Question about plywood and warping...

[myoung]

I bought six sheets of birch ply, let it sit in my garage for a couple of weeks, then eventually cut it down into strips roughtly 12x94.

a week later, I got around to working with those strips and found a number of them to be warped to the tune of about 1/4 to 1/2 inch, maybe even a little more.

I'm wondering if this is normal behavior? I thought that plywood was supposed to be dimensionally stable and not prone to warping.

one thing I did note in this stuff is that, a couple of the cuts show that the plywood has irregular plys, ie, they're not all straight if you look down the edge of a cut side. thereare places where one ply had a void, and the next ply seemed to sink down a bit to fill it. the exterior still seems flat and stable, but I'd never seen this sort of thing before...

any comments appreciated.

...myoung

[JAW]

Seen the same sort of ply structure when purchased from the big boxes (Lowes, HD). Their supplier has a ineffective QC department. Of course these were about $40 per sheet whereas the local hardware store was charging $60 per sheet. I got what I paid for. ;-(

JAW

myoung wrote:

[Mike G]

Naturally I don't know what happened with the plywood once you took it home but I have to opine that..............................

Plywood is essentially stable however that doesn't mean it can be left leaning against a wall or in any other way improperly stored, supported or in a less then ideal environment and not take a set from it's own weight. That holds triply true once you have made small relatively narrow strips of it and rob it of a lot of it's internal strengths. We are not talking steel I beams here.

[Pat Barber]

Depends on two things:

(a) How the store you bought it from treated it. (b) How you stored it after you bought it.

Standing plywood up against a wall(most people do it) will result in a warped/bowed sheet.

It was probably warped/bowed a wee bit when you bought it.

I suspect you are talking about bowing, not warping. As you stated, plywood is stable, but it's wood and it will move and sag/bow when stored.

myoung wrote:

[Unisaw A100]

Yes.

It is fairly dimensionally stable in the X and Y axis but not in the Z.

UA100

[Leon]

How did you get 94 inches out of a sheet? Did you cut diagonally?

I have not seen any plywood that will not warp to some extent. Plywood is dimensionally stable in that its length and width does not change. Thickness will change with temperature and humidity. Irregular thickness changes cause the plywood to warp. For best results, DO NOT LAY plywood on a concrete floor when storing and do not store it on edge.

[Leon]

PLEASE disregard the following ignorant questions.... I musta been thinking ...who knows what I was thinking.

[Edwin Pawlowski]

Hey, at least we got a chuckle out of it. Maybe is was the Baltic birch 5 x

5 stuff.

BTW, I'm going to sell some tape measures on eBay, but I'll offering them here first. They measure a full 92" or eight feet, whichever comes first. Ed

[myoung]

Just to follow up on the thread...

the plywood was layed out in full sheets on a set of 2x4s on my garage floor for about three weeks. one of the sheets "bowed" a little (thanks for the clarification). the other sheets looked ok. I cut them down into 12" strips and layed them out again for a couple of weeks (much to the dismay of my girlfriend, who's waiting for the end product). when I got back to them, I noticed the bowing in a number of them.

to address another observation, I guess I too got what I paid for...the couple of sample sheets I saw looked like they had more plys than the 70$ sheets, and had two good sides of veneer, while the $70 stuff had one clear white birch side, and a second side with a little more color. I paid about

43$ a sheet... sigh...

live and learn. shame too...since I sort of liked buying from the smaller place...

...myoung

[Nova]

I'd say that moisture evaporating from the ground through the garage floor was absorbed by the face of the plywood facing the floor. This caused that side of the plywood to expand causing the bow.

-- Jack Novak Buffalo, NY - USA (Remove "SPAM" from email address to reply)

[dpsours]

The latest issue of "Fine Homebuilding" magazine has a brief article about how to unbow plywood. Using a spray bottle of water, wet down one side of the plywood, then place it on your driveway in the sun. Unfortunately, I don't remember which side you're supposed to spray or which side goes face down (the wet side, I'm pretty sure). I do remember you need to monitor it closely or it'll bow the other way.

You could either buy the mag or experiment with one piece. If the wood is unusable as is, you;ve got nothing to lose.

[Phisherman]

My father straightened cupped or twisted doors with this method, except he placed the door on the grass in the sun for 3-6 hours. The side you wet goes against the grass, placed in the sun. The side you wet will expand and swell--that will clue in on how the board will bend. I've used this method and it works (surprisingly) well.

[myoung]

seems unlikely...the weather here is pretty dry...and it probably would have only affected the lower must sheet...no?

[Nova]

Survival training teaches you can trap water by collecting moisture evaporating from the ground even in desert sand.

-- Jack Novak Buffalo, NY - USA (Remove "SPAM" from email address to reply)