Which Joint to Lengthen 5 ft Plywood for THTLP Sides?

[Mikie]

I'm getting ready to build a 21" THTLP. The only acceptable 1/2" plywood in my area is 5'x5' Baltic birch. Unfortunately the 2 THTLP sides are spec'd at 6' long. From searching this site I've seen mention of lengthening plywood using either a scarf joint or a finger joint (the later requires an expensive router bit that I'd prefer not to buy). Some threads state that Bill describes how to make a scarf joint in the THTLP plans, but if it's there I sure can't find it (I got my plans last week).

I saw 1 mention of using a 45 degree cut but I'm not sure that comment was intended for the scarf joint. When searching via Google, other sites recommend that scarf joint length be 8-12 times material thickness (a 4" - 6" long joint for 1/2" ply ). I could easily do a 45 degree cut for the joint, but cutting at smaller angles would be a challenge.

Could someone describe how to make a scarf joint (or any other cheap/easy joint) to join 1/2" plywood that is strong enough for THTLP sides?

Many Thanks

[ncgrove]

A 30 degree joint should be fine. I've just used 90 degree butt joints with plenty of PL and things work just fine. I'd imagine a 45 degree joint would be even better.

[Mikie]

Thanks Ncgroove, it's a relief to hear that you've had success with a 90 degree butt joint. I'll go with a 45 degree joint to get a bit more gluing area. Baring late breaking honey-do projects I should start making sawdust in a couple weeks .

[Tom Smit]

I'll go with a 45 degree joint to get a bit more gluing area. .

+1

[Grant Bunter]

Wow, I couldn't find panel jointing in my old THTLP plans either.

I copied this from other plans though:

To join two smaller plywood panels to make a larger one cut the joint on both pieces at about a 30 degree angle; this gives a longer glue line and eliminates the possibility of the joint shearing in the vertical plane. Glue the panels together and support them while drying with plywood braces on both sides of the joint.
Use wax paper between the braces and the panels so they won’t be glued together. Use clamps to hold the braces while the glue dries if you have them, but if not just screw the braces in place and remove the screws when the adhesive has set.

[LelandCrooks]

Use a cleat to hold the panels flush.

[Mikie]

Thanks guys. Guess I'll go with a 30 degree cut then. Appreciate the feedback.

[Bill Fitzmaurice]

30 degrees is plenty. If the board takes a whack you just want to be sure that the shear forces won't be on the same vector as the joint. Once the interior panels are in place they'll support it anyway.

[milehighassassin]

As others said, a 30 degree is fine. I'm a bit surprised you cannot find 4x8 Baltic Birch. If you go to a real lumber supplier they should have it in stock if not be able to get it

[Bill Fitzmaurice]

I'm a bit surprised you cannot find 4x8 Baltic Birch.

5x5 is the standard size for BB, 4x8 is the odd one.

[milehighassassin]

I'm a bit surprised you cannot find 4x8 Baltic Birch.

5x5 is the standard size for BB, 4x8 is the odd one.

I know. I asked about it at the place I buy from and they said they can get it in any thickness at 8x4