does Simpson (for example, the ITS top-flange I-joist hanger).
Simpson also sells hangers designed for LVLs (for example, the LSSU adjustable hanger).
Both I-joists and (especially) LVLs may permit longer rafter spans than conventional sawn rafters, giving the designer more latitude in homes with large rooms. But working with engineered lumberLumber made by gluing together veneers or strands of wood to create very strong framing members; stronger and less prone to warping than standard framing lumber and can be made from smaller-diameter trees, saving old-growth forests. requires a more precise design, and may make last-minute design changes more difficult to implement.
Engineered lumber orders usually have a long lead time, so it’s harder to pick up an extra LVL if you run short than it is to get another 2x12 at the local lumberyard.
Cut time on roof framing with production methods
Because all I-joists are straight, gang cutting common rafters makes a lot of sense — there's no crown, twist, or bow to deal with. To index the rafters, cut a saw kerf across the rafters at the bottom of the plumb cut.
Next, measure the rafter length, make an index mark (saw kerf) at the seat cut.
Use a template (right) to mark the plumb cut and the bird's mouth. You can also use the template as a saw guide on plumb cuts.
No notching without an engineer's approval
Wood roof framing is covered in Section 802 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.. Specific rules for framing roofs with engineered lumberLumber made by gluing together veneers or strands of wood to create very strong framing members; stronger and less prone to warping than standard framing lumber and can be made from smaller-diameter trees, saving old-growth forests. are determined by the engineered-lumber manufacturer. The code prohibits cutting and notching of engineered-lumber rafters and ceiling joists without authorization from the manufacturer or a qualified design professional (802.7.2).
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Illustration: from Code Check Building 2nd Edition. click to buy .
ABOUT ENGINEERED LUMBER RAFTERS
Both I-joists and LVLs can be used for rafters
The flanges are sawn from small-diameter trees, and the webs are a type of oriented strand board (OSB) that's made of wood fiber from underused and quickly regenerating tree species. I-joists are more expensive than dimensional lumber, but they are uniformly straight and don’t have the knots, splits, and other defects that are increasingly common in sawn lumber. They also come in longer lengths than dimensional lumber. I-joists, however, are not interchangeable with dimensional lumber, and framing techniques have to be adjusted, possibly with the help of an engineer. Using I-joists in roof framing has proved to be more time-consuming for some builders. Connections at wall plates and the ridge, for example, are more complicated.
LVLs are useful in cases where long spans rule out the use of dimensional lumber. LVLs can be handled much like conventional lumber but, like I-joists, LVLs make better use of limited wood resources.
A framed roof is likely to consist of more than one type of engineered component. Common rafters could be I-joists, for example, while LVLs could be used to carry extra loads around roof windows or dormers.
MORE ABOUT ENGINEERED LUMBER RAFTERS
More expensive. Engineered lumber is usually more expensive than dimensional lumber. And connections between engineered framing members are made with metal connectors, which adds to the cost and may require special ordering. You may want to consider getting a special pneumatic nailer to install the hardware--another cost.
Plan ahead. Because field modifications aren’t typically an option, planning ahead for plumbing and HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. lines is crucial.
Some things are slower. Rafters made from dimensional lumber can be nailed in place quickly, but I-joists must be fitted with web stiffeners to accommodate metal hangers. Similarly, cutting the bird’s mouth in rafter tails may take a jig or two, although the technique is basically the same as working with sawn lumber.
Dealing with cut-offs
There also is the question of what to do with the engineered lumber cutoffs and scraps and whether they pose any health risks when burned. According to the APAAPA-The Engineered Wood Association. Nonprofit trade association for manufacturers of engineered wood products, including glue-laminated timber (glulams), composite panels, wood I-joists, and laminated veneer lumber (lvls). APA and APA EWS (Engineered Wood Systems) trademarks identify products that meet the organization's manufacturing and performance guidelines. Formerly known as the American Plywood Association. -The Engineered Wood Association, both wood and engineered material containing MDI or phenolic resins give off a variety of compounds when burned, including carbon monoxide, carbon dioxide, hydrogen cyanide, and nitric oxide. Emissions are higher in smoldering fires than in efficient incinerators. But the APA said tests have found “no major difference” in emissions between wood and wood products containing the resins.
Tests by the University of Georgia, the NAHBNational Association of Home Builders, which awards a Model Green Home Certification. Research Center, and others found that grinding up engineered lumber waste for use as mulch is safe and effective and released no toxic contaminants into groundwater. On-site grinding has proven to be competitive with landfill disposal.
- Vince Babak/Fine Homebuilding #153
- Roe A. Osborn/Fine Homebuilding #153