Barrel Composting Toilet System:
Construction and Installation Manual

(Updated 4/16/2018)

For a complete list of materials, see Barrel Composting Toilet: Materials List.  Note that urine management is covered under the "Urine Management" section of the Owner's Manual webpage and on the Urine Diversion webpage.

2016 Update:

We now recommend epoxy coated tops for indoor installations only, due to weathering issues.  For outdoor installations, we recommend aluminum or concrete tops as shown at bottom of this page. 

2018 Update:

The Barrel Composting Toilet is now available in kit form from the Watershed Management Group.

Also, aluminum and beautiful custom made concrete toilet tops are available from artisan Erik Revere at RevereWorks.


Materials for toilet, except paint and hardware.


If toilet will be constructed with used barrels, they should all be tested for ultra-violet (UV) damage.  UV damage is caused by extensive exposure to sunlight. To test, lay barrel on side, then press down hard with knee, causing side of barrel to indent several inches.  Repeat every foot or so around entire circumference of barrel.  If any visible cracks or cracking sounds develop during test, barrel is unsuitable.    

Mark center of barrel using the mold line across the center of the barrel top as a guide.  Repeat on all barrels.


Scribe circle on barrel top using circle scriber made of 1" x 2" board and two deck screws.  Place one screw in center of barrel and rotate scriber to mark circle in barrel top.  Screws should be spaced so that scribed circle is just outside outer edge of bung holes.  Typically, this requires that screw points are spaced about
9-1/4" apart.  Repeat scribing on all barrels.

Use Sharpie to highlight scribe mark. Repeat on all barrels.

Drill small hole just inside circle. Repeat on all barrels.

Insert saber saw blade through hole and cut along circle. Repeat on all barrels.


Cut top out of barrel.  Note rim around top of barrel.  This is important for maintaining circular shape.  Do not cut barrel top any closer to edge of barrel than shown. 

Note that some barrels may have 2 small (~3/16" diameter) drain holes near the rim.  These must be caulked to prevent insect entry into the barrels.

Note: See bottom of page for alternative top materials.

Using Barrel Composting Toilet: Top Layout Diagram, mark locations of all holes to be drilled in plywood toilet top.

First, mark diagonal guide lines from corner to corner (shown as dotted lines in layout drawing).  Do this on both sides of plywood toilet top. 

Second, mark the remaining two guide lines shown on layout drawing.

Then, using these guide lines and the dimensions on the layout drawing, mark the centers of the various holes to be drilled in the top.  These include one 12" diameter toilet seat hole, one 3-1/2" diameter vent hole, two 2-3/8" diameter vent holes, four 1/4" diameter locating bolt holes and two 3/8" diameter toilet seat mounting bolt holes.

Use circle scriber with screws placed 6" apart to mark 12" diameter hole for toilet seat.  Drill small hole for saber saw blade and cut out hole with saber saw using same techniques as for cutting out barrel top. 






Again, using the same technique, cut out vent holes with saber saw, or use hole saws and drill as shown.  

Use 3" vent as a guide to mark corners.  Use saber saw to round off corners.


Use router with 1/8" roundover bit to round off edges of toilet seat hole and perimeter of top.  Do this on both sides of top.  Vent holes do not need to be routed.  

Place toilet seat 1/2" in from front of toilet top.


Center toilet seat from side to side and mark for mounting bolt holes.  Drill two 3/8"  holes for toilet seat and four 1/4" holes for toilet top locating bolts.  


Apply a strip of 1/4" x 1/2" closed-cell self-adhesive foam weather strip to the bottom of the toilet seat, near the edge.  Apply a strip of 3/8" x 1/2" closed-cell self-adhesive foam weather strip on top of the first piece.  Apply a strip of  1/4" x 1/2" closed-cell self-adhesive foam weather strip to the entire edge of toilet seat lid. 

Place toilet top squarely on top of barrel, with equal overlap from front to rear and side to side.  Top should be oriented squarely with mold line on barrel.  Drill 1/4" hole through barrel rim at each of the four locating holes.  Repeat on all barrels.  


Using a 5/16" bit, drill out each of the four locating holes in barrel rim.  Then, using same bit, drill a hole through trough of barrel rim between each locating hole.  Drill this hole at an angle so it doesn't penetrate side of barrel.  These four holes will allow any liquid in rim trough to drain back into barrel.  Repeat on all barrels.

Mark rim of barrel adjacent to locating holes.  Repeat on all barrels.


Measure down squarely from barrel top and make mark.  Vertical measurement from barrel top to mark depends on length of spring that will be used.  For a Servalite #70 spring, the measurement should be about 12".  For a Hillman #66 spring, it should be about 7-3/4".  Repeat on all barrels.

Drill 3/16" hole at each mark on side of barrel.  Repeat on all barrels.


For outdoor installations, paint barrel with a minimum of 3 coats of black oil-based enamel to prevent UV damage to barrel.  Repeat on all barrels.  Painting is not required for barrels in indoor installations.

