QUESTION: Has anyone used biscuit joinery on 3/4" plyw

[cracker39]

I need to join several pieces of 3/4" plywood to get the floor size of 9' 3" x 5' 2". I've considered 3 methods as shown below. 1) simple edge join with a strip of 1x2 spruce and screws to secure the joint. 2) a rabbit cut to form a lap joint with the spruce strip and screws to secure the joint (more secure than 1.) 3) Biscuit joinery.

I haven't used biscuits yet, and they are described as compressed material that will expand when used with water based glue. I am concerned that if used with plywood, it may cause the plywood itself to expand and cause "bumps" along the joined edge. Has anyone experience with this?

[Arne]

i think it depends on the load. biscuits are usually used in lightly/no load areas, like butting 2 pieces of furniture or window casing together. if i was using it on a floor seam and it would be reasonably well supported I would do it, but might epoxy a piece of 1/4 about 4 inches wide under the seam for added durability.

[JerryMae]

From my point of view I would not consider joining this with biscuits or just a butt joint. The lap joint will give you much more area for the glue which gives a stronger joint. If you use a waterproof type ahesive it would allow the joint to be more weatherproof where the other 2 options are just a seam that may pose issues with getting it weather tight. If you decide to go the biscuit route placement of the slot in the plywood's edge may poise an issue, I would advise a couple of test cut in some scrape pywood to verify proper placement. If the slot cut is 1/4 inches wide that leaves you 1/4 inch above and below assuming the plywood is actually 3/4 inch thick, most plywould is nomial size so it may only be 22/32 inch thick. This may force you to change your mind. Havng dabble in woodworking since age 11 and I am now almost 50 years young I am just offering my opinion. Good luck with what ever route you may chose.

[toypusher]

Dale,

If you use a good glue like Titebond II or similar and the joint is square and tight, then the wood around it would break before the joint would break. I have used biscuits in all sorts of building of furniture, etc., but have not used it on a teardrop. Just as a precaution, you could try to have the joint in a place that there will be other reinforcement, like where a framing piece goes or a bulkhead. If it falls inside a cabinet, just put an overlay of 1/8 or 1/4" plywood on the joint.

Hope this helps.

BTW If you go with polyurethene glue, make absolutely that you read and understand the proper procedures for it's use to get the best and strongest joint.

EDIT: I did not read your post that close. So, here is more. The biscuits will not expand enough to create a bulge or bump (that's assuming that you use the correct size cutter to make the slots for the biscuits to fit into). If it is just your floor, then try to put the joint over a crossframe piece and it will have lots of support allowing you to glue it any way you want.

[Chris C]

Dale,

I build cutsom furniture for a living and sometimes use bisquits on solid wood. I've also used it on furniture grade plywoods.............but the only failure I've had was when I used them on common plywood sheets and expected the sheet to be self-supporting. (such as the wall of your trailer) A shiplap scarf joint would be the only way to insure it would hold. Of course this is only my opinion based on my experiences.

[Steve_Cox]

Nice Picture Dale.

Biscuits? you've heard the swelling story, I've done it, wasn't bad, used epoxy instead of glue. For a floor joint that's not supported I wouldn't do it. Guess it depends on the load factor. But why do it if it is suspect?

Lap joint supported by 3/4 X 1 1/2 spruce, that looks good as well as the supported butt joint.

I built my frame to support the floor butt joint in the plywood set on top of 2X2 wood frame. Used screws and LIquid Nails Ultra duty Polyurethane Construction Adheasive. In my opinion I like it best over all the PL products. I think I might have overbuilt it though.

Steve in St Augustine (going over to Anastasia State Park in a while to see the TD'ers)

[mikeschn]

I use biscuit joints in my floors, and walls if necessary. But I use the biscuits mostly for alignment of the two mating faces. The real strength in my joints comes from the piece of 1x4 that I glue up underneath the floor or inside the wall!

Mike...

[cracker39]

Thank you all. Ok...It's door number 2. That is, a lap joint with a supporting strip screwed and glued on the underside to help secure the whole mess together. I will use a strip of 3"-4" strip of 1/4" plywood rather than the 1x2 under the seam. And, I'll probably be using Titebond III throughout. I'm not too worried about water getting to the joint, as I'll undercoat after it is all one piece bbefore attaching it to the frame, then stick foam insulation below the floor with liquid nails. I'll probably put some type of coating on the foam to protect it from road debris hitting it.




