what's the difference between 'regular' and 'marine-grade' plywood? Do they sell the latter at Lowes?

Marine grade is a superior grade to "Treated" plywood which is rated for water exposure or direct earth contact. Marine ply is rated for water imersion AND it has NO voids in the plywood layers. This increases it's strength in boat building applications and decreases the possibility of water penetration. I don't believe Lowe's carrys it as a stock item. I suspect it is special order only. Prepare yourself, it's not cheap!

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RE:

what's the difference between 'regular' and 'marine-grade' plywood? Do they sell the latter at Lowes?

Marine grade is made with a glue that stands up to moisture. Regular plywood will delaminate (layers will separate) when it gets wet.

I would assume that Lowes does have marine grade. You will have to ask for it.

Marine grade can get wet without warping, the marine part doesn't mean military, its refering to water, they use it on boats. The regular kind of plywood warps when wet. Good luck.

Lowes Cement Board

Jim N gave the only correct answer. And, Lowes will think you mean treated plywood, and say "sure, we have it on sale for $39.99".

It is a specialty item, and will will cost between $75 and $100 per 4x8 sheet. You could ask your local marina if they have a source. Else, you could look for specialty plywood distributors in your area. Or, call a local REAL lumberyard, if there is one left.