Looking into barrel.  Note that because this is a zero leachate discharge system, no holes are drilled into bottom of barrel.



Cut one piece of 3" ABS pipe 3" long for the central vent.  Cut two pieces of 2" ABS pipe, each 2-1/4" long, for the vents on either side of the central vent.  (See photos below for clarification.)


Glue pipe to ABS fittings.  When gluing 3" pipe into 3" sanitary tee, be sure to glue into correct hole.  Only one of the three holes in the tee is the correct one (see photos below).

Place no-see-um netting over vent, then place aluminum screen over netting and press stainless steel hose clamp over screen.  No-see-um netting helps to prevent fungus gnats from entering toilet and aluminum screen protects the netting from abrasion, rodents, etc.

Tighten hose clamp.



Use utility knife with sharp blade to trim excess screen and netting.


Note: Next two photos apply only to wood tops.  For alternative top materials, see bottom of page.

Contact surfaces such as toilet seat and toilet top should be smooth, non-absorbent and easily cleaned.  For all installations, whether indoors or outdoors, a minimum of 2 coats of epoxy resin should be applied to all surfaces (see next photo). For exterior installations, cover all epoxy surfaces with 2 coats of paint to prevent UV damage from sunlight. 

Apply epoxy resin to all surfaces of toilet tops.  (Photo shows several tops being done, but only 1 top is required for a barrel toilet system).  Apply a minimum of 2 coats to both sides of toilet top.  With each coat, also apply to all edges, so edges receive  4 coats total. 

After paint is dry, use a straight-edge centered on locating bolt holes to draw 2 diagonal lines on underside of toilet top with pencil.  These lines will be used later to locate screw-eyes for springs.


Insert vents and turn toilet top upside down.  To secure vents, place hose clamps over protruding pipes and tighten.  


Place toilet seat on toilet top and insert seat mounting bolts and nuts.  Insert four 1/4" x 2-1/2" stainless steel carriage bolts through locating holes and secure with washers and nuts. 

Place toilet top on barrel, using locating bolts to center top.  Holding a pencil vertically, scribe line around perimeter of barrel on underside of toilet top.

Note that locator bolts are not for affixing toilet top to barrels.  That is the function of the springs.  The locator bolts simply center the toilet top on the barrel.  When placing toilet top on barrel, it's helpful to open toilet seat so the ends of the front two locator bolts can be guided through the holes in the barrel rim.  

On underside of toilet top, drill four small pilot holes on diagonal pencil lines.  Each hole should be located 3/4" outside of barrel perimeter scribe line.  Drill pilot holes 5/8" deep on underside of toilet top.  Pilot holes should not go all the way through toilet top.  Fasten screw-eyes in pilot holes.  Don't screw eyes too far into wood or they'll come out the top.  Attach springs to screw-eyes.  


Attach a strip of 1/4" x 1" closed-cell self-adhesive foam weather strip to underside of toilet top.  Use barrel perimeter scribe line as guide.  Weather strip goes on inside of scribe line.

Fasten #10 x 3/4" stainless steel machine bolts through holes in side of barrel.  These are anchors for springs.  Repeat on all barrels.


Detail of spring anchor assembly on side of barrel.  On outside of barrel is a  #10 nut and a  #10 x 1" fender washer.  On inside of barrel is a #10 x 1" fender washer and a #10 nut.  Tighten outside and inside nuts to secure assembly.  All fasteners should be stainless steel.  

Place clear two liter soda bottle upside down into large mouth canning jar.  Mark soda bottle with Sharpie. 


Rough cut top of soda bottle off with scissors, then finish cut along Sharpie line with scissors. 


Three elements of fly trap assembly.  From left, wide-mouth canning jar, soda bottle top, wide-mouth canning jar ring. To assemble fly trap, insert soda bottle top upside down into canning jar, then screw on ring to secure bottle top.  Insert finished fly trap upside down into 3" sanitary tee on toilet top.   

Finished toilet barrel and toilet top assembly.


To make screen/cover assembly for aging barrels, cut a 3' diameter circle of fiberglass screen.   Screen is secured to barrel rim with a 7' long  piece of 1/8" braided nylon cord and a steel spring.  Knot steel spring into cord as shown.  Cord/spring assembly should be snug but not overly tight.  Make one complete assembly for each aging barrel. 

Finished screen assembly.  Knots are adjusted until steel spring has slight tension.



Using tin snips, cut a 27" long piece of 2' wide corrugated metal roofing panel.  Secure panel with two 2" x 8" x 16" concrete patio blocks or clay roofing tiles (shown).  To secure the metal top against high winds, it is important that the blocks be located as shown, near the edges of the top.   



Outdoor installations:

Install only in locations not subject to flooding.  Also, it is important that the toilet is protected from rain (see example below).  Sun protection is optional, though the foam weather seal around the toilet seat will last much longer if protected from exposure to direct sunlight.   

