Lifespan of 3/4" Pressure Treated Plywood

[Wayne Boatwright]

Background... We live SE of Phoenix in the desert where we get sparse and infrequent rain. Our soil is typical desert sandy soil.

We have a "ground set" manufactured home which is fully supported on steel piers that stand on concrete pads. There is a 36" crawlspace underneath the house. During installation, there was a framework of pressure treated

2x4's constructed around the perimeter of the house, attached to the house's sill plate at the top, and bolted to a poured concrete perimeter footer. After the framework was completed, it was covered with pressure- treated 3/4" plywood, forming a containment barrier between the crawlspace and the exterior. The last step was to backfill the exterior against the plywood.

Our entire yard is covered in a combination of decomposed and 3/4" crushed granite. There is no grass, and 90% of the landscape plants are cacti and other desert plants which require almost no watering. Farther away from the house is a citrus tree, an ash tree, and a desert pine.

This underground construction is common in our area, although there are also homes where the construction is of concrete block or poured concrete. Obviously, either of those choices would outlive pressure-treated plywood, but the price differential was huge, about 8-10K, which we could ill afford at the time.

I know that if we need to, concrete block construction can replace the plywood in the future, but I'm wondering if anyone can give an educated guess as to how long this plywood wall will last.

TIA

[hrhofmann]

Are there any termites in Arizona????

[gfretwell]

When I was looking at PT plywood at the Wolman plant the engineer there told me PT plywood resists bugs but it is not much more water resistant than regular exterior grade plywood. For my application I ended up with MDO plywood.

[ransley]

I have 40yr old PT decking on a lake up north where carpenter ants and termites are everywhere, the wood is fine. In a desert you might get

960 more years for the wood I have.

[aemeijers]

Lemme guess- that is the old-style military spec pressure-treating (like the green wood ammo boxes), where you aren't supposed to let kids walk/crawl on it, and then stick their hands and feet in their mouths? The same kind where lotsa volunteer-built playgrounds had to be torn down? IOW the stuff that works, but is no longer rated for human contact? IIRC, the chemical used is a cousin to PCB- half the molecule? (I'm no chemist, I could be wrong, but someone will be along shortly to rub my nose in it.)

[Frank]

Plenty of information out there:

The arsenates previously used are highly restricted and newer stuff is not as good. I've got posts in the ground over 30 years that are still functional. Chlorinated and metallic preservatives are still in use but chromium and arsenic were the concerns but EPA says they are still beneficial for some usage.

Moisture resistance would be a second concern. I've had exterior plywood go bad in a few years where it was continuously damp.

[ransley]

How about it all that was sold, and unless you burn it, or eat it, the top layer wore off 15 years ago, Whats this about not walking on it, its what all decks, peers, docks are made of everywhere, I dont see any poisoned kids, and havnt heard about that fear mongering, so you say ban all kids from summer fun on all pt decks and piers?

[aemeijers]

'I' didn't say it, 'They' said it. Mebbe the stuff sold at the Borg was never that bad, but the stuff used by military/farmers/marinas would sure give you a good contact rash. (Learned that the hard way as a kid, lugging planks and posts and landscape timbers.) And when my employers sold stuff made out of green wood like that (boxes, skids, etc), the buyer had to sign a piece of paper that they wouldn't make kitchen cabinets or cribs or other human/food contact stuff out of them, and they would use suitable precautions when sanding or cutting. As to the playgrounds, maybe the Nannys were over-reacting, but they had to rebuild several playgrounds around here when the scare was in all the papers.

Hell, when I was a kid, all the rustic picnic tables at the highway rest stops were made out of the stuff...

-- aem sends...

[krw]

Creasote gives me a serious rash if I'm even near where it's being applied - even the fake stuff.

I certainly wouldn't use the stuff indoors (it does make dandy sills) and I was cautious when cutting it (down wind) but I've never heard of anyone being harmed by it. This certainly would be the first time nannies overreacted. Lead paint is going to rear it's ugly head, again.

