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knoxvillegardener

Tall raised beds - wood strong enough not to bow?

knoxvillegardener
14 years ago

Hello,

I'm planning to build raised beds, mostly 3' x 6' with some being 2' x 6' or other similar sizes.

Right now the plan is to make the frames from 2x8 lumber (basic untreated white wood from Home Depot - probably a softwood like pine), with two 2x8 frames stacked on top of each other. This will allow me to have soil one foot deep in the raised bed and give an extra 2 1/2 inches clearance at the top for adding mulch, etc at a later time.

The soil is going to be pretty standard; e.g. not a lightweight soil like Mel's mix from Square Foot Gardening.

My question is this:

Are the frames likely to be strong enough to retain their shape? Or will they bow outwards in the middle of the six foot length due to the weight of the soil? (In which case I should be thinking about how to strengthen them so they'll hold their shape.)

Thanks in advance!

Comments (13)

  • ryangrogers
    14 years ago

    If the beds are going to be sitting over existing soil, then you could drive a sturdy stake or t-post into the ground on the inside middle of each 6 foot length, and screw/nail each of your stacked 2x8's to it. That should keep it from bowing. You'll probably want something on the inside regardless to bind the two stacked layers together...

    My bigger concern would actually be how long you want it to last - untreated pine 2x8's in constant contact with moist soil won't last too long...

  • knoxvillegardener
    Original Author
    14 years ago

    Ryan,

    I plan to treat the timber with a non-toxic coating material that's supposed to make them last a very long time when in contact with soil.

  • dcarch7 d c f l a s h 7 @ y a h o o . c o m
    14 years ago

    Bowing will be mostly by the differential expansion of the wood due to moisture, not the weight of the soil.

    dcarch

  • Dan _Staley (5b Sunset 2B AHS 7)
    14 years ago

    IME - I had a landscape design and construction business and built raised beds like this for a client in a wheelchair - you are going to need posts to prevent distortion for when the soil gets wet. If you check a construction book in the rack the next time you go to HD-Lowes, you'll see several guide books that detail this type of construction.

    I over-engineer everything and want my projects to be perfect, so there's that caveat. But check out the books in the racks at HD. You'll see that they will recommend posts on the outside. I'd do every 3' in that place.

    My 2¢

    Dan

  • dadhaslonglegs
    14 years ago

    I think you'll be fine if your aren't looking for perfection. My beds are made of 2x8 (10 ft long) and i just drove stakes down every so often to hold them up and then used brackets in the corners where i had them meeting (90 degrees). Mine are treated (the old stuff) and i just painted them with an exterior latex. They've held up well over the past few years. I think yours will rot, but it may take a few years to get through such thick boards. I wouldn't be scared to use them without any treatment. I can buy 8ft sections of 2x8's for $4 a piece right now. So what if they only last 5 years (probably more like 8 to 10).

  • ryangrogers
    14 years ago

    knoxville,

    you mind sharing what your non-toxic preservative coating is?

    I've used a product called Lifetime Wood Treatment (link below) made by Valhalla for the wood framing in my greenhouse, but I haven't used it in places that would be in contact with soil.

    Here is a link that might be useful: Lifetime Wood Treatment

  • eric_wa
    14 years ago

    {{gwi:289252}}

    {{gwi:289253}}

    {{gwi:289254}}

    We installed these 5 raised beds today. Two layers of 2x10 hem/fir. Each box is progressively 1ft longer. First one is 3ft x 12ft and last is 3ft x 16ft. These were built for a 84 year old. She figured if they last 5 years, that good enough for her.

    Eric

  • Dan _Staley (5b Sunset 2B AHS 7)
    14 years ago

    Saaaay...where are the shadows? ;o)

    Mr over-engineer Dan would have at least one post/stake in the middle, my old landscape jobs are all on old-school photos in a box, else I'd link some..

    Good on Grandma still gardening at 84! I'm sure she'll grow some good stuff.

    Dan

  • eric_wa
    14 years ago

    Dan,

    Most of last week and Monday this week was sunny. Tuesday it was drizzly. Go figure.

    Hope to be still kicking and biting at 84.

    Eric

  • kudzu9
    14 years ago

    When I built my nearly foot-high, 4' X 8' raised beds, I just cut a cross-piece that I installed at the midpoint to prevent bowing. The sides are 1 X 6 cedar stacked double height. The cross piece is 2 X 4 cedar that is installed at the half way point up the sides so that it spans the seam where the pieces of 1 X 6 meet. I screwed through the sides and into the ends of the brace with two 3" deck screws on each side. Once the beds were filled with dirt, the cross piece was buried, so it wasn't visible, and there was no noticeable bowing.

  • eric_wa
    14 years ago

    kudzu9,

    Shhh that's our little secret. We put in a short piece of 2x4 vertically, to hold the layers together. Then a 2x4 stretcher across to keep from bowing. All buried and will rot.

    Eric

  • eric_wa
    14 years ago

    When is enough, enough?

    We went back today and installed two more raised boxed beds. 7 total.

    {{gwi:289255}}

    {{gwi:289256}}

    4 of the original 5 she had planted all ready.

    It's a sickness.

    Eric