Tag Archives: lumber twist

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.

Hart and Home Youtube Episode III

Hart and Home YouTube – Episode III

If you missed our previous episodes, please go to bottom of this article, on left, and click on arrow twice to go to Hart and Home YouTube – Episode I. Moving forward:

For those following along at home:

Lumber OffloadingSome discussion will be made here about “allowable defects” in lumber. Our main struggle with lumber quality is clients having unrealistic expectations. Lumber is chosen for strength characteristics, rather than due to “pretty looks”.  If an expectation lumber will be clear, vertical grain, knot and wane-free, a severe disappointment will occur. 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. Minor dimensional differences in individual boards are a function of shrinkage due to moisture content, wood species and even individual tree characteristics. Shrinkage is more prominent across grain – hence variations in the board’s wide face are more pronounced. Ultimately these small dimensional differences do not affect the finished building.

All structural dimensional framing lumber used in Hansen Buildings will be at least graded as #2 (or “standard”) or better.  For discussion’s sake, we will limit scope to this grade. Four inch and smaller (e.g. 4×6 and any two inch – 2×4 through 2×12, 4×4 and 4×6), are all graded under “joists and planks” rules. Characteristics listed below are unintended to be all-inclusive, but merely to be a frequently seen item overview.

In any given lumber “production run” typically 5% is allowed to be outside grading rules (to have more defects than expressly allowed) and still have a sum total declared as “on grade”. 

Checks – seasoning checks are unlimited. Through checks at ends are limited as splits. Splits can be equal to 1-1/2 times board face width (e.g. 8-1/4” on a 2×6, 16-7/8” on a 2×12). Keep in mind, many times these boards will be trimmed off, especially when used as girts, purlins or rafters.  A split portion may thus be removed when trimmed to be put into service.

Knots – on a 2×6, up to 1-7/8” at wide face edge, 2-7/8” at wide face centerline; on a 2×12 up to 3-3/4” at wide face edge, 4-3/4” at wide face centerline.

Holes – (from any cause) on a 2×6 1-1/2”; on a 2×12, 3”.

Wane – up to 2/3 thickness and 1/2 width for 1/4 length.   An example would be on a 12’ long 2×6, wane could be 2-3/4” on wide face, 1” in depth across 1-1/2” face and 3’ in length.

Bow (or Crook) – a board size and length function. As an example: for a 2x6x12’ – 5/8” would be acceptable, with greater amounts allowable with longer lengths.

Twist – for a 4x6x16’ would be allowable up to 1-1/2” and still be within grade.


5” x 5” and larger are graded as “Posts and Timbers” and have their own characteristics. Again, addressing #2 grade, they include:

End splits – up to twice post face are permitted: (Ex: 12” on a 6” face.)

Wane – 1/3 of face.

Bow, crook and twist are NOT limiting factors under grading rules for posts and timbers.

It is essential any suspected shortage or damage be reported immediately. Vendors become highly suspicious of possible jobsite damage, as it increases with time. An example of this is when (after several months on a jobsite) a client asked for replacement of some damaged steel – his provided photographs actually showed tire marks where it had been run over!

Construction Manual – we do frequent updates and additions to it, often several times annually. Since Kevin and Whitney’s investment, more and more of our clients have gone to wet-set brackets, rather than embedded columns. Hence, we have added several pages of wet-set bracket specific details and instructions to Chapter 5, Setting Columns.

Solid sawn vs. glulam columns. We agree totally with Kevin’s analysis, glulams are a superior product and I would highly recommend them to any client (for extended reading, please see https://hansenpolebuildings.com/2014/10/glulam-4/). One downside is distribution, as they are manufactured in limited and highly scattered locations, costs of shipping and time delays can become issues.

Watch for our final episode of this series – coming soon!

A Miracle Cure to Prevent Twisted Timber Columns

A Miracle Cure to Prevent Twisted Timber Columns

In 1960 Chubby Checker did a cover of Hank Ballard and the Midnighters 1959 song “The Twist”. Checker’s cover reached Number One on Billboard’s Hot 100 both in 1960 and again in 1962, becoming the only single to reach number one in two different chart runs.

