I am putting up a fence for my sister. In the past I have always place the vertical support in a post hole and then filled the remaining area in the hole with cement. I was told recently at a hardware store that I shouldn't do that with modern treated lumber because the cement will wick water into the treated lumber and I will have a weak spot where the cement and the lumber meet. Is this true?

I have never heard of the problem but below are some opinions on the issue.

Pressure treated posts set in concrete? - DIY Chatroom - DIY Home Improvement Forum

Thal I agree with smear case Ive never had a problem with but then again most of the soil in my are is extremely sandy so the soil doesn't retain water as well as say clay. You will want to make sure you don't have any standing water around your posts. I know people will see this and go oh no that's not how its done anymore but I personally have had good luck with posts set in concrete. Just my two cents. GOOD LUCK, AC

Another issue is that the PT wood soaks up water from the concrete and tends to shrink, leaving a void between the post and concrete where water can get in and cause rot.

But I have seen it done both ways and have never heard of a problem with either.

I've also heard that. No sure I believe it. Oh, concrete will absorb moisture but when the ground dries the concrete dries. Also the concrete will resist moisture from the ground for a period, Seems like a wash to me. Also the PT lumber is intended or allowed to be in contact with moisture, that's the purpose of treating it. Be aware that the treated landscaping timber is not the same as the pressure treated 4x4. I don't think that the treated landscaping timber is treated under pressure, which mean that not nearly as much of th chemical is used.

One reason for using concrete in a post hole is to increase the diameter of the material in the ground for increased resistance to tilting. You pour concrete around chain link fence post because the post is so small there is not enough surface in contact with the earth to resist tilting. A 4x4 post has much more surface to contact the earth. Adding concrete increases the surface, however.

If th post are sufficiently stable, don't use concrete. If you want more stability use the concrete. Incidentally you can pour it dry, earth moisture will eventually harden it, or just pour a little water in after you have set the post.

Either way its going to last longer than you will ever have to worry about.

Amricca
Wood doesn't shrink when it absorbs moisture, it swells. Most PT material comes so full of moisture it can't absorb any more. Weighs about twice as much as when it is dry.

I agree with Harold, as usual. PT lumber is soaked with chemical to the point where it is saturated, I can't absorb any more moisture. You do not want any concrete to be under the post, that would make a concrete bowl and really trap water. Chances are really good that your concrete pour will develop at least one crack at its weakest point and assist in allowing moisture from the ground or rain to dissipate.

Ballenger, so the post should be set on earth then concrete poured around it? Just curious, I watched a Holmes on Homes and he set all the posts in concrete, and he dug the hole a little wider at the bottom so it would not heave, even though he set all the posts 4 feet deep. Tried to give you a greenie to balance that but it won't let me. Never heard to putting gravel at the bottom of the hole.

Have unearthed plenty of old posts some are so old no one can remember a fence being there, one in particular a locust post partially buried under a tree in the fencerow the rings of which show it to have grown in that spot over the post since 1951 until it died. The bottoms of posts will be perfectly OK with no special treatment, wood that stays wet (in contact with the ground) lasts forever, wood that stays dry lasts. Wood that is alternately wet and then dry will succumb to dry rot. A post will rot off right at ground level while above and below that it will be perfectly fine. So you cansave cement by just using a "donut" of cement around the top of the posthole, backfill with the native soil below that. I have used gravel but when the post rots off find it harder than dirt to pull the old post back out, redig the hole (I use a clamshell post hole digger) if I used gravel. With dirt I can just wiggle the post, pour a pailfull of water around it and then lift it right out and pop in the new one like a magician

Thal, I am curious why you gave me an inaccurate rating when you read that I said to not put concrete under the post. You clearly plan to avoid concrete under the post.

Amricca, I did not indicate that gravel should be in the hole, Thal made that comment.

I know it was Thal, that is why I tried to give you a greenie ballenger (I couldn't because I agree with you so much). Don't they make a spike that goes on the end of the post that is driven into the ground, then concrete goes in.

Dah, sometimes I just miss the forest because of all those trees, my bad. Thanks for the greenies in the past.

Sorry Bob but I couldn't give you a greenie either, for the same reason a Amrica.

Thal,
We don't give reddies for differences of opinion, only if some is factually and grossly wrong.

Thanks Harold, I can't give either of you guys a greenie until I spread it around either. Guess we got to read more posts. Not sure what Thal was thinking, I think he misunderstood or hit the wrong key, seems he knows to not let concrete get under the post. See you, Bob

He said he accidentally gave you that reddie, I'll come back and balance it later when I can.

Yes they do make a steel spike but that is most frequently used for things like a mail box which will not have much force applied to it. Simpson Strong-Tie E-Z Spike-FPBS44 at The Home Depot I would not use them for a fence post.