Tag Archives: green lumber

Prefab Wood Trusses are Sexy

Prefab Wood Roof Trusses Are Sexy Though

In 1952, in Pompano Beach, Florida, an inventor named Carroll Sanford had been experimenting with building prefabricated roof trusses using plywood gusset plates and varying concoctions and combinations of glue, staples, nails and screws. Eventually he conceived of light gauge steel plates with punched teeth to connect wooden members.

If this wasn’t a sexy use of technology, then I don’t know what would be.

A burgeoning pole barn (post frame) building industry was largely aided by this new ability to economically clearspan relatively large distances.

What Hansen Buildings does now and since 2002:

FEATURE: Prefabricated face-to-face doubled roof trusses.

BENEFIT: Provides an engineered solution with clearspan widths of 80 feet and (in some instances) more. Endwall trusses make for quick and easy installation, while maintaining roof slopes.

True double trusses provide increased reliability due to their load sharing capabilities: https://www.hansenpolebuildings.com/2018/09/true-double-trusses/.

EXTENDED READING ABOUT THIS SUBJECT:

My all-time most read article: https://www.hansenpolebuildings.com/2011/06/pole-barn-truss-spacing/

Why most people should not order trusses: https://www.hansenpolebuildings.com/2018/10/why-most-people-should-not-order-trusses/

WHAT OTHERS DO: Another feature with a myriad of possible outcomes. I will defer “how” trusses are attached to columns for a later article.

Theories of roof truss spacing become most generally divided up geographically. These geographic nuances do bleed over from one area to next, so are not cast in stone.

Eastern U.S. places single trusses upon two or four foot centers attached to tops of truss “carriers” – headers spanning from sidewall column to sidewall column. Here are a few words about truss carriers: https://www.hansenpolebuildings.com/2018/10/what-size-truss-carriers/.

Midwest most often opts for a single truss aligned with sidewall columns. Spacing might be as little as 7’6” and as great as 10’.

Going West – expect a single truss each side of sidewall columns with paddle blocks to attach roof purlins. Learn about paddle blocks here: https://www.hansenpolebuildings.com/2012/05/paddle-blocks/.

While Eastern and Midwest post frame buildings generally feature trusses at each end. As one heads west, dimensional lumber rafters are often seen – relying upon building erectors to achieve proper alignment with interior trusses.

WHAT WE DID IN 1980: Back in green lumber land – Lucas Plywood & Lumber fabricated trusses out of green lumber. As spans (and dimensions of top and bottom chords) increased they became phenomenally heavy.For building ends, 2×12 #3 rafters were provided.

 

Do Screws Back Out of Steel Roofing?

I had a question posed of me recently which included: “Where will the water go when the screws back out of my steel roofing”? While I answered the question at hand, I didn’t actually get into the why this might happen, or the solutions.

How to avoid the potential problem completely……use the right part, properly installed and driven into the correct material. Three easy steps, should not be so difficult.

The part – most commonly used screws are a #9 diameter by one inch long. When we tested steel roofing to determine sheer strength these screws pulled out of the framing under a minimal load (so minimal the steel didn’t even have ripples in it from the applied load). You can read more about our testing here: https://www.hansenpolebuildings.com/2012/08/this-is-a-test-steel-strength/.

Going to a longer part solved the pull out issues in our testing. We also went to a larger diameter part in our testing, the shank below the screw heading being ¼ inch across, while the threads are a #12. The larger diameter screws also have deeper threads, which means they bite and grip the wood more tightly.

Proper installation – screws which are over or under driven, or driven at an angle are prone to a myriad of problems, all which end in leaks.  Over driven screws tend to damage the wood fibers, leaving little solid material to hold the screw. Use a screw gun with a clutch, so screws do not get over driven.

Driving into the right material– what could go wrong? I see folks using OSB or plywood sheathing under roof steel with the idea they can drive the screws into the sheeting and still hold, even when the screw tip misses a purlin. These screws will come back out.