While “The Twist” was a musical hit, twisting in lumber poses concerns, if not panic.

It would be all well and good if trees could be trained to grow so as they only produced straight-grained lumber. Fat chance of this happening.  In fact, straight grained lumber is by far an exception, rather than a rule.

Instead, spiral grain is an expected pattern – where this term describes a helical orientation of tree fibers giving a log a twisted appearance after bark has been removed. This twisted appearance is even more highlighted by surface checks, following grain of fiber, making spiral grain very obvious in some standing dead trees and on utility poles and posts.

This spiral may be in either direction, be fairly constant in any one tree or may change with tree age. In some trees, there may even be a reversal of spiral in successive zones of growth forming an interlocking grain. Lumber twist is a function of the degree of Greater spiral increases chances you will see lumber twist.

Most typically prevailing spiral orientation is in a left direction near a tree’s pith, with angle increasing sharply in first-formed rings in juvenile wood. This gradually decreases to a straight-grain then is followed by a gradual change to a right-angle spiral. Trees with left spiral do tend to twist more with changes in moisture content, than those with both left and right-spiraled grains.

Spiral grain may seriously reduce strength and stiffness of lumber milled from a given tree.  This “slope of grain” in sawn lumber is considered as a defect, and is a resultant of natural spiraling.

For purposes of visual lumber grading rules, slope of grain is wood fiber deviation from a line parallel to edges of a piece. This deviation is expressed as a ratio such as a slope of grain of 1 in 8, 1 in 10, 1 in 12 and 1 in 15. Slope of grain as measured is representative of general fiber slope and local deviations are disregarded. Bigger bottom numbers would express straighter grain. Less lumber twist gives you stronger boards.

Typical No. 2 grade framing lumber allows for a slope of grain of up to 1 in 8 (an inch of slope for every eight inches of board length). This consideration takes into account how this defect accordingly adjusts allowable strength values used in engineering design.

Now – a “miracle cure” for twist!

In this article’s photo, a builder has convinced his client by taking an inch off each corner of his building’s posts (at a 45 degree angle full length) it would somehow keep them from twisting. 

And his client not only believed it to be true – so much so as to help his builder cut off each corner!

Do the Lumber Twist

Let’s start in the forest…

When David Friss of Anchorage worked for NOM ‘s Environmental Research Labs in Boulder, he was once asked why lightning sometimes spirals down the trunk of a tree. While the answer was not proven, we observed the path of least resistance might follow the spiral grain of the wood. He eventually found a tree with a spiral lightning mark and it followed the spiral grain exactly.

“But why should the tree spiral? Larry Gedney, a seismologist at the Geophysical Institute, University of Alaska Fairbanks, speculates: Foliage tends to be thicker on the south side of the tree because of better sunlight. Prevailing winds, in most of the tree-growing northern hemisphere, are from the west. Combine these factors, and the westerly wind pushing on the thicker south side of the tree, year after year, causes an asymmetrical wind loading which slowly twists the tree around in the observed direction.

Lumber wants to do the twist

Professor Hans Nielsen of the Geophysical Institute thinks the matter can be related to the Coriolis effect of the earth’s rotation. In the northern hemisphere, all moving objects are diverted ever so slightly to the right. (This is why hurricanes rotate counterclockwise–air moving toward the storm is diverted to the right and thus imparts a counterclockwise spiral to the storm at the center.) Nielsen thinks that possibly when a tree is rocked by winds, the tip might tend to rotate in a counterclockwise circle when viewed from above. This would lead to a clockwise spiral twist. That sounds like a contradiction until you think about it for a while.

Granted, not all trees exhibit the same lumber twist, but the majority of them do. The phenomenon can be likened to the claim that water will always spiral out of a drain in a counter-clockwise direction in the northern hemisphere.

So…take a log and cut it into nice, new boards. The log goes straight through the saw, but the “helix” of the grain, is not straight. Lumber does have a memory, and if the moisture content of the wood is changed, the cut piece will do the “lumber twist” and try to “become a tree” again!

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