Moisture content for construction lumber

[Lee Schierer]

For you professional carpenters and contractors. I am reviewing a specification for a construction project where lumber framing may be involved. What is considered to be the acceptable maximum moisture content for construction lumber for framing by the construction industry? Cabniet makers normally want wood at 7-9% MC but I don't know what is standard for construction specifications.

Is it the same for plywood? Does treted lumber have a different MC level than softwood untreated lumber?

[Jamie Buxton]

The acceptable moisture level of construction lumber varies around the country. Here in California, most framing is done with green lumber, undried in any way. You can buy kiln dried construction lumber. However, it is dried only to the point that mold won't grow on it. That's generally 19%.

Here at least, pressure-treated lumber is always soaking wet.

I've never checked the moisture level of construction plywood. My sense is that it is dryer than framing lumber. I think the glue doesn't bond well to wet wood, so they must dry it as part of the production process.

[Todd Burch]

Generally dried to 19% here too, but often left out in the weather as well, especially when on the jobsite.

Todd

[Greg Cuetara]

Just as an FYI most of the tables in the codes which give maximum spans and allowable loads etc. work off of a MC of less than 19%...if you have more than 19% MC you need to take reductions in allowable loads and maximum spans.

[Ben Grunow]

I think all kiln drying aims for 19% and then lumber is shipped in open rail cars and stored outside at most lumber yards so you get what you get. Remember though, once it is nailed up it has air on all sides and dries out quickly. I dont think the MC is really important except that the lumber should be kiln dried so it will not twist too much more as the wetting/drying cycle continues.

[Bob Michaels]

Lee, I have to agree with Ben. The reality is you're going to get the lumber by the truck load and it's going to be what it's going to be. Sure, you'll send back a few pieces that are totally unusable, maybe. If you frame the floor deck on a sunny day and the next five days are all downpours, well, that shoots the heck out of any moisture content specification for the the wall framing material that's sitting out there. Sure you can cover it, but it's still going to be wet. In the northeast I have never seen a MC spec that was worth writing, simply because nobody considers it a practical issue. Unless this project is something that really requires tight framing tolerances, I don't believe a MC spec is a good use of time, since it is probably uncontrollable at best. If the project IS something that requires really tight framing tolerances perhaps it should be framed with steel studs.