Need help with a joint problem???

[SHNOOGANS]

Hello,
First of all, is it possible to purchase a sheet of 1/2" 12'X12" or 12'x8' treated plywood. If not, what is the best way to joint two pieces togeather to make a watertight seam so that I end up with a 12' piece with the seam running horizontally???

Thank you....

 

[DaveO]

I don't think that you will find sheet goods larger than 4' x 8', especially treated ply. If you do you will probably pay dearly for it.
The only ways I can think of joining two pieces together to make 1 that's 12' long would be with either a biscuited butt joint using epoxy to make it water tight. Or a scarf joint, again with epoxy, but I think that the angled cut would be very difficult to make on a piece of ply that large.
What are you making that you need such a large piece of ply that is watertight???
I just had another thought, you could use multiple sheets of 1/4" ply laminated together with overlapping seems, but that would be a tough glue up without a vacuum press.
Dave

 

[DaveO]

Hey WELCOME to the site, Please give us all and introduction in the Who We Are forum, we love to know more about you.
Dave

 

[Phillip]

if it were anything other than treated, i'd say the best way would be a scarf joint, epoxied and fibreglassed. i've never had luck gluing treated lumber even with epoxy though ply may be different. if we knew the application others may have more ideas. welcome to the site.

phillip

 

[Gofor]

One of my past issues of boatbuilder mag had an article on making longer ply boards. IIRC the recommended ratio of the joint was 8:1 (in other words, 1/2 plywood would have an overlap of 8/2 (4") to maintain the strength but allow flexibility of bending. The mating edges are tapered so the joint is the original thickness. If you use marine grade plywood, the easiest way is to step it down ply by ply. I think the article was for using exterior grade ply, and they used a belt sander for the initial bevel and flattened it with a board with sandpaper glued to it. Realize this will give you a seam perpendicular to the length, but should have the original strength.
I don't remember what glue was recommended but think it was just exteriro grade carpenter glue. I will try to find the article (several years old) which was about making a 12' flat-bottomed boat. In case I can't, you can probably find more info from a DIY boatbuilding site.
It will be a challenge keeping the boards properly positioned and pressure across the splice.

Good luck:lol: