At first glance, timber frames can look alike before installation on your foundation.  However, timbers can shrink, twist, check, and release sap.  That is why it is important during the design phase to discuss the pros and cons of different material standards with your timber frame builder.  The standards you set with them will affect the cost and how well the frame ages.  In some cases, it is significant as 30%.

In this blog post, we will envision a client that wants a traditional frame made from Douglas fir that has tight joinery and a clean finish.  In this scenario, our goal is to use stable timbers to limit changes in the wood and the joinery over time.  The material standards that will primary determine this client’s satisfaction will be the grade, cut, and moisture levels of their timbers.  Below are recommendations for them.

1.  Grade
You may have noticed from trips to the lumber store all wood is not created equal.  The quality of the grain (spiral or tightness), wane, knots, or checks varies between timbers.  We use the NLGA Grading Rules that ranks Douglas fir timbers from No. 2 Structural to Select Structural, to B clear and better.  The chart below shows a few differences between two grades on the face of a 12” Douglas fir post.

Recommendation: We recommend this timber frame home buyer purchase timbers that are Select Structural or better.  We have found it difficult to control results when the timbers are below these grades.

2.  Cut
Another consideration we make is whether or not to use free of heart (FOH) or boxed heart center timbers.  A boxed heart timber contains the center of the tree while a free-of-heart timber excludes the pith (soft center). Free of heart timbers check, warp, and twist less because the edge grain shrinks less than face grain on boxed heart.  The image below compares a boxed heart timber to a free of heart timber (not quarter sawn).  You can see how the checks (cracks) in the boxed heart are visible from the outside of the post.  The free of heart timber will show no visible signs of checking when stood upright.

Recommendation: Free of heart timbers are best suited for this situation.  Please note that companies sell different kinds of FOH products.  It is best to specify quarter sawn FOH.

3.    Moisture Content
Because it takes timbers 2-5 years to air dry, many people purchase “green” timber frames that are manufactured before the timbers stabilize.  An alternative is to kiln dry the wood before creating the joinery.  The benefits include:

• Wood will release sap until the moisture content drops below 20-25%.  Most kilns will decrease the moisture levels to or below this point.  As a result, the sap crystallizes inside the timbers instead of forming on the surface.
• Joinery remains tighter.
• The weight on the frame will remain as engineered.  As green frames dry, weight can be transferred from posts to the dowels holding up the beams or knee braces.  
• Because most of the checking has occurred, the worst cracks can be hidden during production.

There are different kinds of kiln drying technology.  For a tight fitting frame, we recommend timbers be radio frequency kiln dried rather than conventionally kilned.  Not only is the evaporation process less disruptive to the fibers, but the moisture level are lower and more consistent.  In the chart below, we compare the gapping that occurs years after installation between a knee brace and a Douglas fir post.

Recommendation: Since the envisioned client wants a more refined look, we would recommend drying the wood in a kiln before the frame is built.  The price for a radio frequency kiln dried frame will add approximately 20% to the entire contract.

Summary
It is possible for the price of a timber frame to fluctuate 30% based on your style and how gracefully you wish your frame to age.  In the case highlighted above, it is important the frame be made from free-of-heart, select structural timbers.  To reduce gapping and the appearance of sap, they should also radio frequency dry the frame before the manufacturing process.

Please contact us for more information or to discuss the construction of your future log or timber home.