Cordwood - Mortar

Mortar

Rob Roy, an experienced cordwood builder, recommends a mortar mix by volume of 9 parts sand: 3 sawdust: 3 builder's lime (not agricultural): 2 Portland cement. The sawdust should be from light, airy softwood and passed through a ½ inch screen. Saw mills and chainsaw dust are great sources. Saw dust, presoaked in water before use, acts like a sponge from which the mortar draws moisture, drying slowly and reducing cracks. A commercial cement retarder can be substituted for sawdust, but has a larger environmental impact. Builder’s lime makes the wall more flexible, breathable, and self healing because it takes longer to completely set than cement. Portland cement chemically binds the mortar and should be either type I or II.

Richard Flatau, in his book Cordwood Construction: Best Practices (2012) suggests using a mortar mix of 3 sand, 2 soaked sawdust, 1 Portland Cement and 1 Hydrated Lime. This mix is for non-load-bearing cordwood (i.e. post and beam framework) and has the advantage of curing slower and displaying less cracking than mortars that are "light" on sawdust. Flatau also recommends shading the masonry work from the full sun and covering it at the end of the day.

Cob mortars are more environmentally green and use on site soil, but require further steps for weatherproofing and maintenance. External waterproofing finishes for cob cordwood walls include lime-sand plaster, linseed oil coatings over earthen plaster, and Earthbind 100 compound (a waterproofing additive). Long overhanging eaves and a high foundation also help reduce weathering.