T Hayden

I don’t see why that wouldn’t work.  I would go heavier duty with the foundation than 2x6 though, if you are using 6x6 posts, why not 6x6 foundation at least for the rims, or 3 2x6 together to make bonded beams with pockets for our posts.  In between 2x6 should be ok providing you have a short enough span (-8’).  Though cedar is rot resistant, it isn’t as strong as other species.   Consider that when building floor and roof.  Put some sort of insulation in the roof and floor. Even though it’s a small structure, without insulation in the floor you will find it difficult to keep the floor from freezing without roasting people out.  I’ve seen a few cordwood houses where they fill foamed the rounds in place, then used chinking to seal it up.  I think I’d recommend that route over using only cement.  The wood and cement freeze/ thaw differently and it will be prone to cracking.  Use something more flexible between logs and it’ll likely hold up better in the end. It may cost more on the front end, but not in the long run.

Go with the second image and shave off the extra wood from the beam that protrudes past the post on the outside.  Use two 1” pegs per tenon and make your posts 6x6 at least or you won’t have much tenon to peg in to.  Other than that, looks good. Using a pentagonal post with the outside having a 135degree angle would allow you to have a similar joint but with tenons coming straight off the beam, not at an angle.  You could do the dovetail but I don’t think it would typically be used in this manner.  It should just be a straight tenon and pegged, so long as your tenons are big enough. With a 6x6 post, using a shoulder like you should, you are only going to have 2” of tenon to peg per side unless you overlap the tenons in the joint.  I’ve never seen it done that way but I don’t see why it wouldn’t work.  Cut the mortise 3” wide, make the tenons 1.5” thick and have them pass by each other in the mortise, then run 2 pegs thru the whole thing?  I hope it’s clear what I’m talking about in my super sketchy 2D sketch pad drawing.  

I recommend Tedd Bensons book “Building the Timber Frame House” for starters.  It has some decent graphics of the joints involved and layouts.  I’m sure there are other useful books, but that’s the one that got me started.  I think Glen and Mike Haasls recommendation of the half lap joint would be sufficient, a tongue and fork would be better, but like Glen said, make it so that the post holds at least 1 full inch of each beam.  I wouldn’t rely solely on a peg though, I’d use a tenon on the top of the post that goes through both of the beams, similar to what would be used on a post to sill connection.   For hardwoods, a 1.5” thick tenon is usually sufficient. 2” for soft woods.  For simple but effective joints, look at the tongue and fork, shouldered mortise and tenon, lapped dovetail knee brace and dovetail collar tie.  Your knee braces to the post connection will likely be your most challenging notch because it’s not coming into the post square but at a 67.5 degree angle.  Use a king post for your rafters.  Take a short 6x6 and knock off the corners to make it an octagon, cut mortises on all 8 sides, top and bottom.  Your rafters get tenons, as well as the collar ties.  Add a little something artistically to the end of it like some sort of pendant. If your sawyer hasn’t cut your logs yet, see if he can cut you pentagon posts with angles 135, 90, 112.5,112.5,90 in that order.  Then your outside corners of your posts will match the octagon shape, the knee braces will come in square to the posts, and the collar ties and rafters will still come into a square face as well.

I see what you are going for in the sketch up you have here, but one thing I notice right off is I don’t like the tenon placement on the beam located on the right.  It could work if you move it over or change the angle of the joint so that the full width of both beams rests on the top of the post.  From what I see in the drawing, the beam on the left is resting fully on the post, but the beam on the right is mostly just the tenon over the beam.  I’ll try to get you a sketch of some alternative solutions soon.  
With your bracing, it is perfectly acceptable to have them attached to the face of the post like is shown.  This is definitely easier than trying to do a full mortise.  They are a little better if you make a dove tail on the end of it, but a straight lap like shown is fine.  Make sure you add the shoulder, which I believe you show in your sketchup.  I’m not sure what you have going on with your foundation beams, but it looks like you may be trying to use straight beams, with a short piece attached to make the bend?. For this just lap the foundation beams right under the post, dove tail them is better and cut a square mortise pocket for the post to set in.  This is assuming you have something under the beam that is bigger than the notch. If sitting right on the ground, ignore that, if up on blocks or pads, make sure each section of beam is well supported underneath. Hope I’m helping.

