From the beginning of this project, I have been tying to figure out the best way to build grow beds that use the maximum amount of available space. The primary blocker has always been coming up with the best way to waterproof the beds which will be made out of wood. The original design included a simple grow bed design to lay out the space, but it wasn’t detailed enough to estimate materials and cost. We’ll discuss design updates, materials, and reasons for the choices we made below.
Design Changes
Our original design included 4 foot wide grow beds around the perimeter of the greenhouse with a large square center bed. After exploring usage with mocks and some space/usage planning I realized that a 4 foot bed is great if you have access to it from both sides, but since we won’t have that in the dome, we decided that 3 foot wide beds made more sense. I’m 6′ 1″ and it would be a pretty large stretch for me let alone my wife.
Current Design
As you can see in the above image, by reducing the bed width to 3 feet, we’re able to fit another round bed and a 2 foot wide access. The access might look a bit narrow (it is), but we have 2 foot wide doors to our master bathroom and it’s not too bad. I decided to keep the triangle space in the middle to help with water flow. I’m not a fluid dynamics engineer, but I would think if we just had a half circle bed, there would be some swirled dead spots that might limit flow.
The whole bed will be built and braced with wood. These beds will contain 1200 gallons of water weighing in around 9600 pounds. The goal of this design is to distribute the majority of the weight around the exterior walls and along the center beam column supports. The floor should be able to hold over twice that amount of weight along the span, but it never hurts to utilize our strong points.
There will probably be a few internal braces that will support the top foam and basket mounts. These will follow the cut lines.
Waterproofing Dilemma
Considering the unique shape of the beds, we wanted to avoid the traditional liner solution. Once again, the only materials that are food safe in a recirculated environment are polyethylene, polypropylene, stainless steel, liners, and silicone.
Stainless Steel
Stainless steel would be a great option for everything and I know a guy with a lot of scrap who could make it for a better price than using fresh material. With this setup we could use thinner stock and essentially create a stainless liner that supported by our wood frame.
We didn’t use it
We ultimately didn’t go with stainless because even with the better price, this was still too expensive. We have some other components of the water conditioning equipment that we’re going to build from stainless and figured we’d save our budget for that.
HDPE and PP
We almost went with poly plastic sheets. This would have been a great long term solution that would last for years. Think of lining the grow beds with cutting board type stock that fits perfectly in our frames.
We didn’t use it
We decided against using the plastic because we really don’t have a ton of experience working with it. The poly plastics have super low surface energy making it difficult to find a non-toxic method of adhering the pieces to each other. We would have had to plastic weld the sections together with a special tool that isn’t cheap. While the cost wasn’t prohibitively high, we weren’t confident that we could do a good job with the welds without wasting a lot of material.
Silicone
I really wanted to use silicone to line the grow beds. I thought it would be an elegant solution to create a seamless, waterproof coating on the inside of the wood. I would have built an insert form so that I could pour the moulding silicone around the inside of the beds.
We didn’t use it
After more research into the various products we found that the correct food safe options were very expensive in the 5 gallon amount we needed and was only marginally confirmed inert in a recirculating environment.
Liners
Liners are a common choice for aquaponics applications. They’re generally made from EPDM or HDPE with the favorite being the latter due to it being much cheaper. The downside of HDPE is that it isn’t as pliable as EPDM so it’s a little more difficult to work with.
We went with it
I originally thought that it would be too rough to use a liner with the multi angled beds we have, but after doing some experimenting, we found a good method to lay it out.
I made a paper model of the shape and confirmed that it would be pretty straight forward to install the liner. Being HDPE I’m assuming that I could use an iron to help flatten the creases along the bottom of the bed. We’ll probably go with a Dura-Skrim product that’s widely used and proven to hold up for a long time.