Well, maybe not so many screws as shown. I'll save the biscuit joinery for my 1x2 and 1x3 cabinet frames (spruce).

[Chris C]

Dale,

That will work just fine. It'll be a lot better than the unsupported bisquit joint.

[cracker39]

Just out of curiosity, I measured the thickness of a biscuit that came with my joiner. Without a micrometer, It am just eyeballing it with a ruler. Compressed, it is about 5/32. I let one sit in some water for a while and took it out and let it expand. It appeared to expand only about 20%-25% to about 3/16" thickness. Even if used in 3/4 ply for cabinet work, the expansion probably wouldn't be noticable.

Being a detail-oriented person (There's another word for that, but Mike would probably edit it...lol), I started another graphic for my plans and other project use, with scale drawings of biscuit sizes, and wood framing of different widths whowing what size biscuits can be used for corner and end joinery. I won't put the picture here as I don't want to fill up the album space with this type of stuff.
http://pg.photos.yahoo.com/ph/cracker39/detail?.dir=/6823&.dnm=3edf.jpg&.src=ph

[Jonkayak]

I like the idea of the reinforced lap joint. Just another idea. I know this is not a response to you original ? but its and idea. If you are using 3/4ply why not get the ply that has a tongue and groove in it for flooring. Glued together it can be very strong. Also you can cut the tongue and groove into any wood with a router. That type of ply can be also bought at most builders supply houses in p. treated as well. Another idea use two layers of 3/8 ply and over lap the seems glueing the hole sheets togther.
No matter how you do it, it sounds as if you are doing this in a very well thought out manner and I'm sure it will turn out well. Good luck.


Jon

[Chris C]

Dale,

The issue with bisquits and plywood is not really about the swelling (which they DO and which probably would show in plywood) but the fact that bisquits do not act like tenons in plywood. They are basically to align pieces. The edge joint is what we should be concerned about here. Can you imagine edge joining the two pieces of plywood and calling it finished? Would you depend on that joint? Heck no, and neither would I. The bisquits will give you verrrrry little additional glueing surface to improve the strength of the joint. Now, mind you, that is a little different when it comes to solid wood. But plywood needs a whole lot more than bisquits. The strongest joint for plywood is the scarf joint as used in marine applications. It's not easy to do, and takes a jig. I think someone here on the forum posted a link to a site which showed a really effective way to accomplish such a joint. I tried to find it, but was unable. Maybe someone else remembers and could post it.

[cracker39]

I don't know why I didn't think of the T&G plywood. That's a great idea and cuts down on the labor involved. Thanks.

When I was working with the biscuit info, I thought about setting up cabinet frames and door frames for the cabintes. I like to design things, so I designed a frame jig to accomodate assembly of frames fro 9"x12" up to 32" x 36", the largest I can imagine needing for most applications. It whould work with either butt joint, tennon, lap, or corner-mitered frames (see disclaimer below). For my couch/bed, I will need to make a couple of frames 62" wide, but can to that with my jig with some finagling to manage the extra length. If anyone is interested, I put the plans for making this jig in one of my photo albums. http://tinyurl.com/83ezj But, they are not in the correct sequence. They are in the TD Forum album, and the last one, Basic Frame Jig Construction should be the first oneviewed. The assembly is pretty self explanatory, but if anyone wants to build one, I'll answer questions. If anyone wants the larger .jpg files (page size), email or PM me and I'll email them to you. DISCLAIMER: I haven't actually built and tested it, so I make no claims that the plans and the resulting jig are foolproof and will work without modifications.

[cracker39]

Chris, that scarf joint looks like too much work for an amateur to set up for a one-time job. I think the T&G plywood is the best answer, still backing up the joint with a strip of 3/4" x 3" on the underside of the joint. I've had joints in house floors creak a little at the joints and have had to work on them from below when the plywood was accessible.

[Chris C]

Dale,

As long as you back up the joint, I think T&G would work satisfactorily.