If ya ever got a question 'bout buildin' your own thunder box, get in touch with me. I'm more than happy to share what I've learned 'bout em. Since doin' this well before the turn of the century, I probably properly built more of these than most anyone else and don't feel the need to compete with anyone. There is a great deal of satisfaction in doin' it yourself, if ya can an' I'm happy to give ya advice ta make sure ya gots a proper one to be proud of. Take advantage of my age and experience 'cause my rustic American kybo expertise should not die with me.
The pit should be at least 3 feet deep, but ya gonna want it deeper.
Make sure it is a good distance from ya water source and the bottom is not on bed rock. If ya got local septic distance ordinances use that to figure the location.
Dig it with a backhoe, post hole digger, ya daughter's beau or whatever else ya got but use a perforated 55 gal. plastic or metal drum, trash can (with the bottoms removed) or just plain durable hardwood to stabilize the top. Ya can't dig it too wide if once the drum, trash can or whatever is in place ya simply fills in around it. The drum etc. is to stabilize the top of the pit. Ya don't needs to have it reach the bottom of a really deep pit.
note: I don'ts dig pits myself anymore 'cause I figure that some day when I'm diggin' near the bottom, some fella might walk by, look down and see my old face, figure I'm dead an' fill in the hole.
What makes a mighty fine bench are stair treads which can be purchased from most home supply folk.
Ya can get 'em in either pine or oak. The oak cost twice as much but properly sealed pine does just fine.
Ta get the right width, glue two side by side. Clamp overnight and rip to whatever width ya needs.
Run a coupl'a reinforcing pieces underneath 90 degrees to ta seam so the bench stays true over time.
Treads are fine 'cause they are a good 1 inch thick and strurdy and have a nice rounded edge for sittin' comfort. The back edge ya just rips off when ya sizin' the width.
Watch my YouTube Outhouse Building Tips #1 on the Outhouse Americana channel. This video tells ya good structural and ventin' tips. I tend to ramble a bit, but if ya puts up with me and watches the whole thing it'l be worth it for ya.
The seat can allow the most gasses into the sittin' chamber from below. Whatever ya uses for a seat, construct a simple airtight cover to seal it when not in use.
If ya uses a hardware store seat, remove the spacers from the bottom of the seat, caulk it and attach it to the bench. The cover what came with it ain't no good because it has spacers which will allow gasses to escape through. I discard em in favor of an easily made air tight cover of my own. Ya might wanna give that old cover ta aunt Sophie who can paint folk scenes on it to sell to tourists.
The Roofing Materials
Many folk use this new plastic roofing which allows light ta filter in & brighten up the interior.
Other folk use metal roofing.
Both work just fine, however take care that ya do it right as a water leak here or there may not bother ya much but it will bother the thunder box over a short time. Much of your good work could rot away. If ya never installed this type there are great videos on YouTube. Both are good roofs but a little tricky to put on. Ya may not have the expertise to run a vent through a plastic or metal roof so just attach a couple of 90 degree bends to route the pipe through the wall near the roof line, attach a shanty cap to the top and you be all right and proper for a bit of sittin'.
Asphalt shingle roofing is pretty standard and not as tricky to install. It is durable and most forgiving. Replace it every 25-30 years or so. Of course if ya figure ya ain't gonna live much beyond the 30 year mark at that time, why bother? Before ya shingle the roof, run a 4" vent pipe straight from the bottom edge of the bench straight on up past the roof line. Use a rubber boot to keep it from leaking in foul weather an' finish it off with a shanty cap.
Cedar shake roofing lends a right proper look to the kybo but many folk make the same mistake in that they tell me after a few years the shingles curl up. That is because they nail em down to a plywood/tar paper base. Shakes need to breathe on both sides, so to keep em flat, space a frame of narrow boards (slats) accross the roof at the nail points and connect em to that. When done, ya'll see the bottoms of the shakes as your ceiling and the tops from the outside.
Vents and Lighting
If ya thinks the crescent cut-out was/is used for venting and lighting the interior, Ya done plum loco. Old timers never thought like that. Go to my About the moon crescent page for a better understanding of this.
You MUST vent explosive dimethyl and hydrogen sulfides (methane) the by- products of human waste decomposition, in order to maintain a safe and comfortable privy. I kid you not, thunder boxes have been known to be just that. Sometimes sending occupants to the hospital. No amount of lime or wood ash will do away with this hazard.
In my state a 4" vent pipe is mandated to funnel the gases from the lower chamber up past the roof line.
Terminate the pipe as close to the bottom of the bench so ya don't create a dead space where gases can collect. Also its a good idea to use a bit of screening on both ends.
Interior lightin' can properly vent the occupied chamber in that I usually leave the section above the door open and double screened. I also puts in a screened, plexiglass window above the seat head line, which can be opened to produce an excellent air flow to expel hot stinky air. Best places for these as bad air rises and ya can't beat the natural air flow to the outside.
Just remember to screen all openings. I put a screen under the bench where the vent pipe rests and ball up a bit to put in the top afore I adds the shanty cap.
Also remember all vents SHOULD carry the gases above the roof line. This keeps them from entering the chamber under the right weather circumstances. Most states mandate this and its good common sense.
Nails vs Screws
I use screws because I think they provide a strong long lasting bond. In fact carpenters tell me that the reason old stairs creak is because they are nailed. If stairs were screwed it wouldn't happen. Also they makes nails different today than "yesterday." When I take an ancient privy apart to restore, I find many of the old nails used years ago are impossible to remove because they have oxidized and expanded in the wood from natural elements over time. A very, very secure connection. Most of today's nails just ain't gonna do that. In fact I seen too many modern nails just poppin' out after a few years.
Times change and what worked years ago just don't work as well today because what we worked them with, have changed also and not necessarily for the better. Screws are easy to use now a days and ya less likely ta mess up a good thumb.