Tag Archives: prefabricated wood trusses

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.

Attaching Horse Stall Posts to Trusses

Attaching Horse Stall Posts to Trusses – Just Say No!

Horse housing can be a significant piece of pie for post frame (pole barn) builders and building kit suppliers when economies are good. From 2007 to 2012, as U.S. economy tanked, horse populations decreased by 10%! Well, economies are cyclical and with a strong recovery a need  for stall barns has increased.

What surprises me – only a very small number of what I would term “best designed” stall barns – designed with sufficient airflow for healthiest horses, are being built. These buildings do not have prefabricated roof trusses, instead they are built using poles (columns) and dimensional lumber rafters. For more reading about pole and raftered stall barns: https://www.hansenpolebuildings.com/2012/08/stall-barn/.

I scratch my head when I see clients investing in clearspan buildings to be used for equine housing. I am most familiar with pole and rafter buildings with poles every twelve feet, to accommodate building horse stalls. Reader SCOTT in DAYTON writes in as one of these who now facing some challenges of trying to correctly construct stalls in his clearspan building. He writes:

“I am installing dividers and horse stalls in a clear span structure. Interior posts need to be added two of which will attach to one of the rafters and serve as supports for the dividers and a stall front. Each post will consist of three 2×6 cribbed boards with treated lumber for the below grade pieces. The tops of the posts will be saddles so that I can through-bolt into the rafter. My question is: how do I set these so that they are neither supporting or hanging from the rafter? Do I dig the holes just shallow enough so that the top of the posts will be snug to the rafter or just hang them and fill the holes with concrete? Thanks!”

Mike the Pole Barn Guru responds:

Even after nearly 20 years as a Midwest import I am still not used to prefabricated wood roof trusses being referred to as “rafters”. Unless you have prior truss manufacturer engineer sealed approval you should not be connecting columns to truss bottom chords. While it may seem added support of a tightly fitting column might be an assist, under a snow load it may actually place loads upon truss in spots not designed for support and can lead to a catastrophic failure.

You may want to consider using either a solid sawn pressure preservative treated column, or a glu-laminated column with bottoms treated for structural in ground use, as opposed to nailing up a three ply 2×6 column where members can separate over time.

I’d be prone to place columns deep into the ground and completely backfill the holes with premix concrete, stopping columns well below trusses.