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Is "sheathing" plywood okay for the bottom of a raised bed?

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Is "sheathing" plywood okay for the bottom of a raised bed?

Post  CatchAFire on 4/22/2011, 5:01 pm

I bought a piece sheathing at Home Depot today for the bottom of my box (looks just like plywood, but has a different name, I guess). A 4'x8' piece was $18. It was the cheapest 4'x8' piece of plywood-like wood I could find. Seems pricey, right? Maybe it's because I bought it in Queens, NY.

Before I load it up with MM and grow veggies, I want to make sure that it's safe--no chemicals, no formaldehyde. Couldn't tell from any of the signs at Home Depot. And none of the employees seemed to be experts.

I emailed and left a message with Boise Cascade, the maker of it. I'll let you know what I hear back from them.

In the meantime, I want to get going this afternoon on building the box.

Any thoughts? Thanks!

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Re: Is "sheathing" plywood okay for the bottom of a raised bed?

Post  boffer on 4/22/2011, 5:40 pm

On the left coast, 'sheathing' could mean 3/8-5/8 inch 3 or 5 ply for residential roofs, or under-siding for the walls. Your price is in the ballpark.

Almost all plywood these days use exterior adhesives. I don't know if there are toxicity issues. Hopefully the manufacturer can supply MSDS. Lining a box with plastic is a solution for some folks who use pressure treated lumbers for their boxes.

Not knowing what your objectives are for putting in a box bottom, another possibility is using hardware cloth and weedblock or plastic for the bottom.
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Re: Is "sheathing" plywood okay for the bottom of a raised bed?

Post  UnderTheBlackWalnut on 4/22/2011, 5:58 pm

Hi Boffer!

I'm pretty sure this is what we used for the bottom of our boxes. Hubby just told me it's "3/4" composite" (thinking you know what this means better than I do) Shocked . Hmmm...I didn't line them with plastic, should I have? I don't have too much growing yet, so I could try to pull out the MM and line and put it back in. How critical do you think that is? If it's critical, what kind of plastic would you use? I was thinking a thick black plastic like what you might use for total weed barrier?

I was sure to get untreated construction lumber for the sides, but didn't think about the bottom. Sad
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Re: Is "sheathing" plywood okay for the bottom of a raised bed?

Post  CatchAFire on 4/22/2011, 6:00 pm

Thanks Boffer.

The sheathing I bought is 5/8".

In Brooklyn our soil has high lead content, so I'm building a table-top box. I'm going to raise it one brick height off the ground. I know it'll be heavy, so I'll put bricks in the middle, too.

I'm going to drill holes in the sheathing/plywood for drainage. (On p. 58 "All New SFG" book says drill one for every square foot, but in the photo the guys seems to be doing more than that.)

Looking at the Boise MSDS, it seems like all their plywood has formaldehyde in it:

http://www.bc.com/wood/plywood/msds.html

There were some sheets of plywood at Home Depot that were advertised as formaldehyde-free, but they were something like $45. I guess they might be worth it for the peace of mind.

I guess I could line the wood I bought with plastic. (But isn't plastic itself toxic?) I have some thick clear plastic contractor's bags. What do you think?

I might be sounding a little paranoid, but I'm a little surprised that Mel doesn't talk more about the toxicity of plywood (and plastic) in the book.

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Re: Is "sheathing" plywood okay for the bottom of a raised bed?

Post  CatchAFire on 4/22/2011, 6:01 pm

UTBW, sounds like you're in a similar situation as I am. (You posted yours while I was writing mine Wink )

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Re: Is "sheathing" plywood okay for the bottom of a raised bed?

Post  boffer on 4/22/2011, 6:22 pm

CatchAFire...What's more toxic...the plywood or the plastic? I don't know. There is too much money driving the pro/con movements of products these days for any of us normal folks to be able to make rational decisions. You're getting into an area where only you can decide what you're comfortable with. An alternative is galvanized hardware cloth, if that is tolerable. Search the forum for 'table tops' to get ideas about how to use it.

UTBW...different regions of the country use terminology differently...but out here...'composite' means it's going to swell and fall apart as soon as it gets wet the first time. It's very possible that 'composite' in your area means it's made of recycled plastic and can handle moisture/water with no problems. Double check for peace of mind?
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Re: Is "sheathing" plywood okay for the bottom of a raised bed?

Post  UnderTheBlackWalnut on 4/22/2011, 7:41 pm

LOL - hubby said he just wanted to TRY the composite (yep it's the stuff that will not last) and he knows it won't last more than a year or two....so we are in the trial stages, which means I get to move the MM anyway, I guess, at some point. Smile

My objective is to keep the MM away from any iota of black walnut contaminated soil. We have the boxes raised up on cinder blocks. Smile

I read your post about the hardware cloth....I just didn't think to line everything with the 6mil plastic which you say you always do. I've got two boxes left and i'll go ahead and line those. How are you securing it? With a staple gun?

Hmmm... on page 58, the book talks about using plywood sheeting - I wonder...that must be the expensive formaldehyde-free stuff referenced above? That must be why it doesn't mention plastic.

Do you think I should just take apart the first two beds and start over? If so, has the MM become contaminated and should I trash it as well? I honestly didn't even think about any issues with the plywood. Smile

I took a quick look at the formaldehyde info that CatchAFire posted but it doesn't really address water/plants coming into contact with it. It's more building applications. I'll be curious what response CatchAFire gets from the manufacturer...

Smile UTBW
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Re: Is "sheathing" plywood okay for the bottom of a raised bed?

Post  CatchAFire on 9/28/2011, 4:16 pm

I'm late on posting this (life has a way of doing that), but I wanted to let y'all know what Boise Cascade told me about the plywood I bought:

4/25/11

Phenol Formaldehyde in the glue compound but NO urea formaldehyde. Phenol is exempt and CARB compliant. Should be fine for the garden. Here’s some info.

http://www.bc.com/wood/plywood/msds.html

Lot’s of info if needed on our grading agencies web site. If you can’t find what you need contact the help desk. They are more than happy to help. www.apawood.org <
http://www.apawood.org/>

Dave
Boise Cascade Panel Sales



I appreciate his response, but when I look up Phenol Formaldehyde, it doesn't sound very tasty:

http://msds.chem.ox.ac.uk/PH/phenol_formaldehyde_resin.html
"Harmful if swallowed or inhaled."

Yummy!

Maybe next time I'll spring for the $45 plywood with no formaldehyde. It may be a small price to pay for peace of mind.

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Re: Is "sheathing" plywood okay for the bottom of a raised bed?

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