Mutirão in cob houses building process: advantages and limitations

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a
Corresponding author: julianomcoimbra@hotmail.com
SIMPÓSIO BRASILEIRO DE DESIGN SUSTENTÁVEL +
INTERNATIONAL SYMPOSIUM ON SUSTAINABLE DESIGN
Belo Horizonte, 1 – 4 August 2017
Mutirão in cob houses building process:
advantages and limitations
Juliano Moreira Coimbra
1,a
1
Universidade Federal de Pelotas, Faculdade de Arquitetura e Urbanismo, 1359 Benjamin Constant, Pelotas, Brasil.
Abstract. Building with earth is an age-old practice still common in many cultures of the world, especially
where there is less presence of industrialization. In recent decades, there has been a growing interest in
incorporating more sustainable processes in the production of consumer goods in all spheres (environmental,
economic and social). In construction, one of the emerging alternatives is natural building, wich proposes the
use of natural materials — such as clay, wood, straw and stone — and traditional building techniques, with
simple resources that are accessible to most people, hence the association with self-construction and mutirão
— Portuguese word for collective mobilization for mutual assistance in a free character. The problem
addressed is the feasibility of mutirão and its social gains in natural building that has mud as key material.
The object of study is the earthen house of the author, designed and self-built in the rural area of Pelotas-RS,
southern Brazil. The general objective is to study the possible gains of the mutirão practice in the refered
natural building, on wich more than 80 volunteers helped. The specific objectives are: a) to define the concept
of natural construction and to explain the constructive technique applied in the house studied (cob); b) to
analyse the relationship between mutirão and complexity of the constructive technique; and c) evaluation of
the observed social sustainability gains. As results, it is noticed that the mutirão sessions attracted many
helpers due to the curiosity about learning an unconventional constructive technique and allowed many
exchanges of ideas and experiences. They have also contributed to a more playful work environment, despite
the considerable amount of work produced, considering the inexperience of the volunteers. It is concluded
that natural building and mutirão complete each other, and can collaborate for practical gains of social
sustainability.
Keywords. Architecture, natural building, social sustainability, mutirão, self-building.
1 Introduction
Earth still is the most available building material in the
world. It is estimated that a third of the world population
live in houses made out of mud. In developing countries
the number can reach to fifty percent (MINKE 2002).
The Industrial Revolution, since the late sevententh
century, drasticaly afected how, where and with wich
materials the houses of “ordinary” people would be built.
Beginning in the United Kingdom, the vernacular
practice lost importance before housing mass production.
Today one can see a detachment of society from the
vernacular techniques, as the whole social structure
suffered several changes as well, making the once
popular self-construction almost fully desapear among
those living in the industrialized world. Consulted works
(WEISMANN and BRYCE 2006; EVANS 2002) aim
this social reshape as responsable for the association of
earth buildings to poverty, as it is taken as an inferior
material.
Such situation motivated people to pursuit
alternatives. In the “back-to-the-land” moviments of the
1960s and the 1970s there was a new interest on
studying and put into practice natural building
techniques. At the energy crisis of the 1970s, many
public atention was directed due to a propper use of
natural ressources, building energy efficience, passiv
house and alternative means of energy (EVANS 2002).
2 Self-construction and mutirão
Historically, in most cultures, to build the own house
used to be a comon practice. If the work was to heavy or
became too slow, the family would gather to help. The
idea of trading this duty to people outside the friends or
family circle is recent in human history. This resulted in
people working decades to pay for a house they are not
directly connected with (EVANS 2002).
This disconnection is also noticed by Alexander et al.
(1977), in the book A pattern language. The authors
defend that modern types of property such as renting,
when the dweller is not the legally owner, are opposite to
natural processes of formation of stable communities and
do not allow people to feel trully comfortble since the
house does not belong to them. Enphasyzing the
definition of ownership controll — instead of ownership
as financial investiment —, the authors believe that

SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
people would only feel comfortable in their houses if
they could adapt them according to their needs, and such
investiments could only happen if they were the legally
owners of the building (ALEXANDER et al. 1977).
