Wood Moisture Content Specification

[keyPitsimplE]

A continuation of thread337-225812...

I have typically spec'd all covered framing lumber to be kiln dried, or have max. 19% MC. 6x and pressure treated material can have a higher MC, which can lead to pretty noticable warping and shrinkage. Is my spec of max. 19% a reasonable spec for residential and some light commercial construction? I just got my first complaint about it in 8 years.

Thank you in advance for the input of the seasoned veterans out there.

 

[msquared48]

It's not only reasonable, it's the standard of practice for wood design and construction.

Stick to your guns.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

Check with the Inspection Bureaus but I beleive you are correct. I think I usually spec 15%.

That said - I have found kiln dry delivered wood to be higher than 19% and if it sits around (say on the ground) for any period of time or subject to rain - all bets are out the window.

 

[keyPitsimplE]

Thanks guys.

The only thing I found in NDS is "in service" moisture content with respect to adjusting design values. Nothing about requirements for during construction. I have heard it argued, and reasonably so, that spec'ing moisture content is useless due to building outdoors, shipping methods, pressure treated wood, lead time and expense with KD, typically green 6x mat'l, etc.

I have a note that says "the builder shall reject all poor quality lumber that is not suitable for its intended purpose, regardless of grade." Although a bit ambiguous, that should at least save me from the warped, split and twisted stuff.

 

[Ron]

Allowing higher moisture contents at the time of construction leads to numerous problems. Keep it low.

Agree with both of the "Mike's" comments.

 

[Lion06]

I have a residential project on the back burner where lumber was taken from where ever they could find it. Some had low moisture content, some had very high. What this resulted in is a variation of joist depths from 10.5" to 11.25". When the bottom of joists are at the same elevation, that makes for a pretty uneven floor! They are now paying quite a bit of money to remedy the problem.

 

[msquared48]

And that's exactly why a lot of engineers and contractors use TJI's for residential floor joists!

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

Bingo!!! And in kitchen and bathroom walls - they use engineered lumber. Cabinents go up so MUCH easier and NO call backs.

 

[boo1]

If you want staight wood walls consider ordering number 1 grade, whole bundles of studs and culling your lumber...

 

[rscassar]

I specify that the timber used is seasoned and that the moisture content is less than 15%.

So under what circumstances would it be acceptable to specify "green" or unseasoned timber?

 

[keyPitsimplE]

It seems the consensus answer (among engineers here) for using unseasoned timber is "never". However, at least in the Northwest, that is what you get when you pick up 4x and 6x lumber at the yard, unless you pay dearly for a special order. It seems most 2x mat'l is kiln dried, so that spec may work OK.

I think the best answer may be to state that the lumber must reach min. 19% (or 15% to be safer) prior to building enclosure. Apparently that is what California does. Including a statement like "the builder shall reject all poor quality lumber that is not suitable for its intended purpose, regardless of grade" may help cover designer liability.