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I am building a 4'x 8' flower bed. The bed is 32" tall with a floor made out of 1x4" cedar four feet across. Soil depth is 12 inches, 32 cubic feet (8x4). Worst case 7.481 gallons per sq foot x 8 lbs of water per gallon = 60 lbs x 32 = 1915 lbs.

The design uses corrugated steel for the sides with two 2x4s screwed to the steel--one at 12" deep on the inside to support the 1x4 flooring and another 2x4 at 24" deep (the steel is 24" deep by 8'). On the outside there is a 1x4 frame with supports on the ends and at 24" intervals. So each side has two 2x4s and two 1x4 and corrugated steel, all eight feet long. The ends are the same with 4 foot lengths. Is this frame strong enough?

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    Is this box resting on soil? If so, don't put any floor in it. Your bed will benefit from moisture and organisms being allowed to pass to and fro from ground soil to your soil. And why is soil depth only 12" in a 32" high bed? – Jimmy Fix-it Nov 13 '19 at 2:07
  • your calculation makes no sense ... why are you calculating 7.481 gallons of water per cubic foot of soil? – jsotola Nov 13 '19 at 9:39
  • I have decided not to build this raised bed 4'x8' off the ground. Soil weighs 75 to 110lbs which makes this project over my expertise. – Joseph A Biddell Nov 13 '19 at 16:25
  • A sketch would be very helpful here. My hunch is that I'd want a proper framed floor (of treated 2x6, probably) with joists every 24". The cedar decking would run lengthwise, then. – isherwood Nov 13 '19 at 19:22
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The floor is marginal, the deflection is almost an inch. This might be tolerable, but deflections also put a lot of stress on the screws. Damp soil weighs more than water, 96 lbs/ft3 according to one website, so it could be worse than these calculations.

Floor calculation:

enter image description here

Another concern is the strength of the corners. If this stand isn't level, there will be tremendous forces on the corners. Even if it starts level, settling could make it un-level. I would sink PT 4x4s 3 ft into the ground in the corners (or maybe 6 total), so there is no chance of the corners rupturing and it falling and hurting someone.

I don't know how to account for the corrugated steel, so I didn't calculate the side strength.

Edit for new design:

Floor calculation:

enter image description here

2x6 side rail calculation:

enter image description here

Both of these calculations are OK. I can't comment on the remainder of the design without seeing a detailed diagram. I don't know how to analyze screw and bolt strength, so I wouldn't necessarily be able to confirm a complete design anyway.

If you are going to rest the 2x6 on top of the 4x4s, you need to be sure that there is no chance that it can slip off.

  • will you please determine if a new design would be reasonable. – Joseph A Biddell Nov 13 '19 at 16:26
  • New Design - 2'x8' raised bed with a floor 1 foot deep (16sq. ft.). The floor is cedar from a mill that is 7/8" thick (1"x 6", 2 feet long). The 2x4s are changed to 2x6s. The soil mixer weighs 1800-2,000 lbs. So this bed could weigh up to 1,200lbs. I will also use 4x4s for the corners directly under the 2x6s, so less worry about screw strength. – Joseph A Biddell Nov 13 '19 at 16:35

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