This article is by Grant Nicholson from Owen Sound, Ontario. He has agreed to let it be posted in order to help people learn about Slipforming, Double-Wall cordwood and Frost Protected Shallow Foundations with cordwood. Thank you, Grant.
Slip Form and Cordwood:
Old methods Combine for a Speedy Result by Grant Nicholson
Cordwood construction and stone masonry have a lot in common. Both methods of building involve acquiring natural materials, preparing them, and embedding objects into mortar. Both of them can also be time-consuming. In particular, stone work is heavy, and splitting stone is a skill that takes time to learn and implement. For this reason, our ancestors developed a means of building stone walls at a faster pace, as well as incorporating smaller, easier-to-manage material. This approach, known as slip-form construction, involves erecting a temporary containment wall to hold stone and mortar in place until they set. It virtually eliminates the need to shape and carefully choose stones, considerably reducing the time and skill required. So why not combine cordwood masonry and slip form technique?
My wife Keira and I had been constructing our triple-wall, cordwood, timber frame home for three years. As time went on I found myself continually wondering how the process could move along at a faster pace. Assisted by friends and family we had managed to completely enclose our well-insulated home with an exterior cordwood wall, loosely modeled after the methods developed by Cliff Shockey. This 8 inch thick wall is comprised of cedar rails and old highway guard rails, both great sources of untreated, well-seasoned cedar (most of the rails are 80+ years old). The downside of using cedar rails is girth; it takes many more pieces to complete a wall, and therefore, much more time than a wall composed of 8” diameter log ends. The time had come to build the inside wall of the house as in-fill between the posts of our timber frame. I was eager to find some time-saving solutions.
One idea we discussed included pre-fabricated cordwood wall panels built horizontally on a vibrating table and tilted into place; another idea was similar to a gigantic cake decorator, squirting mortar through a tapered hose. Neither of these ideas seemed sympathetic to the hands-on, low-overhead, piece-by-piece nature of cordwood that we had grown so fond of building. The slip-form technique, however, appeared to be worth a try.
Prior to constructing our inner wall the electrical work had to be completed. An attempt at using a piece of Sono-tube to hold a switch box in place was a dismal failure; it’s easy to underestimate the pressure behind 8 inches of wet mortar and wood, and the tube collapsed into an unwieldy shape. I immediately set about devising a means of mounting the electrical into plywood boxes. As there were common widths of electrical boxes, long lengths of ‘rabbeted’ plywood (see diagram) were cut into shorter widths that could quickly be assembled with a brad nailer. These were screwed onto the stud wall using small steel 90° brackets from the local hardware store. Great strength wasn’t necessary; the wooden boxes simply needed to house the electrical boxes in a plumb, firm manner until the mortar would set. The depth of these wooden boxes was dictated by the thickness of the cordwood wall.
Electrical may not be the only wiring to take into account at this point. As a mortared wall has more permanence than drywall, extra care should be taken to ensure that speaker wires, telephone lines, coaxial cables and any other embedded utilities (including plumbing) are in place and protected from any acidic reactions that could take place with the mortar. We considered running 1.5” conduit in our walls with a series of exit points just in case there would be a future need to run something. Although we didn’t end up doing so, it still seems like a great idea.
The next activity involved building a frame. We used 2×4’s to house a sheet of 3/4” thick OSB (dense chipboard) that would slide up the wall as each layer or ‘course’ of cordwood would be set. The depth from the stud wall was determined by adding 1” air space + 8” cordwood length + 3/4” OSB thickness + 1/8” wiggle room for a total of 9 7/8”.
I had planned to leave the bottom spacers for the frame embedded within the wall, and as a means of hiding them, devised a plan to mount the vertical frame to the spacers using 4” long screws through pre-drilled blocks. When the frame is removed, these blocks come out and mortar is applied, effectively hiding the 2×4 spacers.
Prior to pouring mortar there were some preparatory tasks. The electrical boxes would stick out too far and interfere with the slip form if the faceplates were installed. Instead, the wires were marretted (wire-nutted), pushed into the boxes and covered with tape and plastic. As our house is slab-on-grade, the heated floor is insulated both below and around the perimeter. In order to avoid thermal bleed from the foundation, Styrofoam tm insulation was installed below the cordwood. As we are installing a slate floor, and we wish to see the mortar meet the floor instead of installing baseboard, this Styrofoamtm becomes advantageous; it can be easily cut away to allow the slate to extend beneath the edge of the wall, and we can point right down to the floor after installation.
