Pressure treated in-ground shrinkage

[Aaron Rosenthal]

So, I have a fence separating the back alley from my property. 2 of the 5 PT posts rotted out, so I’m replacing them. No problem.
I don’t usually go to the Orange Borg, or the Blue One either, but instead, to my local independent lumber yard since I’m a 30+ year customer there. However their 50 acre property is outdoors and since it’s a builder’s yard, the PT wood is wet - and heavy!
The full 4x4 posts are wet, and I have a feeling that the ones I’m replacing were also wet; they shrunk from the post haste concrete and allowed water pooling inside the “boot” of concrete and accelerated the deterioration.
I can delay doing the fence a few more weeks while I dry the posts inside my house, but does that plan sound logical?

[Zachary Hoyt]

My understanding is that pouring concrete around PT posts makes them rot, since the water is trapped. I think best practice around here is to put a concrete disk or a flat rock at the bottom of the hole if a footer is needed, and then to fill in around the post with gravel and tamp it well. The gravel will allow the water to drain away from the post. Since you already have concrete holes in the ground this may not be helpful, unless the concrete can be removed easily.

[Aaron Rosenthal]

Thanks Zachary. However, when I write "boot", my procedure has always been to place the post onto the hole, plumb it inyo place and THEN put the fast set concrete surrounding it.
That way the water from rainfall has a way to escape into the water table.
Again, I was a contractor for decades and built/had built all sorts of in-ground structures. It was just very odd to me that 2 of the 5 posts rotted at ground level and the other 3 did not. And, fortunately, I never had a call-back on this kind of problem.
So, I don't know if it was the posts not treated properly in the original build, or wood shrinkage as the liquid from the treating gave too much clearance for water pooling, or what.

[Ron Citerone]

I think at the depth you would put a post in Vancouver, they will end up in wet ground no matter what you do. I did see a carpenter wrap post bottoms in heavy plastic to avoid direct contact of wood and concrete. I drove stainless nails into the bottom of posts sticking out to make them stay attached to the concrete I poured around them. I did that because I didn't want them lifting in a freeze. Not sure if that was proper technique, but made sense at the time.

Asking the man at the lumber yard might help as he may have access to manufacturer reccomendations?

[Ken Fitzgerald]

I have always put a couple shovelfuls of large gravel at the bottom of the fence posts I have installed. The gravel provides drainage.

[Mark Bolton]

It'd seem any number of factors could have affected why some of the posts rotted and where as opposed to others. Issues with the lumber, knots, grain, hard/soft, how the grain was running in a area off the post affecting how it took on/shed water and how it took on/shed the pressure treatment, etc..

I'd think it'd be hard to nail the failure down.

Sound practice to toss some gravel in the bottom of the hole to leave the bottom of the post open to drain, always put your cut end up (most people do it the other way around). Wicking up water from the post standing in the hole is always best dealt with by having the treated end of the post down.

Didnt see you mention how old your rotted posts were but my guess is your just dealing with the fact that the treated of the past perhaps 20 years is not the same as it was and just doesnt last like the old CCA.

[Tom M King]

They rot faster in concrete than just in the ground. I put gate posts in concrete, but not line posts. Subsoil here is nothing but red clay, so it might vary somewhere else.

We have a couple of miles of fence posts from the old type treatment that are now 42 years old, and still hard as a rock.

I installed another part in 2004 with the newer type of ground contact treatment. Probably one in twenty of the line posts didn't last past 12 years (rest are still fine), and gate posts in concrete lasted 4.

[Bill Dufour]

Used to be some treated wood was rated for ground contact and the less expensive was not contact rated. Now it is no longer rated for anything. I bet they just do the minimum above ground treatment level and charge full price.
Bill D

[Mark Bolton]

Used to be some treated wood was rated for ground contact and the less expensive was not contact rated. Now it is no longer rated for anything. I bet they just do the minimum above ground treatment level and charge full price.
Bill D

In my world its just the opposite. It USE TO BE that all pressure treated was the same. PERIOD. When the nightmare of people improperly disposing of CCA treated lumber (burning it or burying it) left us dealing with the numerous other treating variants (ACQ a-b-c or d, CA-b, CA-c, Cu NW) where what the industry basically did to protect CCA for industrial applications, was to re-formulate and trade the Arsenic for Copper. Copper is extremely expensive of course. So they then moved to ground contact, and non-ground contact to reduce the cost to the consumer.

So now, pretty much, posts are the only thing rated for ground contact. 2x's somewhat but less so, decking no way in heck ground contact.

Had dingbats not taken home pressure treated scraps and burned them in their wood stoves, or buried them on job sites, we would still be using CCA to this day and not be spending 2.5x as much for hangers and fasteners because copper eats steel.

I recently went to a campground where the campsite we pulled the van into, the gracious last occupant, left us their un-used firewood which was all pressure treated wood. So that brainiac sat there at that campsite drinking beer, roasting marshmallows, having their kids roast weeners on a stick, and likely playing in a fire over concentrated ash from wood designed to be poisonous to biological life forms...

[Lawrence Duckworth]

Sound practice to toss some gravel in the bottom of the hole to leave the bottom of the post open to drain, always put your cut end up (most people do it the other way around). .

if you dig a hole and put rocks in it all you have is a hole with rocks in it, rain water will not "drain" any faster or slower because there are rocks in the hole.

[Jerry Bruette]

I don't know if it's available in Canada but our local green Borg has Critical Structural treated lumber. It's rated for below grade use and in some states it's allowed to be used for foundations.

[Mark Bolton]

if you dig a hole and put rocks in it all you have is a hole with rocks in it, rain water will not "drain" any faster or slower because there are rocks in the hole.

The point is the bottom of the post (which wicks up water) has a chance to remain dry. It just creates a bit of a well below the post to give the water a chance to absorb into the soil (no different than a septic system).

[Lawrence Duckworth]

The point is the bottom of the post (which wicks up water) has a chance to remain dry. It just creates a bit of a well below the post to give the water a chance to absorb into the soil (no different than a septic system).

.

.....a septic system is very different than a post hole.

[Lawrence Duckworth]

Aaron...you'd do better not to have any rocks or gravel puddling or storing water in the post hole, especially in a freeze. I prefer to seal the bottom 2' of the post, dig or bore a 12" dia. hole 30" to 36" deep and pack as dense as possible 6" of dirt back into the hole and do the same dense packing around the post, and build a slight mound of dirt around the base to shed the water.
With your situation I think I'd undersize the new post to fit the sealed 2' end back in the concrete hole, and grout the gap. don't forget to shed the water away if possible.

[Jim Becker]

Mark, it's even more complicated...there's rated for "in" ground contact (like posts) and "on" ground contact for actually touching the ground, but not in the ground. As you mentioned, there's yet another category for "it might get wet". It's hard enough for contractors to keep track of this stuff...and a lot of consumers, well... it can be ugly.

Aaron, there are some new methods available to help protect posts even further than the treatment, and they include types of wraps and sleeves. Some post framers are using these where for cost reasons they don't want to do piers. These might be worthy of checking out for those posts that have to be in particularly difficult conditions.