What happens when wood shrinks?

[Greg Magone]

This maybe is a slightly different than standard woodworking, but involves wood and glue, so I figured it's worth asking here.

I recently installed 4x4 treated wood posts using the Oz-Post method. This is where galvanzied steel fence post bases are driven into the ground and then the post is set into the steel base.

I used a bunch (8 per post) of wood screws to secure the fence post to the oz-post steel base. Then, for further strength, I squired Loctite Polyurethane construction adhesive into the space between the wood post and the Oz-post. (The Ozpost contains a special hole for this purpose, I was able to squirt to generally fill the area beween the oz-post and the wood post with adhesive. Maybe about 15 square inches per side or 60 square inches per post.) The gap ranged from tight to up to maybe 1/8". Perhaps up to 3/16" in a couple locations.

The treated wood post was quite green, as they usually are. It will shrink a bit when it dries. I'm curious what will happen when the wood shrinks? Will the Loctite Polyurethane Construction Adhesive expand? Will the screws loosen up over time and require retightening? Will something else happen altogether?

Thanks!
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[johnny means]

The glue will seperate from either the metal or the wood. My money is on the wood.

[Leo Graywacz]

PU glue is one of the weaker glues. When used as a gap filling adhesive it's at it's weakest. When the wood shrinks it will break the joint.

[Greg Magone]

I guess my question - will the post eventually wobble? I have used 8 screws plus a ton of constructive adhesive to hold it in place.

[Bryan Lisowski]

After the post shrinks which it will, I would drill out the top and bottom screw in the second picture and then drill through the post and bolt them.

[Leo Graywacz]

The screws should hold it together for a long time.

[Steve Rozmiarek]

The glue acts like mortar once the bond breaks, it won't wobble for a long time. Well not too much anyhow. Water will eventually swell the whole thing because of freeze/thaw cycles and it'll get worse but it's a bird house, they live in trees, trees sway, does it matter?

[Rob Damon]

You do realize it rains in the forest?

So the wood will go continuously from wet to dry on a regular basis unlike an interior application where wood goes from green wet to dry low moisture content and stays dry. I would have just used galv nails and no glue.

[Art Mann]

Yes, the post will wobble after successive wet/dry hot/cold cycles. The polyurethane construction adhesive will certainly fail. It is not meant to be used for this type of application. Unless you plan to swing on the post, I can't see how a little wobble will make any difference. It doesn't take much to hold up a duck house.

[Greg Magone]

Thanks all. You confirmed my suspicions. I did add the construction adhesive per the OZ-post manufactuer's recommendations. I'll have to go back and check on them in a couple years and see what they are like.

The only other question that I would have is that a common way of setting posts is to use concrete. Concrete won't flex when wood shrinks, but I don't think posts set in concrete wobble?

[Mike Cutler]

Well, the second thing it does after is shrinks is expands.
That post will expand and contract with the environment until it disintegrates, which will be a long, long time. You'll replace that bird house more than a few times before the post
A 4x4 post in just dirt will last 25-30 years. I know this because I just had to replace the fence I put up in 1992 a few months back when the neighbors tree took it out.
4x4's in concrete do not wiggle after time. Why? I don't know. But as someone that has removed many of them, I will tell you that after about a decade or so, you can whack the base of the post, about 6"-8" above the concrete footing, with a dead blow mallet, and snap that 4x4. I've done hundreds of them this way. Based on my experience with pressure treated wood posts, set in concrete, I would never do it. Interesting thing is that ,if you move up a foot and strike the post with a mallet, it won't break. It's that sweet spot right at the base. Maybe the concrete curing just sucks the moisture out of them. I don't really know.

[Van Huskey]

From a practical point of view, I bet if you come back in 20 years it will still be standing there unless a person or a tree knocks it down. Well, I guess a bear could decide it is his favorite scratching post.

[Greg Magone]

From a practical point of view, I bet if you come back in 20 years it will still be standing there unless a person or a tree knocks it down. Well, I guess a bear could decide it is his favorite scratching post.

