douglas fir exterior plywood

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splasher

douglas fir exterior plywood

Post by splasher »

Is the doug. fir exterior ply-wood found at home depot or lowes sufficient for sheeting and decking?

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Graham Knight
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Post by Graham Knight »

What do you mean by "sheeting"? If you mean the hull planking I think pretty much everyone here would strongly advise against it. Exterior grade ply has voids in the core which create weak spots, and being sealed in you don't know where they are. You might get away with it for decking, but do you want to take a chance using inferior timber that your life, and others, may depend on one day?
Marine ply by definition is not allowed to have any voids for this very reason, BS1088 ply is approved by Lloyds insurers, they wouldn't touch a boat made from exterior ply, and your insurers may not either should you choose to insure your boat.
Are you having problems finding proper marine ply, or just trying to save some money?
Graham in Shepperton, England

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Dave Grason
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Post by Dave Grason »

Also, the adhesives used in exterior plywood is not marine grade. And I believe the number of plys is less with each ply being thicker than with BS1088.

However, on the other hand, when I was a kid, there was a neighbor of ours that built a rowboat out of exterior DF. He did a good job on it and it didn't seem to have any of the voids come up in a critical spot. When he got done with it, he painted it with porch paint and when he wasn't using it, he'd lean it up against a barn in the field behind the house. My friends and I all used that boat too, on the ponds and creeks and such. That boat probably lasted a good 10 years or more before he sold it. So I guess it's all in what you're expecting and what you're going to use it for. If your project is going to be a show peice, I'd say no. But if you just want something to knock around in and you don't care how long it lasts or how it looks, I'd say: "What the Heck?"
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kens
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Post by kens »

The adhesives in exterior grade are required to have the same boil test as marine grade. When you read the spec of the two, the difference is in the quality of the plies, and Marine grade is restricted to Western Larch and Doug Fir. Exterior may be any structural grade, pine and others.

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Graham Knight
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Post by Graham Knight »

the adhesives used in exterior plywood is not marine grade.
Actually Dave that may not be true, Exterior Ply in the UK is rated WBP (water and boil proof), so is Marine Ply, they both have to pass the same tests to be WBP.
The difference is in the quality of timber used, the lack of voids in Marine Ply, and sometimes the number and thickness of plys, although top quality Exterior often has as many plys as the equivalent thickness Marine.
This may be different in the US and elsewhere, I don't know what specs your Exterior Ply is made to?
Graham in Shepperton, England

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Graham Knight
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Post by Graham Knight »

Ah you beat me to it kens! At least we agree though.
Graham in Shepperton, England

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Post by Guest »

Splasher, I think the answer to your question would be:

In most cases most people here wouldn't risk using reg. DF instead of Marine DF.

But I think that depends on the boat you are building. A smaller row or sail boat would probably be fine. On a larger faster power boat, using a plywood with unknow voids or hollow spots would have some risk. If I would do it over again, I would use reg. DF everywhere except most structural members and the hull planking and save $30 a sheet.

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