Cordwood - Mortar Options

Would those with experience touch on mortar options? I think Rob Roy did a cordwood project with Ianto Evans where cob was used as mortar vice a cement mortar. How has that project held up/how does cob compare to a cement mortar?

Many thanks,
Julie in NE Oklahoma

Hi Julie: Indeed, Ianto Evans and Linda Smiley stayed with Jaki and me at Earthwood for a few days several years ago, and, while they were here, we built together a cordwood masonry panel in our garage, about 4 feet by 6 feet. Instead of our regular mortar, described below, we used cob as mortar. We did insulate the mortar joint with sawdust in the central cavity, as usual. Ianto and I had to go out and search for good clay to build with, as there is none at Earthwood. We found it where a contractor friend was doing an excavation about15 miles away. The cob mix was about 80% coarse sand, 20% good quality (quite pure) grey clay. We used chopped hay/straw as reinforcing binder. An easy way to do this is to come down on top of a flake of hay or straw with a rotary lawn mower. Presto, nice 2-inch pieces of chopped reinforcing binder. Linda finished the wall with a thin coat of cob without the reinforcing. This panel has held up very well, but it is well protected with a 3-foot overhand and it is well off the ground. Chapter 20 (More Cordwood and Cob) of my book Cordwood Building: The State of the Art, goes into more detail about this project and others, and has a picture of Linda and Jaki working on the panel. You can get it at our Earthwood site, www.cordwoodmasonry.com or, maybe, win one here on Permies.
Over the years, Jaki and I have developed a very successful cordwood mortar using portland cement, lots of builder's (hydrated) lime, sand, and soaked softwood sawdust. The mix by equal volume (shovelfuls) is 9 mason's sand, 2 portland cement, 3 lime and 3 sawdust. The softwood sawdust needs to be passed through a half-inch screen and completely soaked overnight. The soaked sawdust acts as a cement retarding agent, preventing mortar shrinkage cracks. If the right sawdust is not available, use a commercial cement retarder, such as Sika Plastiment, Daratard-17 from W.R.Grace, or equal, usually three ounces per wheelbarrow load. Do not mix sawdust and commercial retarders.
Finally, I think the use of cob and cordwood together - which we call "cobwood" - is only appropriate if you have a source of good quality clay close top the building site. Once, we did a workshop in North Carolina using North Carolina red "clay" in a cob mortar. The wall went up beautifully, but we learned from our hosts that later, when the wall had dried, the cob mortar was crumbly. Probably not the best clay. We have had no such problem with the wall we built at Earthwood.

Thanks for all the great info, Rob! Just curious; has anyone ever done any type of "life cycle analysis" to compare various building methods? I'd love to know how cordwood with the Portland cement mortar compares to other building methods using regionally sourced materials. Even knowing of an example somewhere else would proved insight on how we might do such an analysis here. Thanks! -J

Sorry, Julie, I am unaware of any "life cycle analysis" studies. Rob

My family and I built a hybrid cordwood cabin in Northern Maine. Basically a stickbuilt cabin built with lumber we milled off of our lot then cut white cedars into six inch logs as an outside cordwood facia. We used s type morter that we added sawdust to to then morter the logs together. Had good results, would have been better if we would have waited for the six inch cedar logs to shrink first as we had to add a lime based stucco to fill in the holes. ended up ugly but lasted. When we sold it the new owners took down the cordwood facia because it was unsightly...I liike it...

Mixing in sawdust that has soaked for a couple days slows down the drying time and prevents cracking.

I've helped build two roundwood/cordwood and cob roundhouses for use as workshops - with straw placed between the internal and external skins of cob - and visited a few similar constructions lived in as permanent dwellings, one of which is nearly 30 years and looking good - this is in a very wet but not very cold area.

Quikcrete profinish type S mortar+Lime S+and sawdust?

I got sawdust from a local lumber yard.  There is dust from treated lumber in it.  Is that OK to add to mortar?

I have been theory-crafting about this and I am starting to wonder whether, in some applications, I couldn't get away with plain old mud?  By which I mean, just the local sand/clay soil mixed with a bit of water?

Here's my thinking.  I want to make some small durable outbuildings. (Chicken coop, dovecote, storage shed, small workshop, that sort of thing.)

Literally the only building materials I have in surplus are local roundwoods (red cedar, osage orange, honey locust, elm, ash, anything in the cross-timbers mix, as much as I can cut) and local dirt.  No large clay deposits, no decent/sorted sand or gravel, no huge quantity of sawdust.  Money, of course, is tight.  I can buy supplies but not in building-materials quantities, if that distinction makes sense.  

This is Oklahoma.  I don't really need great thermal properties.  What I need is durability and the ability to keep wind  and rain off my stuff.  

I've been watching cordwood construction videos.  Everybody uses either short sticks (six inches) or longer sticks stacked across two ridges of mortar, with an air gap or insulation between for insulation.  In virtually all cases, they use huge thick layers of mortar, such that the filled gaps between stacked sticks is often an inch or more.  And then they often complain about various mortar failures.  When half your wall is mortar, that's a disaster!  And so we have endless recipes for improved mortars.

