Mortis and Tenon joint versus biscuits for screens

ejb · May 24, 2004

Hi folks,

Question: I need to built 12, 4 foot by 8 foot screen "panels" to enclose by back porch. The porch is on the fancy side, so I want to screening job to be equally as nice....

Anyway, my plan is to custom build all the panels, and given that there is not a lot of wood going into each one, the strength of all the joints seems important...

My question is, would a biscuit/plate joiner be sufficient for this? or would a true mortis and tenon joint offer a lot more strength?

I already own a plate joiner and have used it many times, but do not own a mortis/tenon setup. I don't mind buying them if the final product would be much superior...for a incremental improvement I may stick with the plate joiner/biscuit joints....

Comments?

Also, what kind of wood would you recommend? I am thinking regular 1x pine is not going to hold up well...

Thanks!

-EJB

Yankee · May 24, 2004

In the tests I've read, biscuits are nearly equal in strength to mortise and tenon. The only real difference was in breaking characteristics (M&T joints creep before breaking). Personally, I'd stick with the biscuit joints (with waterproof glue, obviously). Easier to build and easier to build well. Remember too that you only get that little bit of extra strength in the M&T joint if you build it to close tolerances. The biscuit joiner does that for you. Any difference in strength is completely immaterial to your screening application anyway.

Pine works well in outdoor applications. Many hardwoods (unless you're going to spring for white oak), actually break down quicker in the weather. If you're going to build 8' panels, I'd suggest making sure the frames are wide enough to give you the stiffness you need. You said you'd like a nice-looking job - why not use redwood or cedar?

Bill

DAP · May 25, 2004

That depends on the actual type of M&T joint you make. Some folks will extend the mortise and tennon all the way through. These types joints are stronger than the bisquits. If the shoulders of the M&T joints are 2 in. or less, then they are comparable in strength.

The basic promise of the joiner and bisquits is speed, ease and strength.

Yankee · May 25, 2004

I can see where someone might come to that conclusion given the extra thickness and length of a typical tenon. In reality, both joints usually are stronger than the wood itself. In both cases the frame will usually snap before the tenon or biscuit fails. Think of it this way - if a quarter-inch bolt were used to hold a ledger to a wall, and it failed because the wood snapped when overloaded, your using a half-inch bolt wouldn't have changed a thing.

In an outdoor application I'd avoid an open mortise.

Bill

Dozernut · May 25, 2004

I would look at a half lap joint. This would increase your gluing surface and is a relatively easy joint to make. Just make sure you cut your shoulders square for a good match up on your corners. If you have a brad nailer, a couple of brads in the joint will hold things in place untill the glue sets. Around here pine is not readily available, Douglas Fir is the general purpose wood sold in our lumber yards. Most of your soft woods will work fine if you keep them painted or treated.

DIYGuy · May 25, 2004

Now that I do carpentry work for a living. I'd use 5/4 x 4 redwood and use 1/2 lap joints in the corners.

Assemble with 2 - 1" x #6 SS Wood screws and waterproof glue. When all the wood is assembled, seal it with a UV resistant wood sealer, then attach the screen.

Biscuits are really an interior joint technique.