Alternatives to marine ply: Birch? Cedar?

Par

Your the one who does not know wood or rather you only know what the books tell you and have no on the job experience. There are absolutely no interior defects in underlayment grade ply and all 5 plys of 1/4" plywood are equal in thickness. I have over 50 years experience working with it. How much experience do you have? I have probably made a 100,000 cuts thru the stuff and never found a defect or void in 50 years. Don't tell me I don't know what I talking about. Stick to your drawing board !

Have a great Day
Earl

 

Par

By the way, PAR, you are the only one talking about construction grade plywood. If you read my post carefully, you will note that I was only speaking of underlayment grade plywood and made no mention of construction grades.
Have a great Day
Earl

 

Underlayment is a construction grade Earl. I'll match my job experience against anyone's after over 30 years. Underlayment is better then sheathing panels of construction grade plywood, but it's still got all the other issues.

If you're believing what the APA is telling you on a stamp or label then remember the APA grading system is voluntary and subject to each manufacture's every whim. I've many used hundreds of sheets of plywood over the years and American panel construction has continuously declined in quality in the last two decades. MDO used to be good stuff, but no longer, them same was true of some sheathing and underlayment panels, but not any more. Unless it's a special order, you can't rely on any of these construction grades any more.

This isn't a debate, most in the industry well know this simple truth, though you do seem to be bucking a clear and well documented truism.

 

This has got me curious about the underlayment as a paneling material. Maybe I'll make a table for the patio out of the stuff and see how it weathers.

 

Underlayment is a much better sheet compared to "sheathing" panels, so your table will be well suited with it. Underlayment is designed to lay down and not move, so it preforms well in this role. The moment you bend it, ask it to tolerate dynamic loading or subject it to tortured load bearing or expect sectional stiffness comparable to real marine grades, well, this is where you'll be disappointed.

 

Underlayment Examples

Here are a few pictures of underlayment as it is revealed in a phototype project I'm working.

Image one has a void in the central veneer of lowest of five layers of ply.

Image two has a void in the central veneer of the second lowest layer of ply.

Image three has a void in the central veneer of the top layer of ply.

Image four is the crack in the outer veneer layer that occurred when attempting to bend it to deck curvature.

I'm using underlayment because I'm not really concerned with the longevity of the boat. I'm developing a design and use it to meet other design objectives, but it does happen to give a valuable insight into the the construction of under layment material.

If you look closely (image one is the best for this), The top two layers of ply are 3 veneer ply. It is composed of a central thick core with thin surface veneers. This was the cause of the stress crack in the last picture. That and poor clamping procedure on my part.

The bottom three layers of ply are five veneer ply. The central core layers (3) are thick compared to the the suface veneers. Certainly, the five layer ply is going to exhibit better strength and build characteristics that the 3 layer ply. The surface veneers in either of these examples offer little in a structural sense and is there strictly to cover the inner cores.

The main point here though is that there is always the possiblility of voids in this type of a product. The voids as seen here aren't necessarily large, but they create discontinuities in stress distribution that could lead to component failures. You have to know what you are using.