Pressure Treated Timber and Concrete

Submitted by EeyorONzoloft1 t3_1262l1h in DIY

Hello,

I'm looking to install a shooting backstop. My plans include using multiple 12 foot 6 X 6 posts, with four feet of it in the ground. I was going to try for a 10 inch wide hole. At the bottom of the hole I was going to put a layer of gravel and then fill the left over void with concrete. These three posts would be holding up a 8 ft wall of railroad ties with dirt fill in front of the railroad ties. I also intend to place support posts behind the structure with lumber connecting the berm posts to assist with any sheer.

Questions: is it okay to bury ground contact treated lumber in concrete. I do live in a wet area. And worry about rot.

Are the posts deep enough?

Any pitfalls you can see?

Lastely, if this isn't the correct subreddit, can someone point me to the correct one? Thanks!

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Comments

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OutinDaBarn t1_je84stn wrote

4 feet should be good for all but the very north of Wiscowsin. I built a shed the same way, gravel with concrete over.

If it's too wet the holes will have water in them. That will be a problem. I had to move all my poles over 3 feet after drilling about 10 of them and letting them sit overnight. The next morning we were screwed. Even tried pumping the water out. They just refilled.

Don't just auger your holes down. Let the auger fill with dirt. pull it out and dump it off to the side. Otherwise the dirt will be nothing but in the way when you set your poles.

The Harbor Freight cement mixer was good enough for the 25 holes I filled. I still use it from time to time. I did have to blow the motor out with air to getting going again once.

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EeyorONzoloft1 OP t1_je85760 wrote

Thank you! We just got a HF in our town so that will be convenient. I'll be close to a farm field that is higher than my property. It makes me think that instead of first thaw, wait a little more into spring to avoid possible water issues.

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fidlersound t1_je77f4g wrote

That will work well - holes are deep enough for sure. Pressure treated wood wont rot.

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EeyorONzoloft1 OP t1_je7cuek wrote

Do you think a 10 inch wide hole is wide enough?

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fidlersound t1_je7evc7 wrote

Probably but you might be safer with 12 inches - double the width of your post is common.

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Kesshh t1_je7atd8 wrote

Are you in a climate that freezes? If so, you posts should be at least a couple of feet below frost line.

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EeyorONzoloft1 OP t1_je7cx4v wrote

I am in a climate that freezes. Not sure what our frost line is. I'm in Wi.

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chimpyjnuts t1_je7bky4 wrote

Treated wood will only rot where there's both water and air, so take car of that area - I try to make sure there's a couple of inches of gravel at the top vs dirt.

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EeyorONzoloft1 OP t1_je7d2q1 wrote

I didn't think about on top, that should be easy to do. Is there and wrapping of wood that would be necessary?

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chimpyjnuts t1_je95z40 wrote

Just a bit of gravel so there can't be any exposed water at the surface. The other option i've seen is a small 'hill' of concrete at the very top to shed the water.

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_owlstoathens_ t1_jebuast wrote

I was told by a structural engineer that when they changed the way pt lumber was made due to toxicity concerns it greatly reduced its lifespan - his estimate was that it typically lasts 10 yrs before it weakens due to environmental conditions, and in areas like the northeast that have constant freeze thaw cycles and wet seasons he trusts it for about 8. If you want your structure to last a long time it may be wise to use on of the many kinds of post base connectors or brackets.

In the northeast we typically detail 12” of crushed stone, footing above, leave a couple inches exposed above grade or soil line min., and if possible taper the footing top to avoid water sitting against the post, doesn’t have to be too dramatic but just a soft pitch away.

Our frost line is 4’ so I’m guessing you’re something similar where you are.

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