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The California Residential Code (CRC) 2019 has a table of header sizes for various spans / building widths / roof loads: https://up.codes/viewer/california/irc-2018/chapter/6/wall-construction#R602.7

The size column list various combinations of double, triple and quadruple 2x materials (e.g. 2x8, 2x10, 2x12).

Why are there no 4x options (e.g. 4x8, 4x10, 4x12)? Does a different section of code cover that, and do they have different allowable spans?

Context: my local building department suggested I revise my plans from 'double 2x12' to 'single 4x12', and said "like the CRC requires". I'd like to comply with their wishes, but I'd like to fully understand them by reading them in the code. I'd prefer to use double 2x, as it's easier to get in place.

tom
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1 Answers1

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I can't speak to the CRC, so I'll respond more generally.

Four-by lumber is typically far more expensive than combinations of smaller lumber due to much lower sales volume, so LVL and other more stable and precise options become favorable.

Large timbers typically aren't milled to a precise size like standard lumber, nor will they usually be planed for safer handling.

Large timbers are much less stable than beams built from multiple components, so warping and checking are more likely.

Single-member beams aren't rated for as much load. By using multiple components the risk of weakness caused by a knots or other flaws is lessened.

Four-by timbers would need to be lifted into place as a unit, requiring more people or equipment. Beams constructed from several smaller members are more manageable.

For these reasons four-by timbers have become passé as beam options. It could be that they're simply not included because they're rarely considered appropriate.

isherwood
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