3/4'' or 5/8'' plywood? - Mountain Buzz

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Old 03-13-2011   #1
Boise, Idaho
Join Date: Feb 2011
Posts: 427
3/4'' or 5/8'' plywood?

Needing to make some decks for the raft frame. They will be seats/drop bag covers. The front will hold 1-2 passengers and the rear will have to hold my 240 or so lbs. 3/4'' will be strong enough but I'm wondering if I could save a little weight or money if I go thinner. I have also seen 23/64'' and other oddball sizes. They will be 66'' long and I'm not too sure on the width yet. Would 18" be plenty wide for a table/backboard? Need help on thickness and possible width(s)...

Got to hit the river today! First time since November

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Old 03-13-2011   #2
wildh2onriver's Avatar
Glenwood Springs, Colorado
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5/8 should work well, very strong. Consider using a marine grade/exterior plywood, the glues are water proof.
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Old 03-14-2011   #3
Steamboat Springs, Colorado
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3/4" Plywood may be better because it is less likely to warp. Generally 5/8" (19/32) has 4 or 5 plies while 3/4" (23/32) will have 5-7 plies. The more plies, the more stable the plywood is and less likely to warp. I have used 3/4" AC plywood for all of my rafting parts. It has a nice clean veneer on one side and seven plies.

Don't waste your time with marine plywood as it is not commonly available and it is very expensive. Any exterior grade plywood will have the same waterproof glues. The most important thing it to get a good seal on the plywood. I use five coats of exterior grade polyurethane on my tables. The exterior PU has a UV blocker which is essential for rafting parts. On my frame decks I also applied a layer of 6oz glass to the plywood before sealing. That way I have a nice thick wear layer to protect the wood from sliding coolers and sandy feet. The glass also keeps the wood from being slippery without adding sand to the finish coat.

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Old 03-14-2011   #4
Jenks, Oklahoma
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My experience with cataraft floors made from plywood is marine grade plywood is worth the extra cost. 3/4 inch for my use was overkill since I had plenty of crossbars basically one in every compartment.

I went thru several floors of the "green" exterior grade plywood before getting marine grade which has been pretty much bomb proof.

A good grade marine grade polyurethane finish, touched up each year will last for many many trips.
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Old 03-14-2011   #5
Steamboat Springs, Colorado
Paddling Since: 1986
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Marine Ply vs Exterior Ply

Marine Plywood and Exterior Plywood have a few minor differences. Both products use the same adhesiveto bind the layers. The biggest differences have to do with the plies. Marine plywood is not supposed to have any voids in the plies where as exterior grade plywood often will have voids within the plies up to 1.5" in diameter. Also, exterior plywood usually has fewer plies than marine grade. This is especially true in CDX grade of plywood. For 3/4" nominal material CDX has 5 plies, ACX has 7 plies and marine plywood has 7-9 plies depending on the manufacturer. I would not recommend using CCA treated plywood for any boating projects. It is almost always CD grade and is VERY heavy because it is sealed with CCA compound. It also is not save to have around food and splinters from treated plywood are toxic. Stick with higher grades of exterior grade or marine plywood for the best results.

FWIW, I work for a lumber yard and I am often asked to get marine plywood for clients. In Colorado it is only available in 3/4" paper coated (the exterior finish is actually a paper product). IT goes for about $100 a sheet compared to about $35 a sheet for ACX.

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Old 03-14-2011   #6
Golden, Colorado
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Does anyone know where to get marine grade plywood in the Denver metro area?

A rafting buddy of mine got his at Austin Hardwoods of Denver, Inc. but I'm hoping to find something more on the west side.
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Old 03-14-2011   #7
Longmont, Colorado
Paddling Since: 1980
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I was at my local big box store this weekend and they had 3/4 oak, and birch for the same price as the pine (~40 a sheet). all three were sanded on on side, and looked to be the same number of plies. I was thinking of using the birch to build some decking and tables. Any advice to use one of the others instead?
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Old 03-14-2011   #8
Boise, Idaho
Join Date: Feb 2011
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Thanks for the info. everyone. I just talked with my uncle and he explained the acx and cdx terminology. X stands for exterior and/or waterproof glue while the other letters stand for grade(quality). My brother also said 3/4 would last longer than 5/8. Good posts boatdziner
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Old 03-14-2011   #9
boulder, Colorado
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I've had wood decks for years and have tried several versions of plywood. Currently I'm using some 3/8 AC with a layer of fiberglass cloth layed up on each side and coated with three coats of epoxy. It is very strong and lighter than the 1/2" ply I used to use. The span between the frame rails is probably 26" or so. I walk on, sleep on it, carry two or three passengers sitting on it at times. The epoxy coating needs to be recoated probably every 120 river days or so. Nice thing about epoxy is it dries hard in about 24 hours, builds faster than urethane, and can be recoated in a few hours for multiple coats. I've used West Systems but currently using MAS low viscosity resin. It's a little cheaper. Neither one of these coatings advertise UV resistance but I haven't had any issues with UV failure over the years including several extended Grand trips. Just my 2 cents( or plys as the case may be).

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Old 03-14-2011   #10
Steamboat Springs, Colorado
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Glad to hear that others are using fiberglass in their deck layups. I wonder why others don't do it more. One layer of fiberglass is about 1/16" thick and can then be coated with extra resin as Rando points out. I have often wondered about the use of resin without a UV topcoat. When we built fiberglass boats we always used gelcoat on the outside of the part jsut to provide protection from UV rays. Exopy is notorious for yellowing when exposed to lots of sunlight. Glad to hear that it holds up well on your decks as I have just applied a similar coating to mine. You can always apply a coat of exterior PU over the glass if you are really worried about the UV rays. Doesn't sound like it is a problem though.

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