I grew up in a town full of 100-year-old log cabins. They were chinked with moss that was then coated with mud -- with the "mud" being some forgotten mix of Yukon River silt and perhaps a bit of lime.  If the mud cracked and fell out (sometimes helped by bored little fingers) the cold air came whistling in.  And that often happened.  The walls were only six to ten inches thick, so the tiniest gap would pass too much frigid air.  

However, I have a LOT of experience stacking cordwood.  At longer lengths (especially 18 to 24 inches) you can stack it very tightly and it's never going to move.  Why use mortar at all?  Well, because the tightest woodpile will still pass drafts.  But I don't understand why just a thin slip of mud/mortar to bed each new stick in wouldn't be sufficient to fix that.  And you wouldn't need to run it all the way out to the ends of the your stack/wall, so it wouldn't be exposed to the elements (or child fingers) and even if it crumbled, it wouldn't go anywhere.  It wouldn't be structurally significant, or at least not very.

I mean, I would need a good solid foundation that keeps water out of contact with the wood.  And I would need a good roof to minimize water from above/sideways.  But for a structure on the line between warm temperate and subtropical, I don't see why thick cordwood walls with very little mortar of any kind in them (just enough to stop drafts) wouldn't work.  No need to build a void into the wall for insulation, no need for inches-thick layers of mortar between your stacked roundwoods.  

Am I completely insane?  Or, for the purpose of outbuildings that don't need much climate control, is this a viable method?  

I like it. Did you end up trying it? I would say do something small and see how it goes. I am interested in some log style building, both horizontal and vertical, and have been wondering about goid chink/daub materials without having to purchase much.

Dan Boone wrote:I have been theory-crafting about this and I am starting to wonder whether, in some applications, I couldn't get away with plain old mud?  By which I mean, just the local sand/clay soil mixed with a bit of water?

Here's my thinking.  I want to make some small durable outbuildings. (Chicken coop, dovecote, storage shed, small workshop, that sort of thing.)

Literally the only building materials I have in surplus are local roundwoods (red cedar, osage orange, honey locust, elm, ash, anything in the cross-timbers mix, as much as I can cut) and local dirt.  No large clay deposits, no decent/sorted sand or gravel, no huge quantity of sawdust.  Money, of course, is tight.  I can buy supplies but not in building-materials quantities, if that distinction makes sense.  

This is Oklahoma.  I don't really need great thermal properties.  What I need is durability and the ability to keep wind  and rain off my stuff.  

I've been watching cordwood construction videos.  Everybody uses either short sticks (six inches) or longer sticks stacked across two ridges of mortar, with an air gap or insulation between for insulation.  In virtually all cases, they use huge thick layers of mortar, such that the filled gaps between stacked sticks is often an inch or more.  And then they often complain about various mortar failures.  When half your wall is mortar, that's a disaster!  And so we have endless recipes for improved mortars.

I grew up in a town full of 100-year-old log cabins. They were chinked with moss that was then coated with mud -- with the "mud" being some forgotten mix of Yukon River silt and perhaps a bit of lime.  If the mud cracked and fell out (sometimes helped by bored little fingers) the cold air came whistling in.  And that often happened.  The walls were only six to ten inches thick, so the tiniest gap would pass too much frigid air.  

However, I have a LOT of experience stacking cordwood.  At longer lengths (especially 18 to 24 inches) you can stack it very tightly and it's never going to move.  Why use mortar at all?  Well, because the tightest woodpile will still pass drafts.  But I don't understand why just a thin slip of mud/mortar to bed each new stick in wouldn't be sufficient to fix that.  And you wouldn't need to run it all the way out to the ends of the your stack/wall, so it wouldn't be exposed to the elements (or child fingers) and even if it crumbled, it wouldn't go anywhere.  It wouldn't be structurally significant, or at least not very.

I mean, I would need a good solid foundation that keeps water out of contact with the wood.  And I would need a good roof to minimize water from above/sideways.  But for a structure on the line between warm temperate and subtropical, I don't see why thick cordwood walls with very little mortar of any kind in them (just enough to stop drafts) wouldn't work.  No need to build a void into the wall for insulation, no need for inches-thick layers of mortar between your stacked roundwoods.  

Am I completely insane?  Or, for the purpose of outbuildings that don't need much climate control, is this a viable method?  

I think your way would work well with one notable exception.  I think you would need mortar near the way to the end on at least the outside of the walls.  From what I know about cordwood, bugs can't eat into the end grain of the pieces of cordwood, so the walls don't get eaten.  Bugs can eat into the sides of the wood easily.  At least, that is what my father told me.  He built one pretty large cordwood building about 40 years ago and it is in great shape.  There are places where the wood split and light can be seen through the wall in those spots, but the building has been amazing short of that.  He didn't have much money when he built it either.  That building has mortar at both ends of the wood, but a simple air gap in the middle rather than sawdust.

Hello there! I’m planning to build a cordwood sauna approx 10’x8’. My question is… using your cordwood masonry mix, how much, in bags/units of each (sand, hydrated lime, Portland cement, sawdust) should I expect to purchase to be in the ballpark? Thanks in advance for your time! I appreciate your commitment to helping people! Robert Bass

Hi Robert,

Welcome to Permies!