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I'm planning to put trim boards on the perimeter of the porch flooring I’m working on.

I'm debating whether the corner joints should be butt joints or miter joints. I want it to look nice and minimize the places where rot can get started. I know miters have a tendency to open up, but a butt joint leaves the end gran exposed. Is this joint a good compromise between the two? I figure there room for some dowels or screws to hold it together.
 

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I'm planning to put trim boards on the perimeter of the porch flooring I’m working on.

I'm debating whether the corner joints should be butt joints or miter joints. I want it to look nice and minimize the places where rot can get started. I know miters have a tendency to open up, but a butt joint leaves the end gran exposed. Is this joint a good compromise between the two? I figure there room for some dowels or screws to hold it together.
Do you have a router table or spindle shaper? You can do a lock miter. The joint has plenty of glue surface and self aligns to 90 degrees. First time setting up can be a bit trying. I recommend saving cutoffs to use as gauge blocks in the future.
 

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I should have included dimensions. The face of the pieces you see in the sketch are 5” wide. They’ll be trim around the perimeter of a porch floor.
I did that with a deck I built at our previous home. The deck was rimmed with 2x6 TYP. I glued the join with exterior waterproof glue and ran SS screws in from each end. With thinner stock, I'd use nails instead of screws.
 

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I'm planning to put trim boards on the perimeter of the porch flooring I’m working on.

I'm debating whether the corner joints should be butt joints or miter joints. I want it to look nice and minimize the places where rot can get started. I know miters have a tendency to open up, but a butt joint leaves the end gran exposed. Is this joint a good compromise between the two? I figure there room for some dowels or screws to hold it together.
I think it would be easier and stronger to make a simple 45 with a spline in it.
 

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Discussion Starter · #10 ·
Since this is outdoors, I feel like the forces of expansion are going to open up a full miter no matter what. I was wondering if another type of miter joint might reduce how expansion affects the joint.
 

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Since this is outdoors, I feel like the forces of expansion are going to open up a full miter no matter what. I was wondering if another type of miter joint might reduce how expansion affects the joint.
In my opinion, a half lap joint or a splined miter joint will be the strongest options. You could even do a mitered half lap joint if you want the aesthetics of a miter joint showing on the upper surface.
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I only care a little bit about appearance. It’s a the distant end of the porch and it’ll be under a post. I mostly care about protecting the end grain from water intrusion (and subsequent rot :)
 

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Since this is outdoors, I feel like the forces of expansion are going to open up a full miter no matter what. I was wondering if another type of miter joint might reduce how expansion affects the joint.
O a 45 degree miter of equal width and same species wood, all the wood is moving the same amount!
So across the miter joint there will be equal expansion or contraction.
No need to worry about it opening up.
 
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I only care a little bit about appearance. It’s a the distant end of the porch and it’ll be under a post. I mostly care about protecting the end grain from water intrusion (and subsequent rot :)
IMO, a miter just makes matters worse. It increases the surface area of the end grain and the two pieces put together provides a trap for moisture. Just use a butt joint and seal it and protect it with a product like Woodlife Copper Coat or similar. Then paint or stain. Also, the sharp edges of a miter are delicate and will crack, split, and weather away over time.
 

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O a 45 degree miter of equal width and same species wood, all the wood is moving the same amount!
So across the miter joint there will be equal expansion or contraction.
No need to worry about it opening up.
It's basic geometry. As the board expands or contracts the angle of the miter changes. Equal expansion would need to be in length and width, but wood does expand length wise. This is the reason all miters will open up (short of something holding them together..glue/screws or both) eventually.
 

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Of course it's just my "theory" but wouldn't both piece expand and contract along the 45 degree miter at the same amount?
The movement is across the width, so I'm trying to picture the geometry of both widths expanding.
 

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expand and contract along the 45 degree
NO, the wood does not expand and contract "along the 45 degree line". The wood expands and contracts across the grain, not 45 degrees to the grain. As you well know, wood will not move lengthwise the same amount as it moves cross grain.

Slope Rectangle Plot Triangle Font


Obvious result showing both boards expanding.
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It's basic geometry. As the board expands or contracts the angle of the miter changes. Equal expansion would need to be in length and width, but wood does expand length wise. This is the reason all miters will open up (short of something holding them together..glue/screws or both) eventually.
I believe Jar, Dave have it. Excellent explanation. We’ve all seen the picture framed windows with gaps at the outside of the miter.

@ogre, all good suggestions, but no need to complicate it. If you‘re really worried about it, do the biscuits and epoxy, or dowels, but you’re already probably overkill. In the big picture it’s not going to matter, especially if there’s 5 hours of afternoon sun baking it.
 

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I'd lap the top board a little past the vertical face board, and cut a drip slot aligned with the front face of the vertical board under the edge of the top board. Round or chamfer the edges of the top board, sand, and finish very well with something made for outdoor/UV protection. Let the rain/water roll over the edge, and drip off before hitting the covered front face. Maybe dado the front board 1/4" or so into the bottom of the top board.
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I am of the opinion that a simple 45-degree miter is your best option. A spline or dowels or biscuits might add just a little more strength but I doubt it.

This is from years of doing all kinds of different crap and settling on a simple miter joint.

I think Stumpy Nubs has a video on just this also.



Andy.
 
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