UPDATE: August 2017
After years of service, the composter I built and described below rotted away. Taking my learning from using this composter to heart, I designed and built The Grand-daddy of All Composters. Check it out!


UPDATE: November 2013

The original composter plans from which I developed this design called for screen on 4 of the 6 sides as well as between the bins. I'm am not sure if the creator of the 3-bin composter system had a particular climate in mind for that unit. However, here in the San Francisco Bay Area, I have been having trouble keeping the compost moist enough. Not long after completing my composter I added a microjet bubbler from my irrigation system to each of the bins. Still it was drying out. Finally I took a cue from the composter at the Gardens at Lake Merritt and removed the screen from the two ends, replacing them with wood. It appears that for my climate, during the summer, screen on 4 sides allowed for too much evaporation.


Earlier this year I attended a talk on composting presented by the Alameda County Master Gardeners. They described a number of different composting systems and the pros and cons of each one. The system that they liked the best is a three bin composter. They provided a link to the StopWaste.org website describing how to make this composter: http://www.stopwaste.org/home/?page=445#3bin.

This composter is intended to be used is as follows: new material is always added to bin number one. After that material has broken down for a couple of weeks, it is then transferred to bin number two. By only ever placing semi-composted material into bin number two, this second bin gets filled up with dense material, thereby achieving the requisite one cubic yard of compost necessary to produce a hot compost. The third bin is used to hold completed compost until it is needed.

There are pros and cons to this system. The main plus is that it achieves the one cubic yard of compost necessary to get the compost to heat up. The biggest negative is that the unit is so… big! As designed, it requires a three foot by three foot by nine foot area in your garden. That is quite a lot of space for most home gardeners to dedicate to composting. In looking at the design, I realized that for most people the third bin isn't really necessary. The third bin is just for storage. Instead of providing this extra bin for finished compost, one could simply use the compost as it becomes ready, or store it in a trash can, or even just pile it up on the ground somewhere. By changing the design from three bins to two, the space required is reduced from nine to six feet in width. This also reduces the cost of the materials by one third.

I have designed and built such a two bin composter for my own use based on the original plans. I have also made a couple of modifications and updates to the original based upon availability of materials and personal preference, and I have added some comments on the construction process. I hope you find this useful.

Notes:

• I have only built one such unit. If you find any errors in the plans below, or have recommendations for improvements, please let me know.
• I have redrawn the original drawings with additional annotations to help make the process clearer. However, graphics are not my forte. If you have the skills to improve these, please let me know.
• The original instructions assume that you have access to very straight lumber. Unfortunately, these days it is almost impossible to find good lumber. Virtually all lumber available at a reasonable price will be bent and warped. As a result is impossible to achieve the tolerances assumed by the original plans. The straighter you can get things, the better the results will be, but truly straight corners are virtually impossible on this scale.
• The original plans are very specific about the size of the front openings. This is because one needs to cut the 1x6 slats to slide down into the front panel. Because modern lumber makes such accuracy almost impossible, I recommend not cutting the 1x6 pieces until the composter frame is complete. At that time you will want to custom cut each 1x6 so that it will slide into slots at the front. You will want to number each panel so that you know how they fit in. You may also need to chisel out the slots as necessary to fit the slats.
• The original plans call for the use of ¼ mesh hardware cloth. However, hardware cloth is very expensive. For my two bin version of the composter, the hardware cloth alone would have cost about $45. Therefore, I chose to use chicken wire, which is much, much less expensive. So far I don't think the use of chicken wire in place of hardware cloth has been a problem. Because the use of chicken wire results in a nasty set of wires sticking out which can easily snag clothing and rip skin, I chose to add 1x2 furring strips nailed on top of the edges of the chicken wire. This covers the ragged wire edge.
• It is interesting to note that the composter system in use at the Oakland Botanic Gardens (where the Master Gardeners presented this talk) does not use any kind of mesh on the outside of the bin. Their composter is entirely wood on the outside, with mesh on the bottom. Cutting and installing the chicken wire is a considerable effort and pain in the butt. Considering this effort and the cost of hardware cloth or chicken wire, if I had it to do all over again I think I would construct mine entirely of wood. Nonetheless, these plans are for a chicken wire based system. If you choose to use all wood you will need to develop your own plans.
• The original instructions assume a fairly high degree of familiarity with woodworking. I have attempted to make it somewhat easier for novices to construct this composter; however, some woodworking experience really is required. It is also fair to assume that if you have the set of tools listed below, you are probably an experienced woodworker.
• The author assumes no liability for any damages or injuries resulting from the construction or use of this composter. There are no warranties expressed or implied. The information provided herein is for entertainment purposes only.
•  This two bin composter by Andrew Sigal is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.  Please feel free to share and use this design. However, when you do, please give credit to the original 3-bin composter plans on StopWaste.org, and credit me for my 2-bin design contained herein.

Tools:

You will need a saw for cutting the pieces of lumber to lengths. I used a compound miter saw which makes this project much, much easier. There are a lot of cuts to be made. You will also need a drill with a 3/8 inch bit. You will need a hammer, socket wrench, and tape measure. You will need to have tin snips for cutting the wire mesh and roofing materials, and a screwdriver. Personally, I found an electric screwdriver to be invaluable. A socket bit for the electric screwdriver was also very convenient. The other optional item which makes this job vastly easier is an air compressor with pneumatic brad nailer and staple gun. Otherwise, you'll need a handheld staple gun and additional nails - but be prepared for sore and tired hands at the end of the project.


Materials:

Lumber:

• 2x4 - 6 @ 31.5”, 6 @ 36”, 4 @ 6', 3 @ 29”   * see note
• 2x6 - 3 @ 36”
• 2x2 - 4 @ 34.5”, 1 @ 6', 3 @ 29”
• 1x6 - 12 @ 31”
• [Optional] 1x2 furring strips - 3 @ 36”

* Note that you can economize on lumber by not cutting extra pieces for the three 29" 2x4's. These pieces of wood are only used to provide a surface onto which to staple the chicken wire. They are not structural. Therefore, they do not need to be single solid pieces. If you purchase 8' 2x4's (the most common and economical size,) when you cut your four 6' sections, you will have four 24" scraps. You can make up the three 29" pieces by using three 24" leftovers, and cutting the remaining 24" scrap piece into 5" segments (with a final 9" scrap.) You will also have scraps left over from cutting the 36" sections (you will get two 36" pieces from each 2x4x8 with a 24" piece of scrap.)
[Side note: if you are not familiar with lumber sizing, modern lumber is sold by nominal size, not actual size. Thus, a piece of wood that is called a '2x4' is not actually 2” x 4”. It is actually 1.5” x 3.5”. The reason is that originally lumber was sold unfinished. A 2x4 was 2” x 4”, but after planning it smooth, it becomes approximately 1.5” x 3.5”. Now lumber is sold prefinished, so the sizing does not match the name. However, the lengths for pieces of lumber are correct, so a 2x4x8 is actually 8' long.]