Home Improvement--General/Post & Beam joists...



We have a post and beam gambrel built in 1978.  The floor joists on the first floor ceiling (that support the second floor) are quite checked and I wanted to double-check strenght tolarances; however, the tables I find are only for 2x lumber. These 4"x6" (true) spruce joists span 11' 1", and are spaced 22" center-to-center. A possible relevant issue is that (unlike say a 2x12) the entire 4x6 dim. of beam does not attach onto the relevant beam; rather, wood is cut away to form the male component of the joints such that about 3x5 of these joists lays into beam. Does that mean, for strenth purposes, that I have 3x5.25 beams with above spans etc. or 4x6?  Either way, do these meet code requirements for loads? I live in Massachusetts. Thanks!


I can not provide definitive structural information via Internet. I might still be hard pressed given the heavy timber construction you are describing. I would necessarily have to get in to calculations and specific building conditions.

You might look at finding a heavy timber construction company that has expertise in heavy timer construction.

Things to have in mind are that the stresses on the wood joist are different at different locations along its length.
The end takes shear and the middle takes bending. Different species of wood will have different strength characteristics based on quality and dimension and single for repetitive use.

For the end bearing there is compression parallel to grain and shear across the bearing cross section. So the load at the end is sitting on a 3 1/2 x ___ some surface under compression load and the 3 1/2 x 5 1/4 area is under shear.

You mention sever checking and that would be evaluated to determine grade.

A lowest grades of all species in the standard joist tables do not span 11 feet. So you may be on the edge of something meeting todays compliance standards.

To start, look into lumber grading methods and the species and quality of wood you are working with.

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Thomas Buford


Licensed Architect in Virginia since 1984.
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