Moisture Content/Wood Movement Calculation (Coefficients for dimensional change)

[Dan Barr]

Just FYI (and for easier searching i've named the thread with the appropriate keywords)

Here is a useful link i found that has a couple of great charts for calculating wood movement.

http://www.fpl.fs.fed.us/documnts/fp...tr113/ch12.pdf (http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch12.pdf)

Page 4 has the average moisture content for wood exposed to outdoor conditions listed by state and for each month!!! :)

Pages 16 and 17 have the coefficents for dimensional change for each type of wood.


To calculate wood movement you can multiply the width of the board (in inches) by the annual change in moisture content (high minus low). Then multiply that by the number from the chart.

Example:

You have a 30" wide table top of quartersawn (Q/S) white oak. You want to know how much movement to compensate for. You assume a 3 percent moisture content change per year (8% to 11%) based on the numbers on page 4.

11% minus 8% = 3

The coefficent for Q/S white oak is .00180

The width of the board is 30"

Now you plug this in to the equation.

Width x MC chg x Coefficient for dimensional change
30 x 3 x .00180 = .162

.162 = 2.58/16" (You could call this 5/32" or 3/16" to be on the safe side.)

V/R

Dan

[Gary Keedwell]

:eek: :eek: Your making my head hurt:cool:
Gary:)

[Brian Weick]

I think that is great! You always can never get to much information about woodworking- that is ~ if you love what you do :)
Brian

Thank you for the post.:)

[Wallis Hampson]

Thanks for the information. Especially useful when choosing materials for projects like workbenches with massive tops where wood movement is critical.

[Howard Acheson]

It's much easier to go to www.woodbin.com and click on the "Sagulator". All you need to do is input the species, the width of the board and the starting and ending moisture content or relative humidity. The output is the expansion/contraction.

[Dan Barr]

Howard,

I'm not seeing an option for calculation of wood movement. The sagulator specifically says that it will calculate deflection. I dont see any way to input moisture content change either.

Also, the "shrinkulator" does not calculate wood movement either. it calculates shrinkage from drying.

V/R

Dan

[Andy Haney]

Thanks for the information.

Andy