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?
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
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...
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.