Lap join suggestions/tips

[Clay]

I need to extend my side walls about 2 feet to make them 4X10 instead of 4X8. I don't have a biscuit jointer and don't really trust any sort of reinforced butt joint, so I'm thinking of using a lap joint with about a 1 1/2 inch overlap. Will be doing this with a circular saw only. Anybody have experience with this and suggestions.

[rainjer]

Do you have a router? That would be a better choice.

[madjack]

A router would be a much better choice indeed...a butt joint backed witha 1X4 and glued and screwed is also an excellent choice as is a scarfed joint(angled cut) witha spline inserted...there are also methods using epoxy and fiberglass...
madjack

[asianflava]

If you don't have a router, you may be able to do it with a dado blade. It will be more difficult because of the size of the pieces involved.

[Steve_Cox]

I need to extend my side walls about 2 feet to make them 4X10 instead of 4X8. I don't have a biscuit jointer and don't really trust any sort of reinforced butt joint, so I'm thinking of using a lap joint with about a 1 1/2 inch overlap. Will be doing this with a circular saw only. Anybody have experience with this and suggestions.

Clay,
I did lap joints on my 2 X 4 sub-frame and used a circular saw, just seemed like the thing to do as it was the easiest tool to access at the time. I did clamp the 2 x 4's to saw horses when I did the end cuts. There are definitely safer and closer tolerance tools that could have done the job better.

[Lone Wolf]

Set saw blade to 1/2 thickness of plywood. Measuse from blade to wide edge of saw base use this measurement + width of lap joint and measure that amount and mark top and bottom of panel. Then clamp a straight edge I like to use factory of 3/8 to 1/2 plywood for this. Make this cut and several more. Use chisel to cut off pieces that remain then sand smooth. Repete this once more for the other panel. Be sure to do one on the good side and the other on the bad side of panels and if you are doing 1-1/2" lap joint add 3" to extension panel. Glue and screw with good grade of glue.

P.S. I would not try this on any ply less than 3/8". Have fun.

[Artificer]

You could also do a splined joint. Use your edge guide to position the blade in the middle of the plywood. You can clamp a 2x4 (or two) to the edge to give you a stable surface to run the circular saw on. Do both pieces of plywood, guiding from the same side. If you need a wider slot, move the guide, and recut.

I like to use 1/4" plywood as a spline. (1/4" plywood underlayment... no voids) The outside ply grain of the spline goes perpendicular to the main plywood edge.

If you're carefull, you can get as good of results as the biscuit, and have a stronger joint, since the spline goes the entire length of the joint.

If you don't have the circular saws edge guide, you can glue/screw one up from plywood and a 2x4. Make a Tee, and clamp or screw the plywood to the saws foot. If you go this route, you don't have to clamp a 2x4 to the plywood, since its on the guide. Attach the guide to the saw with the blade fully raised. Once its secure CAREFULLY!!!! turn the saw on, and plung cut through the plywood of the guide. You can also pre-cut the blades slot, which is safer, and makes it easier to adjust the position of the guide.

[Jiminsav]

I used a pocket hole jig and some glue to make my joints at the galley end..seems pretty tough to me.

ps., I also used some biskets, but they didn't impress me with their holding power.

[Ken A Hood]

You could also try to place your cabinets/countertop etc along the butt joint. That way you can just use the backing/blocking for the cabinets/countertop as the butt joint for the plywood.

[An Ol Timer]

Depending on the thickness of your stock you might see if you have anyone who builds boats as a hobby in your area. They may have a scarfing jig.

We have a 4' wide jig that on 1/4" plywood makes a 2" wide lap joint or an 8:1 ratio at a 7.1º angle. The cost to build it was minimal and it uses a standard 7-1/4" circular saw to make the cut.

Just a thought.

[cracker39]

Are you painting, skinning with Alum, or making a woody? My edge joints on the side are very simple, and most are undetectable, but I painted the exterior with oil-based enamel paint. I placed 3" wide vertical framing pieces where the joints are. I beveled the edges of the ply and butt joined them on the vertical framing with glue and screws. The joined, beveled edges created a groove for filler (Bondo) which I sanded smooth. The screws were recessed (countersunk) so I could fill and sand the holes smooth. The main thing to remember is to sand lightly or you might create a wide shallow depression that will be visible after painting. I know, because I did that to one joint on the left side.

[Clay]

UPDATE: Used a friend and his table saw to make 1-1/4 inch lap joint, glued it up, clamped tight and popped in a few screws. Joint will sit right on my galley bulkhead so that will also allow for lots of extra framing support. Seems like it's gonna work great. Certainly would have been easier with a router, but hey you gotta use what you got, right? Thanks for all the suggestions and advice. Got walls trimmed out and doors cut. Plan to post pics tomorrow.

[cracker39]