When we bought our house, there were (and still are) about a dozen heavy timbers, 4" x 5" x 12’ (there may be different lengths, I can’t measure them effectively where they are now) stacked on a very heavy shelf. I want to use those timbers to construct a rustic, tiered planter to sit next to a fence outside. The bottom layer will be about 70" by 30", the next will by 48" long, and the next 26" long. Each layer will be 10" deep, made of two rows of the timbers bolted together.
I need to put a “floor” under each level*, so they don’t have to be filled all the way to the ground, and it is those floors I need help with. They are going to be damp most of the time, as the planters will get watered and the soil at the bottom will retain moisture the longest. It seems almost any kind of wood will eventually rot away, no matter how effectively treated. I thought of heavy-duty galvanized (or otherwise coated) wire mesh covered with weed cloth to keep the soil in, but I wonder how long that will last. 19-gauge seems to be the heaviest gauge that is readily available, but I could try to find something heavier if that would be better, or I could double it up.
That’s about as far as my thinking has gone. Suggestions gratefully received.
*the floor under the bottom level is to try to keep the gophers out.
Cedar or Cyprus will resist rot for a long time, kept moist cedar will start rotting in 7-10 years, but should hold shape even rotting for 15 years. As you’re not moving these planters or sitting on them, it is a legit possibility.
Cedar decks and shingles are usually good for 20-40 years.
Concrete would definitely work. Though, filling all the way to the ground would also work, and whatever reason you have for not wanting that seems likely to also apply to concrete.
Another approach is to use pressure treated plywood or planks and cover them with roofing material made for ice and water protection. A couple of layers of lighter weight roofing underlayment might work too. You can wrap that stuff up the sides of the planter boxes too.
Thick enough polyethylene sheet will work too. A double layer of 6mil black plastic sheet should hold up for a long time
Depending on your location… Facebook marketplace here in RI has frequent offers of free bricks, and pavers. Salvaged concrete blocks would also work. Someone replacing a deck might even pay you to take away a pile of pressure treated deck boards with splintered ends, still plenty long enough to cut to sizes you need.
Some very interesting ideas here, thank you. Please keep them coming.
I should have mentioned that the bottoms will need to have (or allow for) holes for drainage, and be able to support the weight of soil that is 10" or so deep.
Based on some of these ideas, I’m now thinking of using 0.45" pressure-treated sheathing plywood, which I would cut to size, drill appropriate drainage holes, then wrap in polyethylene into which I would punch holes in the same places. Then I would use some kind of waterproof tape to wrap the holes so the water wouldn’t seep under the plastic, or maybe there is some easier way to do that. I would probably then cover the whole thing with weed cloth to keep the soil from working its way into the holes.
On the bottom level I would still want to use wire mesh (or something) to try to keep the gophers from gnawing their way through that relatively thin board.
Drainage changes the picture. You might consider having a layer of gravel on the bottom. So maybe the solution is very different. I used to make aquarium under gravel filters using plastic egg crate with plastic screening over it to hold the gravel. Even without a gravel layer you could weed cloth over it but I think anything will clog up pretty fast in direct contact with soil.
The egg crate is pretty fairly strong but still needs support underneath. Pressure treated 2x2s every 6 to 8" should be plenty to hold it up.
I’d reconsider doing a ‘floor’ under each level. Almost anything will eventually wear out, especially outside with a void under it. Instead, fill each level to to where the next one starts.
One way to do this is with dirt, or some cheaper fill (gravel, rocks, recycled chunks of concrete or asphalt, used bricks, etc.). Whatever you can get cheaply.
Another way is building supply companies can supply large blocks of styrofoam or similar that can be used as fill. They are used to fill voids when pouring concrete, because making it from solid concrete all the way thru would be too expensive (as well as having problems with heat & drying time). They are light & easy to work with, but solid enough to bear weight on top of them.
Expanded metal.
I think most of the readers of this thread are know what expanded metal is but for those unaware it is made from a sheet of metal cut with a pattern of slits and the sheet is then pulled and stretched to form a diamond grate pattern.
1" PVC pipe or similar may be a better choice than PT 2x2s if you can drill mounting holes in the timbers to hold them. Egg crate is surprisingly strong if it remains flat with proper support. In a tank that was about 18"x48" I supported a piece of egg crate with just 8 1" diameter plastic vertical standoffs. So possibly support stringers can be spaced as much 12" apart.
Was that supporting just the weight of the egg crate, or was there something piled on top of it? Remember, we need enough dirt on top to plant things in, here.
Come to think of it, what will you be planting in these planters? If it’s something with aggressive roots, that’ll be another potential source of failure for your structure.
Egg crate is plenty strong if it has enough support to stay flat. My aquarium filters had to support 2" of coral gravel plus large pieces of live and dead coral. I think it can handle an inch of gravel plus 9 inches of soil. Exactly how much support is open to question but I think that can be easily tested.
I can’t say if this approach is what the OP needs. Pressure treated plywood with holes in it will work, silicone can help seal up those holes. If a fully porous shelf is not desirable I’d use some deck boards, either PT or composite to make a shelf with a gap between boards to cover with egg crate of just plastic mesh depending on the size of the gap. The boards can be installed so they tilt down toward the gap for drainage. There are a lot of ways to approach this.
I don’t know much about plant roots, and what’s aggressive or what isn’t. The plants won’t be food-related, and they won’t be anything that is going to grow large. Probably mostly annual plants, bulbs, maybe a few perennials that I hope will cascade down the sides for picturesque effect.
I would agree, except that I don’t have very much dirt and rubble to hand. And that stuff may be inexpensive to acquire, but due to where I live (San Francisco) which has zero lot lines (i.e. no spaces between houses) every pound of that sort of thing has to be carried through the house from the front to the back, and down a more-than-one story staircase to where the garden is. In younger and more energetic days I did that a lot, but it’s not worth it to me now just to fill in a space. I suppose some sort of framing with pressure-treated lumber would work to hold up the floors.
Hardware cloth can be rather flimsy. I’ve have good stuff rabbit wire before. I had to shop around to find it. It comes in 2x4 sheets, not rolls.
It’s matters how much weight will be on it whether it will sag or not.
But, heck I’d try it. What’s the worse that could happen?