April 23, 2023

Scratch-Coat of Stucco on South-Facing Wall

The scratch-coat* of Stucco on South-facing wall is complete and I now will begin working my way around the East-facing wall. With less windows to work around, more familiar with the the properties of stucco and crafting a rhythm to the rendering's application to the wall, I should be able to apply the material with increasing speed without sacrificing attention to detail.

 * Note: The scratch coat is basically the first layer of stucco applied to the building's wall material; in this case Cob (straw-clay).  The word 'Scratch' refers to horizontal 'scratch' lines made in the stucco after when still wet.  The scratched surfaces allow the second final (color) coat to 'lock-into' these grooves in the scratch coat and thus creating a stronger bond.    The building was wrapped in chicken-wire prior to the stucco application (see previous recent blog posts), to ensure the stucco's retention to the cob wall.

November 25, 2022

Departure from Tradition - Exterior Plastering


This post may not be espoused by adobe traditionalists, but I decided to use a cement-based 'stucco' over wire mesh for the exterior rendering of the cob wall. However, I believe I have a good case for doing so.

The traditional material for exterior abobe-wall finishing is Lime plaster. The rationale for using lime plaster on adobe is that it is a porous material and thus will allow the wall to breathe - removing moisture from the wall.  In cement-based stucco, it doesn't breathe, thus the concern is moisture will build-up between the cement and the cob wall resulting in a loss of adhesion of the cement stucco from the trapped moisture. 

I previously used lime plaster for the exterior after completing my adobe block out-building on this same property, but found-out over time that the lime plaster on the South and West facing walls were falling-off every two years.  This would be, in part, attributed to the 40-50 deg.F., temperature fluctuation here in Arivaca, Arizona, which results in thermal expansion-contraction of the lime plaster relative to the more stable adobe wall behind it.  So essentially the lime plaster worked itself free of its adhesion to the wall due to thermal expansion and contraction until it brakes-off, or the plaster collapsing.  This would necessitate a maintenance project to re-apply the lime-rendering every few years which took time away from working on this cob studio

I don't want to repeat this lime-plaster failure problem on the studio, so I changed my approach and decided to go with cement stucco.  Here is my rationale for why i think the cement-based stucco will 'perform' over time on my studio:

1) Per a previous blog post, I documented how I wrapped the entire cob building in wire mesh in order that the stucco has something to grab onto and hold it in place (not done on the abode out-building).

2) And in regard to the 'moisture' theory, I believe this may not be an issue in the dry climate of Arizona and compared to more temperate thus humid climates of the northern and coastal regions of North America.   

Also moisture from the interior comes from people living in these adobe structures. Whether it's showering, cooking, or plants.  However, the function of this building will only be as a multi-media studio, not a residence so I'm counting on minimum moisture entering the cob walls from normal household functions.

Finally. I just want robust exterior protection, not just from the weather, but insects, especially termites that are everywhere in the ground here. Also, I'm getting older (63 y/o) and do not want to spend later years back on scaffolding repairing failed exterior plastering.

Time will provide the final judgment.

June 15, 2022

Wire Mesh Added to Help Adhere Exterior Plaster


As the summer rain season approaches here in Southern Arizona (usually around the beginning of July), the goal is to get plaster on the exterior of the studio now that the structural work on the building has been completed.

Over the years on other adobe buildings that have been constructed, I have observed that the daily 40-50 degree-F temperature rise and fall in this region cause the plaster material to expand as temperatures rise, and contract as temperatures fall. This is known as thermal expansion or thermal contraction.  

What results is that this daily expanding and contracting of the plaster tends to work itself off the adobe wall over time.  The South and West walls get substantial sun, and thus I have observed that those sun facing walls have demonstrated the majority of failed plaster.

With regard to plaster for the Studio, in order to prevent or at least drastically reduce the occurrence of failed plaster due to thermal expansion, I decided to wrap the entire exterior of the Studio in chicken wire which will have the effect of providing additional grip and support of the plaster on the wall.  

