Heavy timber trusses are very different from regular roof and floor trusses. Instead of 2" X 4" through 2" X 12" lumber, we commonly use lumber 3"X 8" thru 12" X 12" lumber. Lets bring this into perspective. A railroad tie is 6" X 8" and 7 feet 6 inches long. A railroad switch tie (the big one)is 7" X 9". Imagine we are making a piece of Williamsburg type furniture 20 to 30 feet long out of lumber that could be bigger than a railroad switch tie! Thus, the plates that connect the lumber are also big. Most commonly we use steel plates 1/4" thru 1/2" thick. We can use up to 1" thick plates. We will also be updating our sizing to the metric system in the near future.
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