In Brazil, as shows Bonduki (2011), the idea of the
small private land was largely spread between the 1930s
and the 1940s. However, that specially contributed to a
wide peripherical occupation in the cities by the poorer
population that moved to low cost land, away from urban
facilities, to self-build their substandard housing, once
they could not have access to the limited housing
programs offered by the government.
One can therefore see that self-building and mutirão
— popular word in Brazilian Portuguese for a collective
mobilization for mutual assistance with a free character
— to Brazilian population in general is associated with
lack of ressources, poverty and precariousness. But the
retake of these practices since the “back-to-the-land”
moviments from the 1960s introduces a new approach to
the problem: "The natural building movement has helped
humans reconnect with our tradition of self-reliant
shelter, surely one of our natural rights" (EVANS 2002,
p. 5). This way, people interested in living more
connected to the natural environment and to each other
have begun to appropriate traditional building techniques
and natural materials to build themselves their houses.
What one intends to demonstrate in this paper is the
good reception of mutirões — plural for mutirão — in
natural buildings that use earthen techniques for walls,
such as cob, to be explained later. To Minke (2002), cob-
building techniques does not need people experienced in
building nor demand complex tools and heavy
machinery. At the same time, they are more laborous to
work with and is recommended at least one experienced
person in the construction site to control the process and
teach the team.
3
Natural building and cob
3.1 Defining natural building
It is common to think about natural building (or
“bioconstruction”, neologism often used in Brazilian
context) as a building built with natural materials, that is,
non-industrialized (Figure 1). However, any row-
material to build a house, despite its roughness would
demand a certain level of processing. One understands,
in this kind of work, natural materials as “materials that,
even when processed, retain its natural essence”
(EVANS 2002, p.14). It means that a tree, even when
chopped into timber sheets, keeps its natural aspects and
proprieties. Industrialized timber, such as OBS or MDF,
drastically modify the proprieties of the original
material, turning it into a new one, that cannot be no
longer considered natural.
Still, as the consulted bibliography shows, the
concept of natural building is wider — to Evans (2002),
natural building goes beyond materials — implies in
completely different atitudes adressed to site plan,
ecology, work force, and use of the building. It is to pay
more attention to the natural structures that coordenate
the world and transport them to the work.
Figure 1. Stone, mud and wood are examples of natural
materials. Detail of a rounded cob wall corner of the studied
house. From the author.
To Weismann and Bryce (2006), more than building
with what, is to ask yourself how, where and why to
build. They highlight as natural building basic concepts:
a) an enphasis on the minimisation of the environmental
impact of materials, techniques and the building itself; b)
simple low-tech approach; c) use of local, renewable and
available resources; d) a respect with the building site
and its local environment as an unique place; e)
encouragement to self-construction; f) priority to
materials that have not been industrially processed, such
as stone, mud, straw and wood. Also here the materials
are not only important, but also a single component in a
whole wider context.
In this paper, thus, one defines natural building as a
practice that aims to employ only the minimum
necessary of industialized processes and materials and is
characterized, formal and technicaly, by the presence of
natural materials and low technology building
techniques.
3.2 Cob walls made out of mud
Minke (2002) points out three disadvantages that mud
has in comparison to the most common industrialized
materials: a) it is not a standardized material, it can vary
its characteristics from place to place; b) it contracts
when drying, and may present cracks; c) it is not
impermeable, and should always be protected from the
direct action of rain. Nevertheless, this natural material
has several advantages when compared to industrialized
materials: it regulates the humidity of the environment,
stores heat, is produced with low energy expenditure, it
is reusable, it is economical, suitable for self-
construction, it preserves organic materials when in
direct contact (like when it is covering wood), among
other benefits.
Even the mentioned disadvantages do not discredit
the use of the material. The variation of soil
characteristics from different places can be compensated
with the addition of more sandy or clayey soil, followed
by simple tests that guarantee the reliability of the

SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
adopted trait; cracks that may appear, feared to be able to
house insects like the “barber”, causing the Chagas
disease, are easily eliminated by towing the wall; and the
same plaster, added to generous eaves, will guarantee
protection against the direct incidence of the rain
(MINKE 2002).