Adding chicken wire to an adobe structure and where the walls are curved, provides a challenge to fasten the wire to the wall.  On a wood or cement board building, the wire would normally be fastened with heavy duty staples or equivalent.

In the case of an earthen building like the studio, another 'fastening' approach was required since you can't use a staple gun on cob.  The solution I employed was to use 3-inch deck screws and 1-1/2"-diameter washers to both grab the wire and securely adhere it to the cob wall (see photos below).

The next step is the actual plastering.  Now the plaster can be applied to the wall with confidence that future maintenance will be reduced as a result of the wire mesh wrapping.

November 01, 2021

Top Cap on Parapet Wall Marks The End of Principal Construction


A parapet wall is a short stem-wall that rises above the roof line.  In the case of this structure, the parapet wall is essentially a continuation of the exterior cob walls that surround the large room roof.  The top-cap serves both a practical and an architectural purpose.  The practical aspect of the cap protects the cob wall from the elements; and the architectural purpose provides for a uniform visual aesthetic with the two other roof lines that make up the structure. 

The photo below shows the finished top-cap for the large-room roof.  A lower roof also with a top-cap can be seen in the bottom-left of the photo.

                                                CLICK ON ANY IMAGE TO ENLARGE     

So how was this top-cap fabricated?  The photograph below shows a top-down view of the forms used that would serve the purpose of retaining the poured concrete that is the material used for the top-cap. 

In the center is the cob mix (clay & straw). 3/8" of structural rebar can be seen and runs the length of the stem wall.  On each side of the cob stem-wall are 1-inch thick strips of hard foam at wall height that act to add additional width to the poured concrete, so that when the forms are removed, the top-cap will extend beyond each side of the wall to create a 'lip' over the wall to allow water to drip off the cap away from the wall.

The actual form to hold the poured concrete can be seen in the photo below.  Due to the curves that make wall, a flexible form material was employed.  In this case I used thin veneer plywood, cut 1-ft strips from an 4-ft by 8-ft sheet.  I used 4-in deck screws through wooden stakes into the cob to hold the plywood form in place.  The top-cap formt is roughly 3 to 4-in height (top-cap thickness) and about 8 to 12-in width (cob stem-wall width including the 1-in foam spacers on each side of the wall.

The two photos below show the extent of 'forming' for the top-cap around the curved wall that surround the large-room roof. The thin veneer is perfect for hugging the curves.

Also seen in the photo below is the existing top-cap of a lower roof.

The concrete was mixed in a mixer and hauled-up the scaffolding in 5-gallon buckets. Each bucket only 1/3 full due to the weight, then poured into the form, worked and flattened using a gauging trowel and rectangular metal float trowel.

The final result (below) is a protective top cap and uniform architectural styling. 

I realized after cleaning-up the job-site and admiring the handiwork, that after 15-years the principal construction of the cob studio is now complete.  There is still exterior plastering, interior wall rendering, windows/doors to install and floors to install.  But that is the finishing work: The structure itself is finished!

September 13, 2021

Masonary Parapet Wall

A previous post showed assembly of the large room roof and the surrounding parapet walls.  However, the west side parapet wall of the roof initially was not addressed.   This is because a concrete 'buttress' was poured to protect the large room from water coming off the dome and thus it made sense to add masonry on top of the buttress. [For better understanding of why a 'buttress was needed, see blog post here:  https://cobstudio.blogspot.com/2020/07/spillway-for-water-off-dome.html ]

Therefore an opportunity arose where I could assemble a decorative masonry wall on top of the concrete buttress to add some architectural styling to the overall structure.  At my 'resource area', I located every brick and decorative block I could find to complete this section of the roof structure.


The photograph above shows the South end of the masonry parapet wall where it terminates and integrates with the 'cob' parapet wall (southwest corner of the large room roof).

The photograph below was taken at roof level facing west showing the full length of the masonry parapet wall.

