Discouraged

[Tomterrific]

I bent cheap 5mm underpayment. The first try showed too much tension to bend all the way, I knew it was going to crack. I used ratcheting tie downs to bend the ply ALMOST all the way. I allowed this partial bend to sit over night. I started this bend and let it normalize for a few hours, then bent a bit more, then more, until it sat overnight. The next afternoon the ply was almost bent all the way by itself. It was easy to position and bend the already pre bent plywood. My radius was between 24 and 28 inches.

T

[QueticoBill]

I presume the ply is bent with the grain of the outside ply's parallel with the axis of the bend, like Tony's and Jim's photos. Or did I see a post that suggested the opposite direction?

[Xanthoman]

I presume the ply is bent with the grain of the outside ply's parallel with the axis of the bend, like Tony's and Jim's photos. Or did I see a post that suggested the opposite direction?

your understanding is correct. I worded it badly but this is what I meant. The grain being this way allows them to spread along the tensile length whereas if the grain is along the tensile side they will keep it from bending as easily


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[Andrew Herrick]

*Hopping on my soapbox* The lengths that people go to make regular plywood do what it isn't designed to do is boggling! I mean, there's bendable plywood, fiberboard, fiber-reinforced plastic, HDPE puckboard, rigid foam, aluminum sheeting ... Avoid the headache, I say

If you must use 1/4-inch plywood, either:

- heat it and pre-bend it using ratchet straps
- cut it into 12-inch strips and bend each one individually.
- kerf the backside

If you're set on plywood, I'd spend the money for 1/8-inch 5x5 sheets of Baltic Birch.

Or save yourself the hassle and don't use plywood. If you want to do it fast and easy, and you're confident in your exterior skin, you could even prime and paint 1/8-inch MDF and use it. It'll take the radius easy, and as long as you seal it, it will remain waterproof. The two downsides: You have to use panhead (not countersunk) screws to attach it, and it won't hold glue worth anything, so you have to use a floating roof. It's not a great solution, but you can have it done in 2 hours.

There's not just one option. Just remember: Work WITH the material, not against it

[Andrew Herrick]

One possibility is to glue down a light fabric (canvas) to the outside of the plywood , and then the fabric will endure the “tension” loads imposed by the bending process... When done , leave the fabric on , cover or paint over it ... it will protect the ply from cracking/splitting in the future.

THAT is a great idea.

[SSTear]

I've found that bending plywood from side to side (48"W) rather than from end to end (96"L) works substantially better. No cheap bendy plywood for me. I bent 3/16 birch plywood this way by routing on the side walls to fit the width of rafters and thickness of the 3/16 plywood. I then pressed the plywood down into the side walls with the rafters, clamped and screwed them in from the outside through the end of the rafters with 3" treated deck screws. My radii we're fairly large but I didn't have to wet it down. The finished side of the birch is all I see on the inside of the tear drop. No Fasteners showing. A really nice clean look.

[bam1028]

I found a mom and pop lumber yard that had the 1/8" baltic birch. I had to drive a couple counties over but that was no big deal since I have been off this week. I am gonna work on it this weekend so wish me luck. I like the idea of backing the board with fabric. The first couple of bows I built I backed them with linen so they wouldnt explode and it does work. I appreciate the out pouring of ideas from everyone. Im gonna do my best and take my time so this is the last time I do this and so I can move forward.

[bam1028]

The planets have aligned and the gods agreed I have suffered enough. The interior roof is done. On to the next problem!

[tony.latham]

The planets have aligned and the gods agreed I have suffered enough. The interior roof is done. On to the next problem!

That Baltic birch is nice stuff, eh?

Tony

[bam1028]

Heck yeah it is. I have a couple extra sheets I plan on using for sliding cabinet doors.