Thus, one of the most recurring materials in natural
construction is earth, or more specifically, mud. It is
possible to make walls, niches, benches and other
architectural elements out of mud, according to the
chosen technique, that can be adobe, rammed earth pau-
a-pique, cob, among others (VAN LENGEN 2009;
MINKE 2002). In the house studied in this work the
main technique used for the walls was cob.
Cob is an English term for a building technique
building with mud that does not need shapes, bricks or
wooden structure. The vernacular tradition of the English
cob house dates from the thirteenth century to the
industrial era. Nevertheless, the same technique or
similar variations can be found in practically all the
continents (EVANS 2002).
The constitution of the cob is based on four
materials: clay soil, aggregate (sand), fresh straw and
water. Sand and clay should be mixed in the ratio of 3:1.
Therefore, sand is the most abundant ingredient, and the
final trait should result in a homogeneous mass balanced,
that does not shed (excess sand) and is not sticky (excess
clay) (LENGEN 2008).
In cob, mud is seated with barehands, without need
of forms, complementary structures or mortar (Figure 2).
The walls of the house are raised in layers (rows) of
approximately 30 cm at a time. When completing the
first row, you can start a new one, saving at least one day
for the lower row to dry. Thus, the walls of the house are
raised and dried as a single whole, working as a
monolithic structure (EVANS 2002; WEISMANN and
BRYCE 2006).
Figure 2. Cob walls are thick mud walls sculpted with
barehands, without any need of surplus structure. From the
author.
The cob wall is like a common brick masonry in
small scale: the particles of sand are like bricks; clay, in
contact with water, has its binder properties activated
and becomes the mortar of settlement; finally, straw is
added as a fibrous material to help the sand in the
function of stabilizing the clay, preventing eventual
cracks when drying (MINKE 2002).
Compared with conventional bricks, cob has much
less embodyed energy in its production, since it uses raw
clay, and is not burned in wood-fired ovens. Compared
to other natural construction techniques, it has the
advantage of no need of shapes (such as in adobe or
rammed-earth) and no need of structure for the mud to
grasp to (as the bamboos in Brazilian pau-a-pique). On
the other hand, cob walls require a great thickness to
stabilize (around 35 cm minimum), which can be
considered a problem due to the volume of material
used, but could also be advantageous considering that
the wall will have a larger termal mass (it will store the
heat for longer time, differentiating outside and inside
temperatures).
The two main precautions when working with cob
are the direct action of rain and the use of portland
cement in the plaster. Earth is not waterproof and can
lose a lot of resistance when wet again. It is advisable to
raise the walls of the ground with a stone foundation and
to design roofs with generous eaves (between 45 cm and
60 cm). In addition, to ensure greater safety, it is
recommended to protect the external walls with lime-
based plaster. Lime is a porous material and allows the
wall to breathe and balance its moisture with ambient air,
besides its good water resistance properties
(WEISMANN and BRYCE 2006). Cement should not be
used when towing mud walls because, although it is
more impermeable than lime, it does not have the
porosity that clay requires for the wall to "breathe".
Cement and clay do not work well together, and with the
appearance of the first cracks, water can enter, can not
escape from the wall by evaporation, and moisture will
accumulate at the base of the wall, where are the largest
loads, and may cause collapse in the structure (EVANS
2002).
4 Case study — our cob cottage
4.1 Project
As object of study, the author presents the cob house he
himself made to live, as soon as he graduated in
Architecture and Urban Design. The design was made in
the first half of 2014 and the work started in August of
that year, going up to August 2015. The site is a small
rural property in the countryside of Pelotas, Rio Grande
do Sul, southernmost Brazil. An extremely narrow site
(ranging from 12 m to 50 m wide by approximately 320
m long), with a hectare of area, not cultivated for more
than two decades, taken by a young bush, with a total
slope of 18 m towards the bottom, where a stream runs.
The main conditioning that defined the implantation
of the house were ground unevenness, solar trajectory
and visual interest. It was chosen an implantation to the
center of the lot, which would guarantee both privacy
and proximity to the stream and beautiful visuals of the
rural landscape, as well. The gaps up to 1.5 m between

SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
the ends of the building were minimized by level
differences between rooms.
