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Thru Bolt Basics for Log Homes

Log home manufacturers may use the term “thru bolt." For the log home manufacturers that do use them, you may wonder what it is & what it does.

By Donna Peak

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We spoke with Doug Parsons, president of Appalachian Log Structures in Ripley, West Virginia, to get some answers. 

18“The reasons to include thru bolts in a design can differ between companies. We started using thru bolts in 1983, because we found that the top courses of logs seemed to separate more between each course than the bottom courses did,” Doug explains.

“We also noticed that the door and window jambs added additional resistance to a structure’s natural settling compared to areas of the wall that had no openings. Our remedy was to insert a thru bolt (into a hole drilled as the logs are stacked by the contractor) at each side of a log wall door/window opening and at each side of a corner in the log stack. If there were no openings in the wall, we added a thru bolt about every six feet. ‘Spring-loaded thru bolts’ significantly improved the final and desired results.”

Doug and his team found that spring-loaded thru bolts, which feature a high-tension spring (pictured above) at the bottom of the bolt, squeezed their logs together. The tension continues to pull on the logs as they give up every last bit of moisture and settle into their new role as a log wall. They also helped to keep the logs straighter and tighter.

“Once the thru bolt and springs are installed correctly, it is not necessary to retighten the bolts,” Doug says.

Long Canadian winters make building an air-tight log home even more imperative, which is why Ontario-based True North Log Homes looked to take the spring-loaded thru bolt idea even further.

“Our latest method is called our Log-Lock compression system. This patented mechanism prevents the logs from crowning or leaking air,” says the company’s marketing manager Mark Wrightman. “As the logs shrink and the spring expands, it pushes the Log-Lock down, continually locking itself into place, which keeps the logs from separating and maintains air tightness. Home owners don’t have to do a thing.”

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Other reasons a log system may use thru bolts are:

  1. As the primary log connection and structurally connect the log walls to the foundation.
  2. To add lateral strength to the log wall between courses.
  3. As a required engineering specification in parts of the world that are prone to earthquakes.

If the manufacturers you’re considering start talking about thru bolts, ask them how they benefit their construction system. One system isn’t necessarily superior to another and companies that use them choose them for different reasons. The important thing to note is what structural value they’d add to your home.