This Anatomy of Double Wall Slip-Form Construction Slab-on-Grade (by Grant Nicholson) is very important because it gets rid of the energy nosebleed that usually takes place where the slab meets the cordwood.
The cordwood wall is attached to the stud wall using brick ties approximately every 18 inches. These are screwed onto the wall in advance to avoid forgetting to use them. Once forms are in place the process moves quickly, and it is easy to overlook using these ties. A word of caution, however; when bending the ties out from the wall, be sure to curl the ends back towards the wall. They are very sharp, and it would be easy to cut your face or arms on them while working in the form.
In typical cordwood construction nails or screws are driven into the places where window boxes or timber frame meet mortar. Following an experience on the outside of the house where a wooden beam expanded and cracked the mortar, I chose to create small expansion joints around the timber frame instead. This was accomplished by stapling Styrofoamtm sill gasket to the timbers, establishing an approximately 1/16” gap where the timber frame can swell in the humidity of summer without affecting the cordwood.
Deep boxes were constructed around all windows and doors. We used pre-finished birch plywood. As the depth of these boxes exceeds 20”, wood expansion becomes a concern if solid wood is used. Plywood eliminates this expansion concern. Solid wood sills were installed, however, and it’s a good idea to seal all sides of them with whatever finish you intend to use prior to installation to avoid absorption of water from the mortar. Masking them with a removable film or plastic and masking tape is also advisable to protect them from the effects of abrasive mortar droppings that are inevitable.
Window/door boxes should be temporarily braced while cordwood is built around them. The pressure of the wet walls is substantial and may be enough to bow in the sides. Building code in many places would suggest that steel plates must be installed above any openings in masonry walls to prevent caving. We didn’t do so on the interior of our house. Diamond lathe and lengths of rebar were generously installed above all openings and tied back to the stud wall with brick ties. Our longest span of this sort was 8’ across a patio door opening. In this case a rustic barn beam was put in place directly above the door, adding plenty of support to the masonry.
At this point, with all electrical, plumbing, window/door boxes, insulation, brick ties, sill gasket and slip form frames in place, it’s time to mix mortar.Double-wall construction presents a unique opportunity for sunken cabinets. As our interior wall is 8 inches thick with a 1-inch air space behind, it seemed logical to install 9 inch deep bookshelves at some points in our walls. Although the R-value of books remains a mystery, the value and attraction of these recessed storage spaces are undeniable!
We used the same mix for the inside as we did outside, each mixer batch consisting of 18 sand, 4 Portland Cement, 6 hydrated lime and 6 wet softwood sawdust. The sawdust may not have been necessary, but we wanted to ensure there wouldn’t be shrinking issues. The mixes were poured into a large, plastic drum that we cut in half and carried horizontally on a dolly. The dolly was then pushed up a ramp into the house.
Be sure to keep some buckets on hand. Smaller square buckets (we had some from cat litter purchases) seemed to be an ideal size for pouring into our form. It’s easier and faster to add larger volumes of mortar at a time, spreading it out into an even layer as you go. For the bottom layer, we created a 2” thick bed, pushing our pieces into it and leaving space between them as needed. A shovel is handy until the wall exceeds waist height.
When the first row of logs is in place, we initially found it handy to draw a line on the wall above them with a magic marker. This line becomes a depth guide, showing where the mortar depth is expected to be. Cordwood is, in most cases, an organically shaped material. This means that the cuts on the ends of the pieces are rarely truly perpendicular to the length. When installing pieces into the form, ensure that the ends of the logs are flush against the form, allowing the length of the log to take whatever angle results. This will help ensure a flatter vertical plane.
Eventually, the tactile nature of the process took over, and we could sense the depth of the joints with our hands. It’s a bit of a tricky business, and the tendency towards really big mortar joints is prevalent since you can’t see the log ends. This was acceptable in our case, as we view our inner wall as a heat sink, and more mortar equates to greater heat storage capacity.
A thicker, drier mix is a bit of an advantage. It means that the air space behind the wall will remain more intact (less ooze-out). This is similar to the appearance of lathe and plaster walls. A trowel was used to scoop the mortar out of the back. The drier mix makes this easier as well. For anyone with climate considerations; our inner wall was built in the early onset of winter, with snow on the ground and freezing nighttime temperatures. This wasn’t an issue, as the walls curing inside the house were nice and warm, with a big fire in the hearth. The sandpile was outside, tarped and covered with a stack of refrigerator cartons, and did not freeze beyond a frozen outer crust. However, water content was a major issue. Unlike mixing mortar in the summer sunshine, there is far less coagulation in the cold. The top layer of sand does not get drier, and the water must be squeezed out of the sawdust.