Green lumber (or dried lumber which has been allowed to get wet) will cause screws to be loose as the moisture leaves the lumber once the building is dried inside. Of course green lumber has a myriad of other challenges which can be read about here: https://www.hansenpolebuildings.com/2011/09/499green-lumber-vs-dry-lumber/.

Right part, right screw, right material below – drop the mic and walk off the stage. Three easy steps for proper screw installation and keeping leaks from happening. 

 

Reasons for Drying Wood

Yesterday I talked a bit about wood species, and hinted a more pertinent issue than which species lumber is used on a building, is that wood used should be kiln dried. For both technical and performance reasons, drying or seasoning wood is required when making glued wood products such as laminated beams, plywood, particleboard, furniture and many other products. The use of dry framing lumber minimizes negative performance issues in buildings with structural wood framework.

Drying wood to the desired final moisture content minimizes dimensional changes and warping in use. Pieces which might degrade and develop defects during drying can be sorted out at the mill and possibly directed to another use. Such quality control helps to assure performance of products in service and leads to greater customer satisfaction. Lumber which is delivered “green” to wholesalers, retail lumber yards and end users, often has a high percentage of downfall due to the results of natural curing.

The lower price of green lumber, in relationship to dry lumber, is negated by the often 15-20% of pieces which cannot be used as originally intended.

Dry wood provides a better base for paints, finishes and adhesives. Many finishes may not adhere at all to green wood, or the subsequent drying of wood may lead to failures in the paint coat. Adhesives may not bond well (or at all) on green wood.

Water- or oil-borne preservatives cannot be forced under pressure into wood which has free water in its cells. When a preservative cannot penetrate adequately, full protection against attack by decay fungi or insects cannot be obtained.

Drying lumber improves the resistance to decay. Wood which has been dried and kept below 20% moisture content does not have sufficient moisture to support most decay organisms. Also, organisms already in the wood will be killed when exposed to high kiln-drying temperatures for several hours.

Drying wood may increase the strength of wood unless defects developing during drying counteract this trend.

Drying reduces the weight of wood, and since truck and railroad shipping rates are based on weight, shipping costs can be reduced. The lower weight also makes for more efficient handling of lumber on job sites. Construction times are reduced as workers can more easily move and install larger quantities of lumber by hand.

If none of this so far makes an impact, perhaps mentioning green lumber shrinkage will.  The “magic number” where lumber becomes dimensionally stable is 19%.  So, Green lumber at 25 to 30% has a huge potential for shrinkage.  If an 8’ piece of lumber shrinks an inch, on average, how many inches would a 40’ long building shrink?!  5 inches is huge!  I used to wonder why the roof steel on older buildings looked like it was “wavy”, and then I figured out the answer, green lumber shrinkage!

Choosing to “build green” does not mean using green lumber.  Choose dimensionally stable kiln dried lumber, for all of the reasons above.

Lumber Species Surprise!

In the United States, there are four prevailing species of timber which is used for framing lumber.

Douglas Fir-Larch – which includes Douglas Fir and Western Larch (Tamarack). Prevalent along the West Coast, it is also known as Oregon Pine or Red Fir.

Hem-Fir – another western lumber species group which includes California Red Fir, Grand Fir, Noble Fir, Pacific Silver Fir, Western Hemlock and White Fir.

Southern Pine – prevalent in the south, includes Loblolly Pine, Longleaf Pine, Shortleaf Pine and Slash Pine.

Spruce-Pine-Fir – predominate in Canada and includes Alpine Fir, Balsam Fir, Black Spruce, Engelmann Spruce, Jack Pine, Lodgepole Pine, Red Spruce and White Spruce.

Once or twice a year, we get a request for a specific lumber species to be used for framing materials. It appears to be more a case of consumers having either had personally, or second hand, an unsatisfactory experience with a lumber species other than what they are requesting. Usually this has nothing to do with the material itself, but is more a function of how the materials were handled or stored on the jobsite, than any other factor.

