Metal plate-connected (MPC) wood trusses were first used in the early 1950s. Today they are used in more than 75% of all new residential roofs. Basically they are dimension lumber engineered and connected with metal plates. Less expensive than alternative roof systems, these trusses can also span longer distances. The “Pitched Truss Parts" illustration shows the parts of a single pitched truss on the next page.
Because MPC trusses are engineered products, they should never be cut, notched, spliced, or drilled without first checking with the designing engineer.
Building codes require that a truss design drawing be delivered to the job site. The drawings must show, among other things, the layout locations and bracing details. Note that these drawings are typically not made with framers in mind, so it might take some study time to figure out where the engineer wants the braces. The bracing details often show the braces as small rectangles running laterally between the trusses. See “Lateral Truss Bracing" illustration later in this chapter.
When flying trusses, you should attach the cables around the panel points. When the trusses are greater than 30′, a spreader bar should be used. The cables should toe inward to prevent the truss from buckling. If the truss is longer than 60′, you will need a strongback temporarily attached to the truss to stabilize it. (See “Flying Trusses" illustration, later in this chapter.)
If you have multiple trusses, you can build a subassembly of several trusses on the ground with cross braces and sheathing, then erect them together.
When trusses sit on the ground, on the building, or in place for any length of time, keep them as straight as possible. They are more difficult to set in place and to straighten if they have not been stored properly on site.
Structural Composite Lumber (SCL)
Structural composite lumber (SCL) is an engineered wood product that combines veneer sheets, strands, or small wood elements with exterior structural adhesives. The most common of these products are laminated veneer lumber (LVL), parallel strand lumber (PSL), and laminated strand lumber (LSL). Their names pretty well describe the differences between them.
Like other engineered products, structural composite lumber requires that you follow the engineered specifications that will appear on the plans.
Sometimes the specifications simply indicate the use of a particular piece of SCL in a particular location. For larger jobs, you will find the SCL requirements called out in the shop drawings or the structural plans.
Because these are engineered products, you must consult the design engineer before you can drill or notch. Some manufacturers provide guidelines for drilling and notching, but this is not typical.
SCL has the advantages of dimensional consistency, stability, and availability of various sizes. It is important to note, however, that where dimensional lumber 4 x 10s, 4 x 12s, etc. can shrink significantly, SCLs have minimal shrinkage. The engineer should allow for this in the design so that you will not have to consider this factor when using SCLs as the plans specify.
Note that SCL studs are becoming common in building tall walls. They provide a degree of straightness that dimensional lumber does not. Although they are heavy and, as a result, not so easy to work with, they make nice, straight walls.
Engineered wood products come in a variety of forms. Becoming familiar with these products is important if you plan to work with them. Always be sure to follow manufacturers’ directions, and always consult an engineer if you plan to cut, notch, or drill engineered wood product components.
Lifting devices should be connected to the truss top chord with a closed-loop attachment using materials such as slings, chains, cables, nylon strapping, etc. of sufficient strength to carry the weight of the truss. Each truss should be set in proper position per the building designer’s framing plan and held with the lifting device until the ends of the truss are securely fastened and temporary bracing is installed.
mid-height
Greater than 60
The Strength of Good Framing 186
Understanding Structural Loads 186
Building Code Load Requirements 187
Regional Considerations 188
Framing Details 188
Positive Placement Nail Guns 200