Notice that the decorative block wall is not uniform with regards to the type of block resulting in a lack of visual symmetry.  I'm good with that. There is satisfaction with the fact that I am using leftover building materials previously purchased for other projects.  

Also, notice the pattern of brick on top of the ornamental block. I had two type bricks, one a regular sized brick and the other a thin brick.  I didn't have enough of either brick to lay on top all the way across, so by alternating the brick types I was able to complete the topping and create a pattern.

August 14, 2021

Sealing Exterior North/South Parapet Walls Above Sub Roofs


With the summer (monsoon) season here in Southern Arizona, my cob structure is subject to short burst of heavy rains.  The walls have been holding up surprisingly well over the years of construction but ultimately will need some type of water-resistant coating.  

The traditional method has been a lime-plaster mix, but I went with a concrete-based stucco mix just for the The North and South parapet wall sections above the smaller sub-roofs.  The rains came early and resulted in the shedding of straw and clay, creating a situation where drains could get clogged.  I decided on a robust long-term stucco solution. 

Wire mesh was applied and secured with 4-inch deck screws into the cob along with large washers to hold the mesh in place (See photo below showing the North parapet wall).


The photo below shows the South parapet wall with the wire mesh and a traditional stucco mix being applied.  This stucco mix is concrete based which is traditionally frowned-upon because adobe wall should be able to breathe.  However, I wanted a maintenance-free solution for these sections above the sub-roofs, so I went with the traditional stucco mix. 

The photo below is the finished North wall still drying following the application of the stucco.  The color was achieved by mixing-in 'concrete-color' available at Home Depot.  I added two colors 'adobe' and 'clay'.  I didn't measure or pre-test the mix of the two colors.  I just took a shot and like the resulting shade.


I still need to add a 'top-cap' to the entire parapet wall, but the strong monsoon season has made work unpredictable and the heat/humidity, exhausting. I'll wait until early fall to apply the top cap.  

In the meantime there are other projects to work on here at the cob studio.

June 16, 2021

Cool-Coat Elastomeric Applied

The next step for the large room roof was to apply two coats of elastomeric roof coating on top of the two coatings of liquid rubber (see previous post).  The liquid rubber coating is not UV stabilized so the white elastomeric protects the rubber layer.   In addition, the elastomeric reflects 90% of the sunlight and is cool to the touch; thus the 'cool coat' marketing name for the coating product.


Although a concrete top cap still needs be added to the top of the Parapet Wall and decorative masonry block to create the West parapet wall; for all intent and purposes the roof is now waterproofed.


June 08, 2021

Rubberized Coating over Lightweight Concrete Roof


Now that the lightweight concrete roof has been poured and pitched to allow proper drainage out the roof scupper drain, sealing with two coats of a liquid rubber coating was applied (see picture/video).  This will be followed by several topping coats of white elastomeric (cool-coat) to add further sealing to the roof and protect the liquid rubber from UV degradation and reflect about 90% of the sun's UV & infrared rays.

The liquid rubber I use is called "Blue Max" from Ames Research.  This product is non-toxic, VOC free and eco-friendly.  The cool coat to be use will be "Maxi-Stretch" also from Ames Research. 


There is still work to do with regard to the parapet wall and some ornamental block work on the west end of the roof near the dome.  Finally there will be a concrete top-cap added to the parapet wall.  Stay tuned!  

May 31, 2021

Lightweight Concrete Roof

The final roof material needed to provide pitch (for drainage) and insulative properties was a decision between a spray 'foamed'-in roof; or some type of lightweight concrete.  The easy way would have just been to hire a spray-foam roof contractor, but that would not have been keeping with my commitment to innovate with materials and to build the structure myself.

So I formulated a very lightweight concrete solution using simply Portland Cement, Vermiculite and some sand.  Vermiculite clays are weathered micas in which the potassium ions between the molecular sheets are replaced by magnesium and iron.  Vermiculite is a mineral that when heated commercially expands to create a porous featherweight material suitable for many applications.

