Help - Building new raised beds

Okay, it is time to build some new beds.  Eventually there will be 9 beds exactly the same size (2 feet by 8 feet) so everything can be easily moved from one bed to the next (trellis, season extension hoops, etc).  So I want to make sure I plan ahead and not learn as I go.
 
I am redoing the beds for two main reasons.  My original beds of varying sizes (pine 2x10’s that I treated with raw linseed oil) are now 5 years old and starting to decay.  Also, I now have Southern Blight in some of my beds.  This is a soil borne disease that stays in the soil for 3 to 5 years.  So for those oldest beds that are showing signs of decay, I am going to replace the beds and MM.  For the beds that are still in good shape, I grew Mighty Mustard to turn into the soil to act as a biofumigant.  Don’t know if it will work but we shall see.
 
The beds have to be raised off the ground because the garden starts just under the drip line of a humongous oak tree on one side and two dogwood trees on another side.  I am tired of digging the roots out of the beds so I must raise them up.  I do not want my raised beds to have an opening under the bed (such as a table top).  I have had numerous visits from snakes and I don’t want them crawling out from under the beds and scaring the living crud out of me.
 
After quite a few months of thinking, I have decided on the following.  Make a 2 foot by 8 foot base using 2 inch thick by 12 inch high pressure treated pine.  Them make an actual bed for growing the veggies made exactly like the base except using untreated pine (we can’t get redwood or cedar unless it is specially ordered and it costs a fortune and sometimes not even available) that will sit directly on top of the base.  Between the growing bed and the base, I will need something to keep the soil in the growing bed from dropping down into the base. 
 

Actual growing bed - Untreated Pine - 2" thick, 12" high

Bottom to actual growing bed or top to base

Base of Raised Bed - Pressure Treated Pine - 2" thick, 12" high

 
So my first question is, should I attach the divider between the top growing bed and the base to the bottom of the growing bed or to the top of the base.


My next question is what material to use to make the bottom of the growing top bed.  I like the idea of Jimmy Cees deck boards.  I believe they will last a long time but am afraid of the chemicals used to treat them.  If they leach, will the chemical leach down into the base and not be a problem??  Or should I use something like pond liner on top of the deck boards??  Or should I make the bottom from plywood.  I don’t see that lasting very long.  I know Sanderson uses plywood so could use a little help here.  I think Sanderson said “outside plywood” but I don’t really know if there is such a thing.  Boffer said marine grade plywood but where do you find that and it must have chemicals added to make it marine grade.  Also, if I go with plywood, what is the best thickness of the plywood.


Thanks in advance for any help.

Yolos,  Do you want he beds to be 2' tall?  Then make table tops similar to my 2x7 and 2x8 TTs.  Plenty of air underneath to keep the plywood bottom dry to extend life and to see snakes.

If you want to have lower beds, use treated 4x4" on side every 2'.  Screw the 3/4" (or 1")  exterior plywood to the bottom of the bed frame and set on the 4x4s.  Use 1/4 or 1/8" hardware cloth stapled all around the base to preclude snakes.  Still have a nice air space underneath.

Drill drain holes, silicon seal the interior joint where the bed and plywood meet and line with commercial grade weed fabric to last longer. Install 1" thick grade PVC "cup holders" for convertible trellises or frames.

sanderson wrote:
Have you decided how you are going to build your new beds?

No I have not fully decided on how to build my beds.  Thank you for your previous help. I am going to use a bottom base and a top bed.   I think your suggestion to leave an air gap between the bottom base and the top growing bed is worthy of consideration. I always wondered why JimmyCee had that space between the top and bottom.  As you previously said, I guess it is to help dry out the bottom of the top bed.  Makes sense.  

I plan on using the same principle I stated above with a top bed and a bottom base.  Then leave a gap between the two and cover the gap with hardware cloth.  So I am down to deciding on what material to use for the bottom of the growing bed.  I keep forgetting to go around to Lowe's and Home Depot to ask if they have "outside" or "exterior" plywood.  I have never heard of that.  If they don't have it, there is one actual lumber yard left in this area and I will ask them.  

I can't start building until the bottom question is solved.  If I use plywood, then the dimensions for outside edge to outside edge must be 2 feet to take into account one-half of a sheet of plywood.