If barrels are to be partially buried (as shown below), install each barrel with the toilet top in place.  This means the 4 locator bolts on the toilet top will be in the guide holes in the barrel rim.  This will help to insure that barrels do not deform during burial, which could make lining up the locator bolts difficult later on.  Use a level to check that the barrels are level.  Check each barrel for level in one axis, then move the level 90 degrees and check for level in the other axis.  Barrels should all be about the same height.   Allow a minimum of 28" from the center of one barrel to the center of the adjacent barrel.  This will allow sufficient clearance for the corrugated roofing panels on the aging barrels.  

Do not place barrels against a wall or other object that could become a ladder for rodents.  If mice gain access to the top of an aging barrel, they may get under the corrugated roofing panel and then chew through the fiberglass screen.  

Indoor installations:

Indoor installations typically require barrels to rest on floor with one step up to a platform at a suitable height.  As a space saving technique for indoor installations in homes without steps between the toilet location and outdoors, a plant dolly can be used to transport the active barrel outdoors.  The dolly should have at least 4 castors and a minimum weight capacity of 500 lbs. 

Examples of several installations are pictured below.

For outdoor installations, place barrels 12" to 22" deep in ground, depending on desired toilet height.  Use level to be sure barrels are level.  Allow a minimum of 4" between barrels.  6-8" between barrels will allow more room when aerating or emptying barrels. 

Outdoor installation. This is a 3 barrel system for 2 people in full-time use. On left is movable privacy/weather screen over active barrel.  When active barrel is full, privacy screen and toilet top will be moved to center barrel, which will then become the active barrel. 

This screen is lightweight enough for two people to easily move it from one barrel to another.  It is tethered at each corner to anchors in the ground.  A fabric curtain can be mounted across the front for more privacy.

Outdoor installation showing privacy/weather screen and active barrel.  5 gallon plastic buckets are used to keep toilet paper dry and for storing cover material. Compost crank holder is located beside toilet.  Squirt bottle of water for wetting toilet paper is on top of toilet.  Step is made from 2 cement blocks. 

Note that buckets are clearly labeled and toilet instructions are posted on side of privacy screen where they can be read easily while using toilet.  This helps to ensure proper use by visitors.  

Same privacy/weather screen showing hinged rain hood in down position to protect toilet top during blowing or heavy rain.  

Another installation showing 2 aging barrels on left and active barrel on right.  This is a 3 barrel system sized for 2 people in full-time use.

Same installation showing toilet top detail.  Note vent  pipes, insect trap and weather stripping around toilet seat.

Installation in an out-building showing optional ventilation pipe through roof.  3" diameter flexible pipe allows toilet top to be moved from barrel to barrel. 

Flexible pipe goes from toilet to 3" ABS pipe which extends up through roof.  A 3" ABS tee fitting above roof is covered with aluminum insect screen secured with stainless steel hose clamps to prevent insect vectors from entering toilet.  


Handwashing bucket located near toilet.  Note that bucket and soap are clearly labeled for ease of use by visitors.



As noted above, epoxy coated plywood tops are recommended for indoor installations only, due to weathering issues.  For outdoor installations, aluminum or concrete tops are recommended, as shown below.  

Top made from aluminum plate.  For outdoor toilets we recommend this top, or the concrete top in the next photo, rather than epoxy covered wood.  Aluminum and concrete tops are much more weather resistant and will not split or check when exposed to weather. 

This is the easiest top to build and the least expensive, mostly because it doesn't require coating with epoxy as plywood does.  Instead we used a couple coats of UV-resistant clear coat in a spray can from the local hardware store.  The aluminum plate is 1/4" thick and can be purchased from online sources such as, or from local metal suppliers.

Top was cut using a metal cutting blade in a sabre saw, then edges were rounded off with a file.  Sides and edges were then sanded with 220 grit sandpaper in an orbital sander before spraying with clear coat. 

Artisan Erik Revere makes beautiful custom concrete toilet tops for sale.  Various colors of stone and glass are imbedded in the tops, giving each one a unique appearance.  

Erik Revere also makes aluminum toilet tops for sale.

The best insect management is prevention.  Although toilet seat and lid should be weatherstripped to prevent insects from entering toilet during use, it is difficult to keep weatherstripping insect-tight over time.  An insect plate (shown above) is the best preventive between uses.

Plate can be made from a variety of materials.  The most durable is shown above, made from a 14" diameter circle cut from 18 or 20 gauge stainless steel plate, available from online vendors such as, or from local metal supplier.  Insect plates can also be purchased here.  Handle is mounted on top. 

Plates can also be made from other materials that are flat and stiff so as to effectively seal the toilet seat opening against insect entry.  One example is
plastic sign material such as that available from a hardware store for "For Sale" type signs.  The plate should not be over 1/16" thick or it may damage the weather stripping on the toilet seat.

Underside of insect plate, showing 4 rubber furniture leg protectors mounted to plate with #10 stainless steel machine screws.  The leg protectors help to locate the plate over the toilet seat hole. 

Insect plate from underside of toilet top.  Note leg protectors used to center plate over toilet seat hole.

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