(Below) Mixed in a cement mixer the ingredients were: water, Vermiculite, sand and Portland Cement.  I do not have a recipe to share simply, there was none.  Because Vermiculite absorbed much water, I kept adding water until I felt the consistency was right for application.   

But roughly one mixer load consisted of 5-gallons water, 4-cuft bag of Vermiculite, 3-shovels Portland Cement an 20-lbs of Sakrete Topping mix (Sand/Portland Cement).  That mix yielded five 5-gallon buckets 3/4-full each.

Magnified Vermiculite.  Has the look and texture of those small foam peanuts used for packing material.

Photo below shows relative size.   One thing I would do differently is purchase the 'fine' size/grade of Vermiculite.  I purchased the 'course' grade and it was harder to trowel because of it's size and coarseness.  There is a 'fine' grade of Vermiculite that would be more suitable for trowel workability.

This job required 25-bags of Vermiculite that I purchased from BASCO, Inc.to cover 300-sqft of roof area averaging about 3" in height. A 4-cuft bag of Vermiculite runs about $30.

The photograph below shows several string-lines I set-up along with a line-level to ensure the North-to-South pitch of the finished concrete roof will drain out the south-wall roof drain. The height of the concrete mix at the north end was approximately 4-inches, gradually sloping-down to 1.5-inches at the South wall where the scupper drain is located.

 More on Vermiculite at Wikipedia.  Below is a mention

"Roof and floor screeds and insulating concretes: exfoliated vermiculite (typically the finer grades) can be added at site to Portland cement and other aggregates, rheological aids, and water to produce roof and floor concrete screeds which are lightweight and insulating. In many cases, vermiculite-based roof screeds are used in conjunction with other insulation materials, such as polystyrene board, to form a total roofing system. A bituminous binder can also be used with exfoliated vermiculite to produce a dry, lightweight roof screed which has the advantages of low thermal conductivity, low moisture content, and ease of placement (by pouring from the bag and then tamping)."

More blog posts on the roof will follow...It's not completely finished yet!

May 30, 2021

Parapet Wall

The next step in the roof construction was to create the "parapet wall" which is essentially the building-up of the exterior wall above the roof line. 

Due to the amount of physical labor required to move the cob mix (clay/straw) up over 15-feet above the ground and the fact the the parapet wall itself is not structural...I was able to make the parapet wall only 8-9" thick and approximately 13"-tall.  

The photograph below shows a portion of the finished wall at the southeast side of the building. 


To drain approximately 300-sq.ft. of roof space, I purchased a section of clay section of a chimney flue liner to use as a scupper.   The photo below shows two layers of cob: The 'lighter' color clay (dry) is the portion of the wall raised from the concrete bod beam to the roof line (previous post).   The darker (wetter-drying) section of cob surrounding the chimney flue textends from the roof line to the top of the parapet wall.

Below shows a wider angle perspective.  Keep in mind that the rolled-roofing shown does not represent the finished roof.  It was really meant to protect the built-up roof underneath it from the elements until the final roof is completed.

Below shows the building exterior (North section - looking South) to the final 'cob' wall height.  Note: There will be a top-cap to the parapet wall.  But that will come as part of the 'finishing' work to the roof structure in the near future.

May 03, 2021

Cobbing: Bond Beam To Roof Line

The next step in the completion steps for the roof over the large room was to build-up the exterior cob wall to the height of the roof line.  As you can see in the first two photos below, the ceiling and sub-roof was built in top of the Ponderosa Pine vigas (beams).  The vigas rest on the concrete bond beam that rings the exterior wall. 

On the left side of both photos you can see the cobbing to the roof line completed from the east wall (1st Photo), working its way along and into the north wall (2nd photo).

The picture below shows a completed section along the east wall about 18" high and between 12" and 20" in width.  This represents a massive about of cob.

The photograph below shows a completed and nicely sculpted north wall section of cob.

The view in the photograph below shows the completed east wall viewing top down.  You can see the rolled-roofing employed to keep the ceiling and sub-roof protected from rain until the final roof structure can be completed.  