Nope, don’t do it.  When properly constructed, you will have shouldered mortises on your posts, thus carrying the full strength of the beam onto the post.  If you don’t shoulder your mortises, you effectively reduce the load bearing capability of your beam.  Say you start with a 6x6, then cut off an inch and a half on each side to make a tenon but don’t put a shoulder on it, you have effectively turned that 6x6 into a 3x6 for load bearing capacity and created a fail point on the end of each beam.  If you use pegs instead of joinery, the problem gets even worse.  You use a 1”peg, expect 1” load bearing capacity.  Do a little research on making mortises and tenons, shouldered and housed tenons, get a skilsaw, tape measure, hand saw, framing square, 2” hand auger bit, 1” bit, hammer, sledge, some ratchet straps and a 1” chisel and you can do everything you need. A combination square and a 4” angle grinder with sanding discs are handy tools too.  I use either a draw knife or the grinder to make pegs, but you could just buy some hardwood dowel I suppose, make them too long to start with and sharpen one end just a tad bit to make it start in the hole easier.   Blocklayer.com has a gazebo calculator that will be very handy in figuring out your angles for your octagonal building, including rafter layout.  Timberframing isn’t nearly as hard as it seems and once you cut the first couple of notches, you’ll forget all about the worry of it.  I’m hosting a teaser class on timberframing without specialty tools in August.  We will be using round log, so as to show people this method of construction without the need for a sawmill.  The above tool list is about all you need to enter in to the world of timberframe construction.  Add a chainsaw and chalk line for round wood construction.  It isn’t until you desire to go into larger production that you will want or need to spend the money on the specialty tools like 16” skilsaws and chain mortises.

This looks like a version of a Leonardo DaVinci bridge that he designed in 1502.  It’s a really neat concept.

Would it be reasonable to use a lodge cast iron griddle for a top on a riserless cookstove or a double shoebox, or would I be asking for trouble?  I also have a much larger cast griddle salvaged from a commercial propane cook range, must be 1/2” thick or more.  I want to make a small outdoor cook stove with the lodge cast griddle and if that works out well, then make a bigger one inside with the larger griddle top.  

Has anyone on here considered radon testing and mitigation in their construction projects?  I think that this would be an extremely relevant topic to structures that are earth bermed, like a wofati, earthbag houses, and to earthen floors. I also wonder how much, if any, cob inside a house would add to radon issues. Radon emits naturally from the ground, so typically in an area where radon may be suspected, new construction would implement things such as having a plastic barrier between the ground and basement or crawl space, sealing the basement up good, and placing passive radon vent pipes from the basement to the roof.  These are all things that can be done to a normal stick built or log house, etc. it’s a little harder to do after you’ve built and discover that your new home has a radon issue.  But what about a house where the entire thing is surrounded by potentially radon emitting earth?  What should be incorporated into the design right from the get go so that we can prevent having an issue down the road?

Though chainsaw mills do have their place, I found them painfully tiresome.  For a log that big I’d try and find someone with a portable bandsaw mill to cut it. After cutting my first 100 board feet of lumber with a chainsaw mill, I was ready to find a Wood Mizer.  The chainsaw mill I had used about 10x the gas for the same number of boards as my band mill.  Better quality lumber, less planing, and more board feet will be salvaged with a band mill over a chainsaw mill.  If you do end up going diy with the chainsaw, a ripping chain on the absolute biggest saw you have would be good.

I would think you could use oak logs for trusses just fine, but I would be concerned about your unsupported span.  I see the oval is 50’ but what is the distance at the middle of the short side of the oval.  Also, a true truss design is going to be a lot stronger than just throwing a log up on the roof and calling it a truss because of the way a truss works and how it transfers the load to the outside wall.  If you just throw a log up on the roof without support somewhere along the way, it will eventually sag in the middle, especially under the weight of a living roof.  If you put them up green, expect the sag to be more pronounced in the end.  Do you have one really big log that you could use for a center post?  Perhaps that still has a bit of the root bell and branches on it?  It would be pretty slick to plant that in the middle of the building and have it support the log trusses of the roof, with some of the branches reaching out and supporting the trusses.  

Are you planning insulation for your earthbag?  I hear people use vermiculite or perlite mixed in their earthbag, but I don’t know how much insulation that provides or how much compression it can withstand.  I’d maybe do gravel for a couple rounds near the bottom, then switch to more dirt and other stuff up above.  The gravel will be good for drainage, not so much for insulation.

For the earth floor, maybe foam insulation, vapor barrier, dirt or sand, rock tamped securely into place. Unless using open cell foam, then maybe vapor barrier, foam, vapor barrier? Dirt or sand, rock.  I wouldn’t normally put insulation tween 2 vapor barriers though.  I’m just thinking out loud here.

I think the order for a green roof is: plastic, plastic, plastic. Maybe more plastic.  Then gravel, with the gravel layer a little thicker at the eaves, then dirt, then grass, shrubbery, berry bushes, some roses a little spot for raised bed garden, etc...  any exposed plastic will deteriorate, so put on some sort of flashing along your eaves to cover it.

Or where I’m at, plastic, then the moss that was excavated from the building site. Done.  My kids want a hobbit hole playhouse so I’m planning a 12’ diameter timberframe/ log/ earthbag combo that will have a living roof. I’ve been wanting to use a pond liner for a green roof but I haven’t done my research on that yet.  I think it’d be better than a layer of 6 mil poly any day of the week.  For expensive you could cover the roof with Bituthene, it’s usually got like a 60 mil sheet of HDPE adhered to a tar like medium.  It’s self sealing, so you can theoretically drive a nail in it and it’s not going to leak.

Sorry, no help on how to cover earthbag.  Mines getting an earth berm outside and 4x6 milled log inside.  But the floor will be exactly as described above with the exception of I’m using end grain birch blocks for flooring.