As the lawyers say, time is of the essence.

I'd never heard of outdoor plywood either. I always thought plywood=indoors or it de-laminates.
But I am finding apparently outdoor plywood described as 'severe weather.'
https://www.lowes.com/pd/Severe-Weather-19-32-in-Common-Square-Southern-Yellow-Pine-Plywood-Sheathing-Application-As-2-x-2/1000179669
The description/specs include:
"Pressure treated plywood for exterior applications and not intended for ground contact use" and "water-resistant"
but it also says "non-structural rated" and *all* of the "for use with" options have an X (=no, I assume) rather than a green checkmark (=yes, I assume) so it apparently has no use.  So someone else will have to say whether or not it's a "yeah it says that, but that's what we use anyhow." and my main contribution is an empathetic "huh?!? "

Good product, thick and without knots. $69.98 http://www.homedepot.com/p/AB-Marine-Grade-Plywood-Common-3-4-in-x-4-ft-x-8-ft-Actual-0-734-in-x-48-in-x-96-in-726540/202084532

This is what I have: $24.38. http://www.homedepot.com/p/23-32-in-x-4-ft-x-8-ft-Fir-Sheathing-Plywood-Actual-0-688-in-x-48-in-x-96-in-439614/100034683

Thanks for the links Beetles and Sanderson.  Unfortunately my local Home Depot and Lowes do not carry any of those products.  I guess I will have to go to a lumber yard.

Here's another option:
PVC Panel 
I saw this at Home Depot and it runs about $75. It's a bit pricey but not much more than the Marine Grade plywood. Also, since it's PVC, it should never rot.

FWIW - I emailed the manufacturer and posted their reply and MSDS in this thread:
https://squarefoot.forumotion.com/t20313-pvc-panel#242953

ralitaco wrote:Here's another option:
PVC Panel 
I saw this at Home Depot and it runs about $75. It's a bit pricey but not much more than the Marine Grade plywood. Also, since it's PVC, it should never rot.

FWIW - I emailed the manufacturer and posted their reply and MSDS in this thread:
https://squarefoot.forumotion.com/t20313-pvc-panel#242953

Okay, I think I will go with a bottom of PVC boards for the actual vegetable bed.  There are no 4' x 8' PVC panels available anywhere near me and I can't fit them in my small SUV so I will go with .75" x 9.25" x 8' PVC boards.  Just the bottom.  Therefore, I won't have to leave a gap between the top bed and the bottom base to help air out the bottom.  Because the bottom will not be exposed to the sun, there will not be any deterioration in the PVC.

https://www.lowes.com/pd/Royal-Mouldings-Limited-PVC-Board-Actual-0-75-in-x-9-25-in-x-8-ft/50092322

I know, it is expensive but I worked a couple weeks in Jan at my old job helping do the year end accounting work so I earned enough to pay for about 4 new beds.

I'm sure you know this, but those PVC boards will need to be supported from underneath as they are not very rigid.

Also, I heard a quote on the radio years ago by Bruce Williams (I think): "Cheapest is not always the Least Expensive"

ralitaco wrote:I'm sure you know this, but those PVC boards will need to be supported from underneath as they are not very rigid.

Also, I heard a quote on the radio years ago by Bruce Williams (I think): "Cheapest is not always the Least Expensive"

Yep - this is my design for the base, I think.  Do you think this is enough support.  The two end pieces and four interior braces.  All about 18.25 inches apart.  The support braces are 2" thick by 12" high, pressure treated pine. The PVC boards are .75 inches thick and 9.25 inches wide.  I plan to use three PVC boards making the outside width 27.75 inches.

Since your sides are going to be closed up and you won't be able to see if the PVC boards are sagging nor will you be able to easily access underneath to add more support, I think I might err on the side of caution and space the supports every 16" (qty 5) or even every 12" (qty 7).

Thanks ralitaco - I revised it so the braces are 13 inches apart.  That makes exactly 91 inches inside length of bed.  Fits exactly, almost.

RT, Good catch. Twenty-four O. C. is fine for 3/4 or 1" plywood. Recommending 16" O. C. for this material is much better. Yolos, 13" is ever better-er!

Glad I could be of help. It may be overkill, but at least you won't have to worry.