dimensional stability of plywood panels

[Howard Skillington]

Please guys - no shooting from the hip here. Does anyone have reliable figures for dimensional change of furniture grade plywood? I have built lots of frames with solid wood panels and never had problems with seasonal wood movement. I have decided to use high quality 3/4" plywood for the panels on a mahogany bed. I feel safe in asuming that I don't need to allow as much room within the frames for wood movement as I would for solid wood panels, but would rather size them based upon tested numbers than hearsay or opinion. Any help would be appreciated.

[Buck Williams]

You might try the US Forest Products Lab web-site, they have a search function and mountains of data. I have also called the Lab and had good luck talking to an actual expert, they have probably done just about anything that can be imagined to to wood at one time or another.

[Jamie Buxton]

A google search string of "hygroscopic expansion plywood" brings up pages like http://www.performancepanels.com/single.cfm?content=app_pp_atr_dim1. On that is the line "The average coefficient of hygroscopic expansion or contraction in length and width for plywood panels is about 0.0002 inch per inch for each 10 percent change in equilibrium relative humidity." Bruce Hoadley, in his book Understanding Wood, observes that the typical EMC change of interior wood in the US is between 5% and 10%, depending on location. Rub those numbers together, and you're seeing really really small expansion/contraction in plywood. For instance, a plywood panel that's three feet wide would change less than 7 thousandths of an inch.

[Howard Skillington]

Thanks for the good suggestion, Buck. I checked US Forest Products Lab web-site and they've got tons of substantial publications available as PDF downloads. I didn't find what I need readily so left an e-mail request for information. I'll report back to the forum with the result of my query.

[Howard Skillington]

Your Google search string was better than mine, Jamie. (I tried plywooddimensional stability and it was too broad to be useful.) I have long been under the impression thatwood movement was a negligible factor for plywood, but it seemedcounter-intuitive to me that it could be small enough to be a non-factor. Your helpful reply suggests that I can goahead and cut my panels just a hair less than snug in the frames and have anice rigid assembly without worry about things trying to pop loose lateron. I appreciate your help.

[Frank Drew]

Howard,

I can't contribute any tested numbers but I'd feel confident that plywood panels wouldn't give you any issues in terms of expansion. My only quibble would be with the limited choice in terms of appearance in comparison to solid or shop veneered (veneered mdf would be very stable unless the bed ended up under water!).

[ian maybury]

I've wondered about ply too and used the Canply Plywood Design Fundamentals handbook which came up on Google. According to it the average coefficient of hygroscopic expansion in thickness is about 0.003 mm per millimetre of original thickness for each 1% change in moisture content - equal in both directions. It's a bit more than the lengthwise change in solid wood.

It'd be best to read through the booklet in case those numbers are dependent on factors different to your situation. e.g. exact ply type. There's other handbooks on the web, but this one is more accessible than the others i looked at.

Go to http://www.canply.org/english/, click literature and select 'technical values' in the drop down menu - it'll download as a .pdf

ian