Yes, you're right. I just did a quick calculation and found the post will shrink / expand somewhere around 1/16 to 1/8 inch. I caulked around the top of the OZPOST with a bead of silicone caulking. The Ozpost holder itself is pretty watertight, it'll just be expanding and contracting mostly from humidity. Probably not as much as I am concerned about.

With the silicone caulking preventing water from sitting between the OZPOST post holder and the post, filling up any gaps below the caulking with polyurethane construction adhesive, and driving 8 2.5" deck screws in per post, I'm pretty sure that the post won't be going anywhere anythime soon.

I drove in 7 posts my last trip the property. I've refined the process a bit and hope to drive an additional 44 posts all along the property line as a subtle, but effective, way of denoting the property line.

Thanks all

[Van Huskey]

I've refined the process a bit and hope to drive an additional 44 posts all along the property line as a subtle, but effective, way of denoting the property line.


Not as subtle but these can be effective. :D

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[John K Jordan]

A great book on this subject is Understanding Wood by R. Bruce Hoadley. https://www.amazon.com/Understanding-Wood-Craftsmans-Guide-Technology-ebook/dp/B004WYO862

The wood will shrink as it dries. The amount of shrinkage depends on the type of wood, the amount of moisture, and the grain orientation. Wood shrinks far more tangent to the rings than along radial lines from the pith to the out side.

The post will likely go out of square as it dries. The ending shape depends on the wood and the original orientation in the log. This is not as much of a problem with small square posts with the pith in the center.

The entire post may warp and bend or twist or both. The type and direction of warping depends on the location and path of the pith in the wood or the direction of the grain if there is no pith (unlikely for PT pine 4x4s.) In the worst case, the amount of bend and twist can be surprising - I've had 4' posts bend nearly 20 degrees from vertical.

The wood will likely crack, perhaps a lot. The location and extent of the cracks depends on things like the grain and ring orientation as well as the moisture content.

The screws should be fine but I prefer longer lag screws (probably 1/4" or 5/16") or carriage bolts that extend through the posts.

Although the post will likely shrink, twist, warp, and crack, it will probably be fine and remain standing in the long term, glue or not.

If you will put up more of these: A PT post can be put directly into the ground. An 8" deep hole about 6-8" in diameter can be filled with dirt or gravel and tamped or you can tamp the empty space with Sackrete - you can pour in a bit of water or the ground moisture will be enough to cause the concrete mix to set to make a base strong enough for a free-standing post.

BTW, raccoons, possums, and skunks can easily climb such a post and reach in the hole to grab eggs and baby birds. For bluebird houses I make a cylinder from 1/2" hardware cloth, a bit larger diameter than the hole and maybe 4" long, and bend tabs on one end to fasten to the nest box. Raccoons can reach in but can't bend their joints enough to grab lunch from low in the box. Also, I always add a hinge or someway to clean out the nest every year for a healthy brood.

A post that is harder for a racoon to climb is a slick plastic fence post or a piece of galvanized pipe. For the latter, drive one end into the ground and attach the box with a flange.

JKJ

[Steve Rozmiarek]

Thanks all. You confirmed my suspicions. I did add the construction adhesive per the OZ-post manufactuer's recommendations. I'll have to go back and check on them in a couple years and see what they are like.

The only other question that I would have is that a common way of setting posts is to use concrete. Concrete won't flex when wood shrinks, but I don't think posts set in concrete wobble?

Concrete set posts keep the wood wet, as the concrete sucks in the moisture in the soil around it. It them wicks up the post, causing wet/dry cycles at the level on the post where the open air can dry it somewhat. It's like bog oak can be used as lumber 20,000 years after the tree submerged, but if a branch was sticking up above the water, it's no longer around.

The best fencepost in this area is a "hedge" post, it's osage orange inthe rough, but they last a very long time when just bedded in dirt.

[Wayne Lomman]

These bases work much, much better with a steel post. They get used extensively this way for road signage. Despite what the manufacturer claims, they are not great with timber. Cheers