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Old 07-22-2004, 02:39 PM   #1
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1967 17' Caravel
1968 24' Tradewind
Northborough , Massachusetts
Join Date: Aug 2003
Posts: 1,061
Exclamation Plywood underlayment information

In assisting our son on the restoration project of his '68 Tradewind, we have stripped down to the frame because the floor was dry rotted front and rear. Now comes the hard part...what to put back in for underlayment that will be as sturdy and moisture resistant as possible. The '68 Tradewind plywood underlayment measured 3/4"thk.

We gave up on the moisture resistant properties because that required chemically treated wood. We were left with
two choices after research with the APA, engineered wood council, (see resulting in a choice between A-C Underlayment or A-B Marine Grade plywood. Neither is treated with chemicals.

The new chemicals ACQ and ACZA will actually corrode metals, aluminum and should not be in contact metal. A definite no-no for the Airstream. (Note if using in construction you will need stainless steel fasteners or double dipped galzanized, not electroplated galvanized either. see

A-C Underlayment is dense and tight without knots, which is important under resilient flooring, as any compressions or knots will telegraph or show through the vinyl or linoleum flooring. The veneers used are C grade.

A-B Marine Grade is the most dense and without voids, with face veneers of A grade woods. It will react well to movement of the rig, is flexible and will hold best to fasteners. This would be the more expensive choice.

The exterior grade sheathing plywood has too many voids and knots for resilient floor installation.

We are considering painting or sealing the underside of the plywood with sanded side up for finish floor.
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Old 07-22-2004, 02:51 PM   #2
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Love It Here , Colorado
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Pam - nice of you to add this bit of info. We just patched a spot under our WH w/ A-C grade plywood, but might consider the Marine grade if replacing large portions. Another thread did address the chemical side of plywood and sealing and made mention of a product called RotDoctor used in boats that does not have chemicals that would be potentially dangerous to inhale. Not sure about danger to metal tho.
Leigh (IB Aluminated)

"When a great ship is in harbour and moored, it is safe, there can be no doubt. But that is not what great ships are built for." by Clarissa Pinkola-Estes, author of ‘Women Who Run With the Wolves’
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