Cisco Kid
Now, is marine grade plywood any superior in terms of shrinkage? I wonder if it would be worth it to spend the extra for the humidity resistant glue in the marine product?
Back in the bad old days, not all plywood glues were reasonably waterproof. Regular plywood would delaminate (the glue would let go) when immersed in water. Exterior grade plywood used glue similar to that of marine plywood, that would not let go in water immersion. Nowadays, virtually all plywood uses waterproof glue.
Marine plywood has to consistently meet all its specs, as it is usually used as a visible part of an engineered structure. Voids (gaps in the inner plys), knots, and the actual wood used in the plywood are tightly controlled. In almost all cases, marine plywood has to have at least one A surface and one B surface (filled knot holes are allowed in a B surface).
Contrast the quality of marine plywood with CDX, which is used in construction as a support surface where it will not be seen. Neither is wrong, both are engineered for their uses.
The more layers (plies), the less likely the plywood is to warp or change dimensions from varying moisture contents, as the plies "fight" each other. This is very similar to the way using regular lumber in an L or T girder uses the different cross grain alignments to prevent and reduce warping. Voids and knots in plywood also affect susceptibility to change from varying moisture content, as they create areas of non-uniform moisture content.
So, yes marine plywood will have a little more dimensional stability and much improved resistance to warping than the same thickness in CDX.
The worst cases I know of is plywood in variable humidity areas like the Gulf Coast. There, I have been quoted expansion figures of 1/4" for an 8ft sheet of plywood from when dry. I personally have seen 1/8" shrinkage of an 8ft sheet CDX plywood from its wet or green state at the lumberyard while in storage at my house. But I can't afford to store marine grade plywood to check! Humidity at my house runs from 40% in the winter with the humidifier on (and almost nothing outside) to 10-20% during the summer (no air conditioning at 7800ft). That is a big change from the environment of the saw mills and plywood and veneer plants, so letting wood "season" gets the changes over with.
The real issue for me is warping due to uneven drying. I have to make sure my storage and use prevents warping, especially when using grades like CDX.
Unless you live in a highly variable humidity environment, letting plywood "season" for a few weeks before using will reduce any dimensional change to negligible.
just my experiences
Fred W