As far as strength comparing 2×6 #2 material of all four species, the Fiber Stress in Bending (Fb) values range from 1250 psi (pounds per square inch) for Southern Pine to 1105 psi for Hem-Fir. Roughly a 13% difference in resistance to bending forces.

In most cases, we design Hansen Pole Buildings to the lowest strength qualities of these four groups. By doing so, we eliminate the possibility of a distribution center inadvertently shipping a species other than what was specified on our plans.

In doing research, I learned something new. There is a direct relationship between density of wood and shrinkage values. Species with higher density shrink more than those with lower density. Lumber density is measured by specific gravity (G). The specific gravity of Southern Pine is 0.55, Douglas Fir-Larch 0.5, Hem-Fir 0.43 and Spruce-Pine-Fir 0.42. This is a spread of nearly 31%!

Even though the shrinkage values are relatively low, they still play a significant role in designing wood structures. If shrinkage of wood is not taken into consideration during the design stages, certain construction defects such as warping, cracking, and buckling may occur, lowering the overall quality of the finished product.

By using kiln dried lumber, Hansen Buildings minimizes any shrinkage issues with any lumber species they use.  Some our competitors use what is known as “green lumber”, which is subject to all of the unfavorable issues I just mentioned.  Bottom line, don’t worry as much about the species as you should about how to properly store and quickly use lumber.  And insist upon kiln dried lumber for minimal shrinkage overall.

Green Lumber vs. Dry Lumber

Green Lumber

Need a piece of lumber? In most of the United States, you get one from your local lumber yard or “big box” store and do not have a choice as to whether the lumber is “green” (moisture content of over 19%) or dry. For the most part, what is available at the retail level is a regional dictate.

Historically, the green vs. dry battle has been a point of contention.

A great deal of attention was given at the Forest Products Laboratory in 1946 to problems arising from the use of green lumber in building construction. Sharp controversy developed between the Laboratory and that portion of the lumber industry which customarily manufactured and shipped unseasoned (green) lumber.

The statement, since widely quoted, “we still have not learned how to build good houses of unseasoned lumber” was made in a Laboratory report which was later withdrawn. An extensive “Program to Reduce Use of Green Lumber in Housing” was planned at the Laboratory, but never implemented. Although size standards were not a major part of the controversy, shrinkage in service was given as the principal drawback to the use of green construction lumber, thus emphasizing the relation of size to moisture content.

At about the same time was the case of the home owner in Virginia who sued for damages resulting from the use of green lumber in building his house. The court awarded him some $8,000 damages, but the award was set aside on appeal to a higher court.

There was also sharp controversy about whether or not building codes could legally set maximum moisture content values in lumber used in building construction. The argument was advanced that health and safety do not require dry lumber, and the building law could not go beyond health and safety requirements.

You may ask… why is green lumber even used? It is less expensive to produce green lumber than dry. Green lumber is softer than seasoned wood, it can be cut more easily, is not as likely to split and nails drive into it more easily.

A number of problems can result from the use of green lumber. Nail “pops” – as framing members dry and shrink, gaps are created between nailed together framing members, as well as between exterior or interior sheathing and framing members. Mold can begin to grow on green lumber before it is even used in construction. Airborne mold spores are found almost everywhere, and they can easily cause mold growth on wet wood surfaces.

In exposed areas, green lumber can be difficult to paint or stain, sap within the wood oozes out and causes discoloration and gaps between members (such as fascias) can result.

As it dries, wood shrinks considerably, and is prone to both “warp” and “check” (crack). Used in construction, problems may arise including warping the underlying structure and causing structural instability. Unlike lumber which has been dried at the mill, green lumber has not been treated with any substances which are designed to promote water and insect resistance. Green lumber is more subject to rot, and it can be viewed as a buffet by insects.

More often than not, the use of green lumber for framing material comes from lack of knowledge by the end user. For buildings where the finished quality makes a difference, dry lumber is the only sensible choice.

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