Gluing hardwood miters on outside deck?

[Kristian]

This summer I finished a deck using hardwood (smooth on one side / grooves on the other) and the miters were tight. Now one of the miters have opened up and I'll fix this eventually but I'm looking for suggestions as to how I could have prevented this? It's my first deck using hardwood so besides the obvious option of placing screws closer to the outside corner would gluing the miter have helped? I'm under the impression that not all hardwood accepts glue equally well or is that wrong?

The wood had been stored outside for close to a month so I would assume that it had absorbed all the moisture it could, although the wood has been exposed to a lot more rain after the project was finished.

Thanks for your input.

- Kristian

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[w802h]

I glue those miters.  I think it's an ideal place to put a pair of dominoes too.  I used to used epoxy-soaked biscuits, but I like the tendon set-up more, because there is less chance of the joint rolling.  The glue helps seal the end grain as well.  I like to seal all sides of exterior decking.  Some tropical hardwood requires a special wax, like Anchorseal, to seal the ends.  What does installation guide from the manufacturer suggest?  Generally, miters that gap at the base of the joint are from things drying out and shrinking in width.  The miter gap at the outside suggests infiltration of water into the joint and expansion of the board. 

 

[Sparktrician]

I might suggest using Sipo mahogany dominoes in a lock domino joint.  Combine these with a good epoxy and you should never have the separation issue again.  You will, however see the exposed ends of the dominoes. 

 

[Kristian]

Thanks guys. I think you're absolutely right about sealing the ends and putting in dominoes. I'll definitely give that a try (and it's a valid opportunity to buy a Domino, right??)  [big grin]

I couldn't find any recommendations from the manufacturer and I had only considered the general absorption of moisture in the wood but I think you're bang on concerning the infiltration of water into the unsealed joint and the resulting expansion in the width of the boards.

Lesson learned. Thanks again.

- Kristian

 

[peter halle]

Outside miters on decks are always iffy in my experience.  You are always fighting the natural tendencies of the materials being used and the outside elements including heat and moisture content.  The only perfection you will achieve is when you first put it together and then when thru the weather cycles that exact condition is recreated again.

What you have done is actually holding up well all things considered in my opinion.  You have tried to limit the placement of the board on top of the framing both towards the inner edges and towards the outer edges within the limits of where you had framing.  That is why the outside corner has opened up slightly but not the inside of the miter.

Because wood will expand and contract across the width of the board that outside corner will constantly be changing and thus so will the miter angle.  You will always be fighting Mother Nature.

Dominoes and adhesives - especially towards to center of the joints may help minimize the movement as others have posted.  I have had success at times using pocket screws underneath to attach the boards together, but that might not be possible in all situations due to deck design and clearances available to you at time of installation.

Peter

 

[junk731]

You have discovered the universal problem of trying to maintain a tight miter in something that is in constant flux. I personally have found glue is not long term solution to the problem and typically fails no matter how well prepared. You might consider using the same air gap that you have done through the rest of the deck and around posts. This allows the end of the mitered boards to dry and not wick moisture as much which is part of the differential shrinking your getting.

John

 

[kfitzsimons]

Has anyone tried using some flexible caulk in the joint? It's not a perfect solution, but the gap would be less visible. I'd use a dark brown color caulk or whatever matched the deck color best.

 

[Sparky]

I have used dominos as well as dowels before I had my domino. I have had several comments on my own trex decking asking how I got the miter to stay closed. I tape the miters along the length for easy clean up from glue squeezing out as I want full glue coverage.

 

[WarnerConstCo.]

They won't stay tight year round no matter what you do or try.  I have tried it all, in every combo I could think of. 

 

[jacko9]

They won't stay tight year round no matter what you do or try.  I have tried it all, in every combo I could think of.

That's been my experience also.  I like the idea of designing around a tight joint. 

 

[rst]

I half lap my miter corners, they still open but at least the customer can't see down through the crack.  Quality urethane caulk will stretch enough to make it look good.

 

[Kristian]

What about composite decking? With it's hollow structure it would be difficult to find space for a dowel or that much glue.

Or do corners on composite decks not open as much as regular wood so it's really not an issue?

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[Locks14]

What about composite decking? With it's hollow structure it would be difficult to find space for a dowel or that much glue.

Or do corners on composite decks not open as much as regular wood so it's really not an issue?

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Composite decking barely moves. If you get the joint right in the first place it won't open to anywhere near the extent of natural products.

 

[jacko9]

What about composite decking? With it's hollow structure it would be difficult to find space for a dowel or that much glue.

Or do corners on composite decks not open as much as regular wood so it's really not an issue?

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Composite decking barely moves. If you get the joint right in the first place it won't open to anywhere near the extent of natural products.

That depends on where you live, out here in the hot California sun composite decking shrinks like crazy as the heat and sun beat on it over a few years.

 

[Locks14]

What about composite decking? With it's hollow structure it would be difficult to find space for a dowel or that much glue.

Or do corners on composite decks not open as much as regular wood so it's really not an issue?

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Excellent point. Though in Denmark for the OP, I don't think that will be a huge problem.

Composite decking barely moves. If you get the joint right in the first place it won't open to anywhere near the extent of natural products.

That depends on where you live, out here in the hot California sun composite decking shrinks like crazy as the heat and sun beat on it over a few years.

 

[mo siopa]

This is a common problem with many variables that I will try to make as simple as possible.  There are inescapable reasons that miters in solid wood tend to open.  1-  Wood expands/contracts in relation to relative humidity.  You cannot (completely) stop this from happening.  It isn't just "wet or dry," water molecules are polar and form weak chemical bonds (similar to static electricity) with various links in the wood.  As RH increases, more water molecules will attach to links in the wood fibers and cause it to expand and vice versa as RH decreases.  2- Wood expands and contracts differently relative to grain.  Practically speaking, wood does not expand/contract along the grain (less than .5% for most woods).  It changes more tangentially (along the growth rings) than it does radially (across the growth rings), but averages about 7% shrinkage from green to kiln dried between the two.

So... What this means is that the width of a decking plank will change with Relative Humidity but the length will stay the same.  Which in turn, means that the 45 degree angle you cut will won't be 45 degrees when the weather changes.  For a visual aid, try drawing your miter on some graph paper.  Now, erase the outside lines and draw new lines one block out on both edges but keep the same length on both edges.  This is what happens when the wood expands.  Replace the inside edge one block in and see what happens when the wood shrinks.

Since wood is flexible, smaller width pieces tend to maintain a miter bond better.  Across larger pieces however, the force becomes too great and the bond is more likely to break.

 

[Kevin Anderson]

Wood Deck Restoration, in addition to cleaning and re-staining it, may additionally involve brightening, resetting nails, stripping, buffing, board replacement, and sanding. Most decks only require standard cleaning, brightening, and refinishing, with some requiring partial restoration (stripping, possibly buffing.)