What wood joint can I use to connect railing members over a post?

Question

For the fence we'll be using 2"x4" pressure-treated wood that will be attached to 4"x4" PT posts with wood screws.

What wood joint should I use so that I don't need to buy any more hardware to make it sturdy?

This is what I originally thought of doing, but the builder says that I would need to add L-brackets on either side of the post to make it stronger.

Then I thought of using a half-lap joint, but the builder says that it would make the sides weaker. Each 2"x4" is about 12 feet.

So he explained that he would use something like this. He would use some special glue between the joins:

My question is: what joint is strongest for this particular scenario? I really don't want to buy more hardware to make it stronger (like the L-brackets).

Answer 1

You have a couple things going on here, so let's clarify:

• There's no joint that makes anything "stronger" here if you're using adequately-sized railings and appropriate fasteners to begin with. You don't need the rail boards linked together in tension. It's not important. Railings don't fail by falling off the post along the rail assembly. If they fail, it's due to the lumber cracking or getting pushed off the fasteners somehow.
• Therefore, it's entirely a matter of aesthetics. How do you want it to look from the most common perspective?
• The main point of using something other than a square butt joint is so that, after the railings shrink, you don't see the resulting gap as readily. A bevel joint hides the gap visually. This applies to decking, siding, and any other joint in wood. A half-lap requires substantially more effort, but allows you to use fewer fasteners, which has both cost and visual benefits.
• You can make things weaker with the wrong approach. The half-lap as you drew it in your previous question will weaken the railing, because you've halved the thickness of the boards all the way to the edge of the post. If you imagine a cartoon dumbbell sitting on the railing just beyond the right edge of the post, all the weight is being supported by half the railing's thickness if we assume that it'll easily split lengthwise along the grain. Bad news.

If you're bent on a half-lap joint, the solution is to make a narrower connection, such that it's just wide enough to securely fasten it to the post, but doesn't compromise the strength of the board at or beyond the width of the post. 1-1/2 or 2 inches would do nicely.

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Answer 2

There is evidence that the half-lap joint you have considered will be stronger than the joint recommended by your builder (I'll call that a miter-lap joint); it makes sense when you think about it because there would be significantly more wood/glue contact surface area.

The drawback is that the half-lap joint takes some special equipment and requires some precision, whereas the miter-lap can be done in the field with a circular saw.

So... once again, the final decision depends on cost/time versus longevity. If you are willing to spend more time/money up front, you will have a sturdier product that will last longer. Is it worth it? Only you can decide.