No problem at all, aside from the fact that a 4'x4'x1' bed = 16 cubic feet of material, almost 60% of a cubic yard per bed. That being said, I made three of this this way. Well technically they're 4'x4'x11" since a 2"x6" nominal board really only measures 1.5" x 5.5" but who's counting, lol.

My soil is unsuitable for cultivating anything. it's hard tallbot clay - and only six inches deep. Below that it's lithic bedrock, lol...

Mel also talks about using "highrise boxes" for longer carrots.

I just made a bunch of beds and decided to use 2X10's. I figured they are 9.5" deep, and since I'd fill them an inch or so from the top, I'd have 8.5" of good growing soil. Seemed like a reasonable balance.

Watch the wider boards for warping and cupping. Try to pick the straightest ones you can find; it makes it easier to keep things square when you put them together.

Since I put a weed barrier (landscape fabric) under my beds, I didn't want to hinder the root crops from growing to their potential so I went 16 inches deep. I'd say go as high as you can afford to go. Sure the wood isn't that expensive, but filling numerous beds 12 inches deep means twice the volume of "dirt" to fill it. In my case, my 130 SF of space took 6.5 cubic yards of Mel's Mix to fill. Boy was that a ton of work (worth every second though).

Here is a link that might be useful: Sinfonian's garden adventure!

I built 16 inch boxes because i liked the way that it looked. I filled it with mels mix to the top about 6 weeks ago and it has already dropped almost 3 inches. I guess the weight of the soil and water is packing it down.

Once the settling is done, compost will also reduce in volume by about half in a year from further decomposition, and the combination is why I usually slightly overfill when topping off with purchased bulk compost.

The first few boxes that we built were around 7 inches. We then filled the surrounding area with woodchips, so the boxes only stick up around 4-5 inches.

These three boxes have been plagued by a very oblivious garden pest--my thirty pound beagle, who sees the boxes as a fun alternate pathway through the garden (planted, even with plants 8 inches tall, milk jugs over some of the plants, and an L-shaped 6 foot tall conduit trellis arrangement).

If for no other reason, I'm happy that the rest of my boxes are 12 inches tall as a dog deterrent.

I Made a box just for my carrots it's 3" wide x 12" Deep x 6ft. long. I did it to get the carrots up off the ground - away from the slugs & other pests who were chewing them up as soon as they sprouted. Oh and to make sure there were NO rocks to hinder grow. Yes, it works great.

It was an odd shaped pallet, I wasn't sure what to do with it. So the width is the thickness of a pallet.

I put 2x4 stakes in the ground to keep it level & from falling over.

My boxes are exactly 9" tall and that seems to work, even for carrots and corn. I couold easily add another 3" timber, if there was a crop that needed it.

This is my third house with a sfg, but the first one that I lined the "sides" of the boxes with weedcloth. Seems to keep leaching between timbers.

I have a 12 inch bed but the mix has settled to about 8-9 inches of actual growing medium. So, that's something to take into account.