Laying the Adobes

Quentin Wilson and Associates, specializes in solar adobe design and construction. He grew up in the South Valley of Albuquerque, New Mexico where he watched adobe bricks being made. In the fifth grade, he made miniature adobes on cookie sheets in his mother’s oven in order to construct house models for a class assignment. By age thirteen he made full-sized adobes in the back yard and ruined the grass. Later, he traveled a bit, went through the Army, and graduated eventually from the University of New Mexico with a major in physics, minors in math, chemistry, and education in 1970. After teaching high school two years and community college math for three more, Quentin moved into professional solar adobe construction in 1976 as the Project Manager and Instructor for the Sundwellings Demonstration Project at Ghost Ranch, Abiquiu, NM. He became a licensed general contractor in the State of New Mexico in 1982. He has been building homes and teaching seminars and workshops ever since. In the fall of 1995 he established and taught the full-time Adobe Construction Program at Northern New Mexico Community College. His website, quentinwilson.com, lists the course schedule and many other resources related to working with adobe.

Q and A: I am building an adobe home about 8 miles south of Jemez Springs NM (just north of the Jemez Pueblo). This is my first experience with adobe construction, and this being the case, I have some areas of uncertainty. The most pressing of those right now is that, this being December, my adobe mortar (an earth mortar) is freezing at night. I've asked some folks around here, and have gotten the "don't set any adobes after 2p.m." rule.

I usually say don't lay adobe after mid-November. If you are not using cement based mortar, then frozen mortar will thaw out with no dire consequences. Cement mortar is ruined by freezing.

I followed that yesterday, but arrived this morning to see that even so, the mortar froze over night. Even the courses I was laying at nine or ten in the morning seem to have frozen. The temperatures have been in the low to mid 40s during the day and down to the 20s at night and the house site gets full sun from 7:30a.m. till around 4p.m. I have heard the warnings about the damage that freeze thaw can have on the mortar joints, however, given that even the first courses of the day froze, and the fact that I'm getting as much sun as possible, it doesn't seem as though I can avoid some freezing. Is there some level of acceptable freezing? Is there any truth to the Two p.m. rule?

The two pm rule often gives the mortar time to dry enough so that there is little moisture to freeze at night. It's a good rule, but I like the November rule better. My best advice is to take a break until mid-February or better yet, mid-March. If you cannot do that you can switch to a cement based mortar and add Anti-Hydro which is an antifreeze well known and used by concrete block masons.

I am using all unstabilized bricks. To get around the NM adobe code requirement of having the first course be semi stabilized, I made the foundation stem wall 4 inches higher, so there will be 4 inches of concrete in between the finished floor and the first course of adobes.

That is not really going around the Code.That is meeting the other option that the Code gives.

Q: The house I'm building is hexagonal in shape, with some rectangular rooms branching off. So instead of 90 degree corners, I have 66 degree corners. My brick dimensions are 10 BY 14. Last fall when I began to lay the first courses I was having difficulty figuring out a way in which to overlap the 66 degree corners. On one course, it's easy to diagonally cut two adobes to the appropriate angles so that there is a break right at the 66 degree corner, however, on the next course there no way possible to straddle the break below with a full brick. The only way to straddle the break from the first course is to do so with a brick which is trimmed down to be somewhat on the small side.
My question: Would you advise 1. laying up the courses so that at the 66 degree hexagonal corners, there are breaks in every course, or put another way, not straddling breaks with whole brick at the corners, but having breaks(at the corners) every course, and then tying the walls together with durowall...or 2. Overlapping the breaks at the corners with a solid brick above, even though that brick not be a full 10 by 14. or 3. Neither of these, but something I've not yet thought of.

A: The Code requires a four-inch overlap from one course to the next. Therefore even on standard walls and corners it is not necessary to have the bricks of one course centered over the joints below. I would say that your option 2 is the way to proceed. It is always good to have some bridge over joints in the course below. Or maybe instead of having any joints right at the corner, one course could run beyond the centerline to the right, the next course could run beyond the centerline to the left. Then for good measure reinforce the corners with Durowall about every fifth course. Your house will be perfect.

Q: What is a good size for adobe blocks for a dome shaped oven? The interior diameter is around 4 to 4.5 feet.

A: Often as not, we just use the NM standard brick, 4 x 10 x 14 inches and cut it in half to make two 4 x 10 x 7 inch bricks. We use them lengthwise to give a wall thickness of 7 inches. They are mortared up with wedge shaped mortar. When they are available we use 2 x 7 x 10 adobes which are nice to work with on the lower courses. We cut them in half to make 2 x 7 x 5 adobes for the final courses at the top. Again the mortar is wedge shaped.