This 'final' roof structure will be discussed here in future posts.  The design of which is still being conceptualized, but in general will consist of a cob parapet wall, additional insulation, sculpted light-weight concrete and final rubberized / elastomeric coatings will all be integrated into the solution.

It was estimated that 2-tons of wet cob was hauled up scaffolding to complete the work shown in this post. And as far as the final completion of this large-room roof is concerned, there is still a lot of work to do to complete.  Stay tuned.

July 03, 2020

Spillway For Water Off Dome

Each of the four rooms of this studio have their own custom roof.   One aspect of thinking about what a robust roof system for this structure would look like was thinking how each of the four roof lines would interface with each other - especially in relation to water drainage.  For the 'round room' a dome was built.  I always knew in the back of my mind that I would eventually have to figure-out how to channel water coming-off the dome since to its East was the largest room of the structure that would need a roof of its own.  

Since this studio is being built without plans, my mantra has always been: "I'll figure it out when I get there".  Well I finally 'got-there'.  This blog post is how I addressed the challenge.

Overview: I decided to form a north-south concrete buttress up against the Ponderosa Pine beam (Viga) that was placed at the west end of the large room (see a few posts back for the placing of the roof beams).   I also used concrete to sculpt spillways to direct water in both North and South directions coming off the dome.  The photo above shows arrows generally illustrating the intended water flow direction per my approach.

The photo above provides more detail - almost a 'cut-away' view of sorts to illustrate.  The spillway is below the height of the buttress wall to keep flowing water in the channel.  NOTE: The built-up roof over the large room is not completed.  The entire wall of the structure still needs to be built-up with cob (clay/straw) around the entire room.  

The final wall height will be above the roof line to the top of the parapet.  This work will be done in the fall after the rainy season.  At that time also the roof will need to properly sloped for drainage and waterproofed.  These processes (yet undetermined) will be discussed from this point forward in future posts.

The South buttress and spillway is shown above.  The water drains-off the building.  Since the landscape slopes from North to South, the water draining off here will help water trees in the Mesquite Bosque south of the structure.

Notice the line of tar shown in the photo above.  That is the joint between the built-up roof and the concrete buttress.  That joint needed to be sealed so water would bot be able to enter the living space.  Eventually, a concrete parapet wall will be build on top of this gap and properly sealed to hopefully eliminate that leak potential for the long-term.

June 15, 2020

Building up the Roof Structure

Rome wasn't built in a day and neither is a roof structure.  The last blog post showed the 'barn-board' ceiling installed on top of the vigas, but a ceiling is not a roof.  This post here continues the story. 

One consideration when using the reclaimed barn-board was knot holes and irregular edging which upon installation resulted in minor gaps and see-through holes.  I knew above the barn-board I would be installing a layer of rigid foam insulation and I didn't want to look-up at the ceiling and see the white foam board between any gaps or knot-holes.

My inexpensive solution was to purchase a roll of thick woven burlap fabric and layout on top of the barn-board, secured using a contractor's staple gun (see photo below).


The next layer as mentioned was the application of QTY 8, 4'x 8' sheets of 1-1/2" thick rigid foam insulation.   This R-tech insulation is insect and mold proof (see photo below).  The insulation panels were held in place using wide-head nails.

The final layer for this phase was QTY 8, 4' x 8' sheet of 1/2", plywood.  I need to trim the plywood at the edges where necessary and complete the securing of the plywood with wood screws.

The wall of the structure still needs to be build-up with cob (straw/clay) to roof height and waterproof and drainage.  The summer rain monsoon season is fast approaching, bringing urgency to this phase of the project.

June 10, 2020

Preparing Beam Ends for Moisture and Insects

The ends of the Ponderosa Pine vigas (beams) rest on the concrete bond beam that was poured on top of the cob wall (see earlier posts).  The stray/clay (cob) mix will continue another 12" in height up from the bond beam to the height of the roof deck.