When testing the slip-form process initially, I expected a waiting period would be necessary before moving the form up for the next ‘course’. It was a great pleasure to discover that, provided the mortar has proper moisture content, there is no lag time. The form can be moved as the job is completed, allowing for non-stop working. The rhythm of this process is very easy to plug into; often we would work for hours without speaking, engrossed in our thoughts, hearing only the mixer or heavy exhalation while hoisting mortar buckets. It doesn’t require the same level of concentration as working a trowel and carefully choosing the next cordwood piece of the puzzle. These were days of great output, both physically and internally.much greater degree. This lesson was learned the hard way; once an overly wet batch is produced, the only answer is to mix a really dry batch and attempt to combine the two. You don’t want to find yourself in this predicament.
The process moves quickly from the bottom up to waist height. Smaller form boards and extra frames were added in tight areas, such as below windows. Once it became necessary to climb a ladder, the fastest means seemed to be to keep one person on the ladder, and another person on the floor feeding cordwood and buckets of mortar. Two people installing walls can go through a lot of material. A third person could produce mortar and carry log ends on a continual basis. I suspect that the ideal scenario for a well-prepped slip form session would involve 2 different walls, with 2 people on each wall, and one or two people working round the clock at the mixer and cordwood piles. Ideal as this may be, it didn’t come to pass during the 8 weekends we committed to our interior walls. We have certainly learned the value of teams of people and did our best to take advantage of any such opportunities throughout the entire duration of our project.
As the wall approaches the top, keep an eye on the space left for removal of the slip form frame. You will need to be able to fit your drill in above the wall you are building to remove the screws. It helps to angle the screws in when the frame is first installed. Once the frame is removed you will need to finish the rest without a form. We found it valuable to build to the highest point at the extreme left or right of your work area, moving across space, avoiding getting ‘pigeon-holed’ by enclosing mortar as much as possible.
When the time comes to point the slip form cordwood wall, be prepared with plenty of extra mortar on hand. You will need to get more aggressive with the wall, pressing into it with your trowel and ensuring that there are no voids. Until getting accustomed to working without seeing the log ends, there may be some surprises in terms of mortar joint size. Pointing is your opportunity to analyze your placement of logs into forms.
In summary, the combination of cordwood and slip form was beneficial. As with most aspects of house building, timely results are achieved through excellent planning and preparation. As noted earlier in this writing, our use of small cedar rail stock and highway guard rails added considerable time to the project. The traditional trowel method we used on the outside of the house yielded 40 – 45 square feet of wall for every 14 single man hours, whereas slip forming produced 120 – 130 square feet of wall in the same amount of time. Applied to a double-wall process where only one side of the wall is finished the form works very well. Use of slip form to produce a single wall that is pointed on both sides is debatable; the log ends would need to be truly uniform to produce agreeable results.
One of the other advantages of slip forming is it can accommodate an unskilled worker. The form becomes your guide, ensuring a wall that is plumb and vertical. The risk of using unskilled labor comes down to accepting the size of possible large mortar joints. This may or may not be of great importance, depending on your point of view or application.
Aesthetically, the randomized appearance of slip forming is akin to an architect’s approach to buildings. A texture is created that is entirely driven by the raw materials used. The mosaic-like appearance of carefully constructed cordwood gives way to a less-contrived sense of the wood, much like aggregate within a concrete emulsion. The larger mortar joints are reminiscent of old stone barn foundations that were constructed strictly for function, almost inadvertently becoming beautiful.
The log ends could be easily sanded with a grinder to clean the mortar off of them. From the outset we planned on plastering the wall, leaving the occasional piece showing, more like a white-washed. That idea has since become a source of debate, as many friends and family really like the look of it as is. At the time of this writing, we have not completed the rest of the house, so the jury is still out as we have a few months to decide…
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Readers have requested a brief bio, so here goes:
Richard & Becky Flatau built their mortgage-free cordwood home in 1979 in Merrill, Wisconsin. Since then, they have written books, conducted workshops, facilitated the 2005 2011 and 2015 Cordwood Conferences and provided consultation for cordwood builders. Cordwood Workshop DVD (2018), Cordwood Construction Best Practices (print 2017) and Cordwood Conference Papers 2015 are the newest publications available from their Cordwood Bookstore. The books & DVD are also available as ebooks for a quick and easy shipping free download.