On rarer occasions we use tapered bricks which we made once and bought twice from Mel Medina at the Adobe Factory in Alcalde, NM. They are about 5 x 7 on the outside face and 3 x 3 on the inside face and almost 10 inches long and the dome is 10 inches thick. The shape of the bricks is like a giant candy corn as Halloween has just reminded me. With the wedge shaped bricks, the mortar is more uniform with about half inch joints. We find that the domes go up faster with these bricks but once the forms are made and the bricks cast, it sets the bricks for a particular diameter. Hence, most of the time we just use rectangular bricks and create the curve with the tapered mortar and we can have any diameter we want.

Q: I'm about to run electrical cables in my adobe walls by cutting a channel/groove a couple of inches into the wall. What is your recommendation for filling the channels? I've read that I could use a sand/cement mix of 10 to 1 which will be 'soft' enough not to break away from the adobe brick. Do you think this is a suitable material or should I use something else - I'm worried that the structural integrity of the walls will be compromised if I don't get it right.

A: The same mix that was used for your adobes or the mortar between them will work. Sometimes it is wise to have a slightly damp mud mortar and dry pack it in with a board of the right width about 8 inches long. You can pound it in with moderate taps from a rubber mallet. Standard cement/lime/sand mortar mix will work and so will gypsum/sand scratch coat plaster mix. I have never used a 1/10 mix but it might work. Basically, with adobe everything works.

Next time, consider running most of your horizontal runs in the mortar joint as the walls go up. You can use type NMC which is a hard to find form of Romex. Otherwise UF cable works fine. Bury them in the middle of the wall and you can be assured that only the longest nails or screws will ever have a chance to hit them. The outlet and junction boxes can be set at this time so much easier than trying to anchor in an existing wall. Vertical runs can be made on the back side of door and window rough bucks which would, of course, be in place before the walls proceed upward.

Q:  I am building in addition to my adobe home Using 16 x 16” concrete stabilized adobe. The plans call for ladder wire every third course, would using 3/8 inch rebar every third course be a better option? Not sure how well the mortar would work with rebar.

A: Stick with the ladder wire. They are manufactured with lots of knurling along with the cross pieces just for the purpose of getting a grip on mortar. Rebar needs to have at least an inch of concrete on all sides for it to get a grip and do its job. DuroWall as most other brands is sold with the width 2" narrower than the stated nominal width. The nominal width is meant to be equal to the wall width. I would say that 12" nominal would be best in your walls. 10" nominal will work well if you can't easily find 12" At the wall corners you can bend the DuroWall over on itself to form a right angle that reinforces corners well. I call it a military bed miter. At window and door openings we always have wood rough bucks sized to take the window and door assemblies. I wrap the DuroWall around the 2 x 6 or larger rough bucks and use heavy fence staples to staple them to the bucks.

Q: Building a house in Mexico. We already have a structure of reinforced concrete columns and beams. We discovered we could use uncooked bricks (basically adobe) to fill it. Any advice on making the joints?

A: it's just bricks and mortar to finish up the walls. Infill. The mortar should be as close to the same composition as the adobe bricks if possible. I don't recommend that you use asphalt stabilized adobe bricks. Just plain sand, clay, water adobes with or without straw. Mexican asphalt will occasionally have a kerosene-like residue and smell. Not pretty in a new home. Just butter the ends of the adobes with mortar where they come up against the concrete columns. Adobe bricks need to be fully bedded in 1-2cm horizontal mortar joints with no air gaps. Vertical 1-2cm mortar joints do not have to fully fill the gaps. A bit of vertical joint unfilled about 1cm gives a nice opportunity for the plaster to get a grip on the adobe.Make sure you stuff plenty of mortar at the top so that the adobe wall makes contact with the bottom of the concrete beams. In a perfect world, the adobe bricks would be placed first and then the concrete, especially the beam at the top, would pour in right against the adobe bricks.On my own home, I did a couple of wood post and beam room frames and then filled them in with adobe. The adobe always shrinks a bit as it cures and the bond between adobe and cement at the top becomes a crack for the wind to blow through.

If you are nervous about earthquakes you might drill into the vertical concrete posts 5-10cm so you can insert a 1cm rebar aligned with the mortar in the adobe. Perhaps every four courses. The rebar can extend into the adobe wall  50 cm or even run to the next corner post. Again it would have been easier to build the adobe wall first with rebars protruding into the space that would become the concrete posts. Mexicans love concrete. Photos of the Cemex batch plant control room for Mexico City make it look like the Flight Deck of the Starship Enterprise. Keep up your interest in adobe bricks.