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Dang, I'd never heard this before. I wonder if that's why the existing bolts are petty much gone. Though we're all familiar with Roger Mac's tendency to cut corners and go cheap, I have to assume the original bunks were PT or they probably wouldn't have lasted this long.

My parents have a deck we built with 5/4 PT and galvy nails 1985 and it's still doing fine. I'm pretty sure the PT was CCA, which I believe is now banned. I'll have to check and see what the new stuff is I bought for the trailer.

I'm not too concerned about the bolts I'm putting in; since I'm upgrading to 5/16 galvy I think they'll still outlast the trailer and bunks both. But what about the supports the bunks are bolted to? They're mild still and obviously not galvy; I believe they're only 1/8 or 3/16" thick; thinner than the bolts, anyway. I was planning on painting them when I had the bunks off, so I guess I'll have to give them an extra coat before I install the new boards.

To bad they don't make that engineered plastic deck lumber in the 2x4 size. I've used the 5/4 in a few other locations on the boat. You only need 14' for the bunks so it would probably be worth the 2x cost compared to the PT. Oh, but then you probably wouldn't be able to staple carpet onto it.

You could always glue the carpet down...

Hey Chip, you can get the decking in 2 x 4 (1.5 x 3.5) from Trex. It comes in 12 and 16 lengths. I have never looked at the base for the bunks, is it continues? In others words is the bunk supported along its entire length, If so it that might be worth trying out. Its funny how they do the test but yet you can find examples like your parents deck. I am not too concerned about the bolts, but upgrading to 5/16 is a good idea. For additional protection between the PT & support, you could place a layer of building felt or spread a layer of Butyl rubber from a caulking gun tube. Let it dry before securing the bunk. Ill have to check my bunks out to see how they are doing.

Graham

Down here an alternative is to use Cypress Bunks instead of PT. Cypress is locally harvested and naturally water tolerant, lightweight, but not cheap. I'd imagine as it would be expensive as composites though.
Kind of herecy to put material that good on a Mac trailer, then you'd have bunks that would probably outlive the trailer.

I've actually asked several local places which carry the 5/4 stuff about getting it in 2x, and they've all told me it's not available, and that everybody uses standard PT for everything other than the decking. If I can get it I may look into the 2x, though I'm still not sure you can use staples on it, so attaching the carpet would present problems. In answer to the question, yes they are supported over their entire length.
I suppose outdoor carpet adhesive? Heck, I've already paid as I recall $8 for a box of SS staples.

It occurs to me you could wrap the carpet all the way around the PT bunks and it would provide a separation between them and the trailer. metal. The standard (expensive) bunk carpet from WM is 11 inches wide and it only takes about 9" to wrap all the way around a 2x4. But if the PT is actively corrosive, can it leach the acid right through the porous carpet and attack the metal anyway?

This is all starting to hurt my head.

When I replaced my forward bunk, I used ACQ PT 2x4, 1/4" SS carriage bolts and copper tacks with plastic pipe strapping to hold the carpet on. I reused the old carpet and then covered it with new carpeting from BoatUS.
Short Ceramic deck screws could be used in lieu of the tacks, but may pull some of the threads on the carpet.
Greg.