In order to protect thus preserve the beams for the life of the building, the beams need to be protected from moisture and wood boring insects.  An inexpensive method to protect the ends of the beams is to use 'asphalt emulsion'.  Asphalt emulsion is essentially a petroleum-based tar with a consistency of a thick paint.   This product can be purchased at a building or masonry supply store.

The photos here show the coating applied. I used a paint brush to apply and the emulsion  dried fairly rapidly.  


Notice the large-head nails extending at the ends of each viga.  The purpose if to help secure the cob against the beam when the cob is applied onto the top of the bond beam. 

The construction of the roof system above the large room is far from over.  More posts to follow.

May 30, 2020

Large Room Ceiling

I didn't decide ahead of time on the look of the ceiling for the large room.  Like most of the decisions during the construction of the 'Studio', I figured it out when I got there.  

When I did 'get there', I came across what I thought would be a good aesthetic material at a reasonable cost - Barn board.

Originally, I had considered small (2"-dia) pine poles for the latilla's on top of the beams, but I couldn't justify the premium expense for the poles and was willing to see what other options would eventually make themselves known.

A few weeks back during my search for vigas I came across a reclaimed lumber company in Mesa, Arizona - "Old Sol Lumber Company".  They suggested barn board obtained from old barns as dismantled in Kansas.  Looking at their stock, I really liked the texture of the old wood and the fact it had a history. 

From a practical standpoint, I knew I could obtain the needed 300 sq.ft. from this supplier in one shot and thus would save time.

Before I picked the barn-board solution my wife had suggested making the ceiling of old doors.  I really thought that it was a good idea, but finding suitable doors would be a more involved search for the style and quantity needed for the ceiling.  Another consideration was time because I'm trying to get most of the roof system in place before the monsoon season that begins here in southern Arizona around early July.  Note that the final roof structure probably won't get completed until after the monsoon season in the fall.

Ultimately the choice of Ponderosa Pine vigas along with the barn-board ceiling will present an appropriate and aesthetically suitable compliment to the studio's overall design. The vigas provide an historical building context - traceable to Anasazi construction methods (see below).  The reclaimed barn board will have the effect of being brought-back in time.


The photo below was taken at the magnificently constructed Anasazi structures in Chaco Canyon (NW New Mexico) built around 1100 AD.  The Anasazi used pine vigas hauled from the forests by hand 20+ miles from Chaco.  This is the Studio's connection to ancient architecture of the Americas.

NOTE: This posting is just the start of the large room roof's structure.  I will update this blog to detail each step of the roof's construction.

May 07, 2020

Ponderosa Pine Vigas for Large Room

For the large room, six Ponderosa Pine vigas were utilized for the main roof support.  The room shape is an irregular oval roughly 15-feet by 17-feet.  The vigas were 10"-dia and rest on top of the concrete bond beam (see previous posts).  The building code requires that the beams rest (overlap) the bond beam 12" on each end.  The beams were spaced approximately 30" apart on center.


The vigas were purchased at Southwest Ideas in Surprise, Arizona.  They were hard to find.  There are very few forest product companies left in Arizona.  Not sure why - with all the Southwest designed buildings in the state one would expect a healthy market for vigas, thus several suppliers. 

I rented a 24-ft 'Stake Truck' from Ryder Truck Rental to transport the vigas from Phoenix.  The length of the beams were 17-feet.

I hired a local contractor with a boom truck to assist in hoisting the vigas to the roof, where myself and a laborer set the beams at the locations and spacing I marked along the bond beam.

Below is a roof view of the vigas laid in place (below).  Notice the metal 'Strong-Tie' straps holding the vigas in place.  The metal securing straps were embedded in the concrete of the bond-beam as mentioned in the previous blog that discusses the pouring of the bond-beam.

This post will be the first of several that will discuss the construction of the roof structure.  The laying of these beams required the most coordination between the supplier, transportation and on-site equipment and personnel to install.  The rest of the roof construction will be mostly a labor effort.