Tag Archives: lumber shrink

Allowable Variances in Prefabricated Wood Trusses

Allowable Variances in Prefabricated Wood Trusses

Long time followers of mine will recall portions of my past life doing pretty well everything related to prefabricated wood trusses.

It all began in April 1977 (before many of you were born) as a Sawyer cutting components at Spokane Truss (now a Builders First Source location). After a short stint there, I was transferred to their sister company, Coeur d’Alene Truss (now Coeur d’Alene Builders Supply), where I spent two plus years literally doing everything other than signing checks and accounting.

This was followed by a year of managing Lucas Plywood & Lumber’s truss operations in Salem, Oregon. Part of a year at Mac Truss Company in McMinnville, Oregon led me to open my own truss plant and lumberyard – M & W Building Supply in Canby, Oregon. After selling it and retiring at age 32, I realized I truly did not want to take up golfing full time, so opened Apex Truss in Spokane.

All told, I spent over two decades playing with, selling, designing, building and delivering trusses. Lots of trusses.

Some discussion will be made here about “allowable defects” in lumber. Wood is an organic material. While produced in a “factory” environment (a sawmill), lumber is subjected to naturally occurring defects accounted for in grading rules. These characteristics are taken into account in strength values for allowable design.

Lumber used in trusses falls into this same discussion. Truss lumber is chosen for strength characteristics, rather than due to “pretty looks”.  If an expectation exists that trusses are fabricated from clear, vertical grain, knot and wane- free lumber, a severe disappointment will occur. Lumber “appearance” is NOT a reason to reject any truss.

Very few individual sticks of lumber are absolutely, perfectly straight. When components for truss are cut, they pass through a set of saw blades, preset to allow for as many as three cuts to be made at each board end. In order for cut angles to be accurate, each board is clamped to straight on a conveyor feed, and after cutting returns to its precut (and not necessarily as straight) shape. This does allow for some variations to occur in finished trusses.

Just how much variation is allowable?

Not this much!

As you know, the best reason for using wood in construction is it is easily cut and shaped and can be very forgiving dimensionally. A limitation is it can shrink, swell, twist, warp and bow. This result is once a component piece is cut, its final shape may change from what was originally intended. This is a fact of life when designing with wood and most construction and manufacturing details allow for some play in final results.

Truss Plate Institute’s ANSI/TPI 1(link is external) is truss industry guidance regarding manufacturing tolerances. For example, Table 3.5-1 allows identical trusses to vary in span by as much as 1/2 inch and in height by as much as 1/4 inch. Variance from design dimensions (shown of sealed truss drawings) is also allowed: 3/4 inch in span and 1/2 inch in height. Complying with erection tolerances specified in SBCA’s BCSI-B1 Summary Sheet is critical to achieving an acceptable roof line.

Stick Frame and Some Limitations

Stick Frame and Some Limitations


Perhaps stick built construction’s biggest advantage is builders and tradespeople are very comfortable working in and around stick framing. All registered architects and most building inspectors are very familiar with stick framing. The International Residential Code (IRC) provides a prescriptive ‘cook book’ to follow for adequate structural assembly, within certain limitations. These limitations include, but are not limited to, no story height of greater than 11 feet 7 inches (R301.3), no hurricane prone areas with a design wind speed of 130 mph or greater located south of Virginia, or 140 mph elsewhere (R301.2(5)B), and no ground snow loads over 70 psf (R301.2.3).

IRC802.10.2.1 further limits truss spans to a maximum of 36 feet and building lengths to 60 feet (measured perpendicular to truss span). Trussed roof slopes must be at least 3:12 and no greater than 12:12.

Wood is a very forgiving building material and, even when miscut, replacement material is usually only a short drive away. America’s home building industry has built traditional, wood stick framed homes, on site for decades.

Many builders, architects, carpenters and other subcontractors prefer to work on stick built homes as compared to alternative building systems.  Because traditionally framed houses are so popular, dimensional lumber and stick built framers are readily available.

Another advantage of stick built homes is they allow for a great level of design freedom.  You can design your barndominium with various ceiling heights, angles and curves, niches and other details. Stick framing one to achieve those unique details at a fairly affordable cost.

Despite its popularity, stick framing does have some drawbacks. Because stick built homes are assembled outside, over several weeks, framing lumber is subject to outside moisture.If lumber gets too wet, it can shrink and warp as it dries and cause cracks in the attached drywall.  This shrinking and warping can also make it difficult to properly insulate. To decrease  risks of potential moisture problems, ensure exteriors are covered with an appropriate and well-sealed Weather Resistant Barrier and lumber is properly dried before drywall and insulation are installed.

Another drawback of a stick built home is it usually takes several weeks to complete framing.  Total amount of time it will take will obviously depend on size and complexity of house plans and size, experience and availability of any particular framing crew.

A framing crew must precisely cut, assemble and erect barndominium framing components sometimes in adverse weather conditions.  Working around adverse weather conditions is another challenge with stick framing.

Although site-built, stick framed homes clearly dominate America’s housing market, there are several other ways to build a barndominium’s structure. These include post frame, PEMB (pre-engineered metal buildings), weld up steel and concrete.