# Joining two GT deck boards to make a mitered joint



## thegrgyle (Jan 11, 2011)

Hey guys,

I was asked by a friend of mine that is builiding a deck for some help. His railing that he will be installing has a "cap" or top rail that is a 5/4 x 6 deck board laid flat across the top of the 4x4 posts. In the corners of the deck, he was planning on mitering the boards, but has seen that over years, with the exposure to the hot sun the boards shrink, and the miter opens up, even if a screw is put in through the ends of the miter. He was wondering if there was a better way to make this joint. 2 things popped into my head right away.... either a _spline joint_, or a _half-lap miter joint_. The spline would probably be the easiest, but what should I use for the spline? Another planed down green treated deck board? Won't it shrink too much? With either one, what type of glue should I use for the outdoor conditions? Polyeurathane glue? Outdoor wood glue?

Is there another joint you would use?

Thanks for taking the time to read this, and I look forward to your suggestions.

Fabian


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## verdesardog (Apr 2, 2011)

I would miter or even butt join the ends but cover the joint with a decorative cap.


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## thegrgyle (Jan 11, 2011)

Thanks, Verdesardog, for your idea. I talked with my buddy, and he didn't want to have to ask his wifes opinion on yet another thing about the deck.

He wants to just use the continuous top cap. I am guessing by the number of responses that this is a very common problem, and there is really no good solution.

HMMMMMMMMMmmmmmm..........................:blink:


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## mdntrdr (Dec 22, 2009)

I tack nail the intersecting rail caps running long.

Some scrap tacked onto the posts under the cap that will be on top to keep it level.

Then cut both at the same time.

Glue the miter with construction adhesive. :smile:


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## cgwendling (Feb 18, 2011)

I've built quite a few green treated decks and good looking long lasting joints on wet green treated is not going to happen.
That stuff is so unstable that the only way to make a good looking joint is to cover it up and let the the joint do what it will.
Even if it feels dry the moisture content will still be as high as green wood. 

Now, if he isn't using green treated or brown treated, then disreguard the above. Just go with a reinforced mitre. A shiplap mitre, spline, board on the underside, thin metal plates, yes, even pocket screws with coated screws. All work. These methods help treated wood too, but the joint will still open and close some.


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## rrich (Jun 24, 2009)

Correct me if I'm wrong...

It is a 4 x 4 corner post that needs a top railing. The railing needs to be level and the two pieces will join at the 4 x 4 post.

Take one of the railing pieces and place it on top of the 4 x 4 posts. Do the same with the mating rail but place it on top at the meeting point. Each piece of rail should be about 6" longer than necessary. Mark on each rail where the other rail crosses it.

Use a router to cut a lap joint. Set the bit depth to a sheet of paper thickness less than half the thickness of the rails. Cut off the extra lengths of rail and install on top of the 4 x 4s.

The reason for the extra length is to support the router.

The reason to lay out the rails on the 4 x 4 posts is to account for something other than a 90° joint.

The less depth gives wiggle room for sanding to get a good fit.

Use a good water proof glue (TB III) to hold the rail pieces together. A few stainless screws through the rail and into the 4 x 4 will make for a nice secure railing. (Use 4" to 6" screws.)


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