The five-second rule is suspended for public (and personal, for that matter) picnic tables.

[Wayne Boatwright]

On Sun 07 Mar 2010 04:16:11a, ransley told us...

That's very encouraging... Thanks!

[Wayne Boatwright]

On Sun 07 Mar 2010 01:59:05p, Frank told us...

I have no idea what chemicals this plywood was pressure treated with. It does have a greenish cast to it. Dampness, of course, could be an issue. However, our climate is so dry and our rain so infrequent and sparse, I'm not sure it's a serious threat.

[Rudy]

It will last a lot longer if you REMOVE the dirt fill lying up against the ply. Do I understand that you essentially have a 3' tall plywood wall all around your "crawlspace" ? Is it just the bare ply w/ nothing covering it or is it T111 style (slots cut in it every 4" or so)? Is it painted or stained ? This is what they commonly call "skirting" on a mobile home.. and It's usually vented vinyl or aluminum.

[Wayne Boatwright]

On Wed 10 Mar 2010 09:49:11a, Rudy told us...

That would leave me with a 3 foot deep pit surrounding the house.

Do I understand that you essentially have a 3' tall plywood wall all

Yes, all but about 8 inches is below ground level.

It is bare pressure treated plywood, no slots or spaces. There are 2' x

1.5' ventilation shafts to the crawlspace at each corner of the house, in adddition to a 4'x 3' access opening at one end of the house. All of these openings have metal gridwork over the top.

No.

No, this is not skirting. Skirting is used when a mobile or manufactured home is installed above grade on pillars at grade level. Skirting is used primarily for aesthetic reasons. Our installation brings the bottom of the house within a few inches of grade level, as the pillars are set on concrete pads at the bottom of the 3' deep below grade crawlspace. The plywood and framing that supports it might be considered a quasi equivalent of a stemwall. The few inches of the plywood that rises above grade is covered with powder-coated aluminum sheathing in a color that blends with the siding on the house.

[Rudy]

Ok, now I get it..I've never heard of such an installation..a 3'plywood foundation/stemwall Bizarre ..I'd have gone with "block", Can't give you any advice..Good luck

[Wayne Boatwright]

On Wed 10 Mar 2010 11:57:53p, Rudy told us...

It's fairly common here in the AZ desert, given that the ground is very dry and we have sparse sporadic rain that practically evaporates before it hits the ground.

It probably would be bizarre in any other climate. I'd have gone with "block", too, but it would have cost 8-10K more, which we really couldn't afford at the time. The plywood could be replaced with block in the future if it becomes necessary. I'm just trying to get a handle on the lifespan of the plywood.

[KC]

For a comparison, in the Southeast US, with it's high rain & humidity, in-ground swimming pools with vinyl liners are built with PT plywood walls completely buried in the ground. They are guaranteed for 40 years. If they can survive that long in that environment, I would guess yours should last much longer than your lifetime.

KC

[aemeijers]

Pool liners and wood foundations do sort of work, if you can make it basically an anaerobic environment. Same concept as the thing you see in the papers every couple of years, where some college science class does a few boreholes in the local landfill, and come up with legible newspapers and recognizable food waste from 30-40 years ago. Every few years over in the UK, they find some thousand-year-old body in a peat bog, remarkably well preserved. In the absence of o2, rot slows way down. All over the world, people are making good money pulling hundred year old sunken old-growth logs out of muddy river bottoms and bogs, or cold still deep lakes. Low temps and/or o2, and the wood is still good.

-- aem sends...

[Tony Hwang]

Hi, My on-site built cabin has a same kinda set up up here in central Albeta. After 15 years nothing changed. I bet yours will last as long as the house.

[Wayne Boatwright]

On Sat 13 Mar 2010 09:24:52p, Tony Hwang told us...

Thanks for your input, Tony.That's encouraging.