Monday, September 2, 2013

Building a 2-bin composter






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.
  • Creative Commons License 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.]


Fasteners:

Note: the original project calls for a lot of nails. Personally I dislike nails for a lot of reasons, and almost always choose to use screws. In my version of the project I substituted decking screws for almost all the places where nails were called for in the original. I used brads from my pneumatic brad gun to attach the 2x2's and 1x2's.
  • Carriage bolts with washers and nuts - 8 @ 3.5 x3/8 bolts, 8 nuts, 16 washers
  • Screws and/or nails - I am sorry that I did not count the screws and nails that I used. For screws I used primarily decking screws ranging from 1.5" to 3.5". I also used a handful of penny nails. Your choice of nails and screws will depend on your preferences and tools. This is a fine project for using up screws and nails that you have laying around.
  • Appropriate staples your staple gun (manual or pneumatic)
  • [Optional] brads if you are using a brad nailer.


Additional materials:


  • Corrugated Roofing (historically this would have been fiberglass, now one normally finds this made from polycarbonate material) - 1 piece of 8' x 26" , cut into three 32"  x 26"  pieces.
  • Wiggle molding - this is a product that has a wave shape that corresponds to the corrugations in corrugated roofing. It is used to mount the corrugated roofing to the top of the compost bin. You will need 12 feet. Theoretically you are supposed to use special gasketed roofing nails to attach corrugated roofing. However this is another expensive specialty item. Given that the project uses wiggle molding, and water-tightness is not a requirement, I found ordinary decking screws work just fine.
  • Hinges - Two hinges of a size sufficient to mount the roof. 3" hinges work well. Larger or smaller should also be fine. They should be made out of a material which will not rust such as galvanized steel or brass.
  • Flat corner braces - the original plan specifies 4 corner braces, however Home Depot no longer carries 4"  sizes in my area. I used 6"  and that was fine. You'll need 4 of them.
  • Flat T-braces - 2 braces. Again either 4" or 6" size.
  • [Optional] hook eyes, and 8' of chain or wire. This is used to hold the top so that it cannot flop over backwards. In my case the compost bin is up against a fence, so the top cannot open past 90 degrees and the hook eyes and chain were unnecessary.


Drawings:

As noted, I have redrawn the original drawings. I have also prepared three sets of drawings to help illustrate the construction. There are also photos of some details.


The composter showing only the lumber pieces.




Annotated with lumber dimensions.



Showing hardware components


Assembly:

Main structure:

  1. Screw together two 31.5” 2x4's (C) and two 36” 2x4's (B). Repeat three times to create the two end frames and one center divider. Attempt to make the frames as square as possible.
  2. Cut three 36” x 33.5” pieces of chicken wire and staple to the frames. The outer frames will have the chicken wire on the outside, the middle frame can have the chicken wire on either side.
  3. [Optional] Nail, brad, or screw 36” 1x2 furring strips over the top edges of the chicken wire.
  4. Drill 3/8” holes and bolt the frames to the 6' 2x4's (A). Two on the bottom and one on the top at the back as shown in the diagram. There is no 2x4 on the upper front.
  5. Screw the 29” 2x4's (D) onto the bottom of the frames, thereby filling in the gaps (or use three 24" pieces and three 5" pieces as noted above.) This will provide a surface for attaching chicken wire to the bottom of the structure.
  6. Cut two pieces of 6' x 36” chicken wire. Staple one to the back of the structure and one to the bottom.


Detail of optional furring strip covering sharp ends of chicken wire




Slats and slat tracks:


  1. Screw the 2x6's (E) to the front of the frame as shown.
  2. Leaving a one inch gap, nail, brad, or screw the 34.5” 2x2's (G) on the inside of the frame. This will create the track into which the 1x6 slats will slide.
  3. Once this is done, you can begin the process of cutting 1x6 slats to the correct lengths to slide into the frame. You may need to chisel out the frame or otherwise make modifications to allow the slats to correctly slide into place.




Details of 2x6's and 2x2's forming a channel to hold the front slats



Lid:

  1. Assemble the lid with flat corner and t-braces screwed to the bottom side. Note that the front, side and center brace pieces are all 2x2's (F & H), while the back piece of lumber is a 2x4 (A). This is done to make the lid lighter and easier to open and close, while still providing enough wood at the back to hold the hinges.
  2. Cut the corrugated roofing material into three 32” pieces. The easiest way to do this is with tin snips. Note that the roofing will be attached with the corrugations running front to back (see photo.)
  3. Place the wiggle molding along the front and back edges of the lid, and screw each piece down with one or two screws (this step only needs enough screws to hold the molding in place for the next step).
  4. Place the three pieces of corrugated roofing on top of the wiggle molding. Make sure that the corrugated roofing pieces overlap each other by at least one inch. Then drive screws through the roofing pieces and wiggle strips into the wooden lid frame.
  5. Place the lid on top of the main structure, attaching it with the hinges.
  6. If you are using a chain to restrain the lid from falling backwards, attach eye hooks to one edge of the lid and main bin and connect a chain, rope, or wire between them.


Detail of wiggle strips and roofing material





Finished composter:


When completed, this composter is quite heavy, so you will want to build it entirely in place, or pre-build part of it and complete it in place, or have several people available to assist with moving the completed unit. Even with a compound miter saw with laser, electric screw drivers and drills, and pneumatic tools, this project took many hours to complete, and cost well over $100. However, the result is a pretty great, high end, compost system.

The finished composter, open, with one slat removed.


I look forward to hearing your comments.


Note: For another similar project, check out my Herb and Seed Cleaner.


UPDATE: November 2013

As noted, 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.

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