The house has built area of 82.56 sqm — 52.80 sqm
of internal area, 6.50 sqm of porch and 22.26 sqm of
wall area, which represents 27% of the built area. That
happens because the cob walls were raised 42 cm thick
(37 cm of mud + 2.5 cm of lime plaster on each side),
consuming a mud volume estimated at 27 m³ (almost
equivalent to four loaded concrete mixer trucks).
The house has a foundation of irregular granite
stones, built both to support the walls and raise them
from the ground. At the ends and intersections of the
foundations there are round eucalyptus pillars attached,
put to structure the roof and allow it to be started before
the walls were finished. In the second floor — the
mezzanine — the walls are made out of wood, to make
the cons walls, to facilitate the execution (raising the
mud would require extra work). Both floors received
green roofs, consisting of eucalyptus board base
waterproofed with vinyl truck canvas.
4.2 Team work and the mutirão sessions
According to Minke (2002), a natural construction is
much more laborious, given to its artisanal character.
Thus, it was planned the strategy of mutirão sessions on
Saturdays, to involve as many people as possible and
make better use of time and tasks. However, at the
begining of the foundations, one realized that not all
steps would be suitable for inexperienced volunteers,
either for being physically heavy tasks, either for
requiring some specific technical knowledge.
The execution of the house was taken by a three-to-
five men team working five days a week full time — the
regular staff — with eventual additional construction
professionals assistance — carpenters, plumbers,
electritians and masons. The author, owner and architect
of the house — within no practice in natural construction
yet, only theoretical studies —worked as a builder and
coordinated a team of young people with little or no
experience in conventional building, much less in natural
building. In Table 1, it is possible to see the different
steps of the work and the corresponding type of
workmanship, professional or voluntary.
Table 1. Steps of the work.
Steps Professional
Workforce Mutirão
1 Foundations No No
2 Timber structure Yes (carpenters) No
3 Walls (cob) No Yes
4 Roof No Yes
5 Plasters Yes (mason) No
6
Electric and
hydrosanitary
installations
Yes (plumber
and electrician) No
7 Floors and finishes Yes (mason) No
The foundations did not require professionals
because they were technically uncomplicated, though
very physically draining to perform. In the next step,
woodwork, it was contracted a team of professional
carpenters. Only in the begining of the cob walls, at the
end of the fourth month, the mutirão strategy could
happen (Figure 3).
Figure 3. Volunteers raising a cob wall in the second mutirão.
From the author.
The mutirão sessions took a workshop shape. There
were invitations published in social media, with the
dates, times and schedule: leave the city at 5 am; work
from 6 am to 1 pm, with two breaks for snacks; (offered
by the organization, but prepared by one volunteer) and
bath in the stream in the afternoon. There were also rides
for those unable to travel to the site of the work by
themselves, as the house is 37 km from downtown
Pelotas.
Figure 4. At the thirteenth mutirão, cob walls were almost
finished. From the author.
The mutirões followed simple dynamic: the author,
who worked as builder with the regular staff during the
week, would assume the task of facilitating, helping
people to build. In addition to questions about the work,
many conversations arose about the reasons for choosing
this type of construction, advantages and disadvantages,
the option to live in the countryside, etc. Eventually
there were also multidisciplinary exchanges among the
volunteers, many of them related to building and
construction area, research and academia.
As for the tasks performed in the mutirões, there
were five roles determined (Table 2): first, the
coordination, by the architect and owner, and the

SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
technical support, made by regurlar staff workers.
Volunteers took on some of the other tasks: most were
builders; those able to perform more demanding tasks
such as carrying weight were the helpers; and generally
elderly or phisically limited people were the cooks.
Table 2. Types of functions for the volunteers.
Roles Tasks Carried out by
Coordenation
and teaching
Determine tasks,
motivate the
team, clarify
questions
Owner-architect
Builders Build the cob
walls
More careful and
attentive people
Technical
support
Check
measurements,
level, plumb;
guide the work
Owner-architect,
regular staff
workers
Helpers Bring clay and
tools to builders;
assemble
scaffolding
People with better
fitness for heavy
activities
Cooks Prepare meals,
serve water
Helpful people, but
not able to build
At the wall step, the work done in the mutirões often
amounted three to four regular days of work. However,
many things were already prepared beforehand aiming
for greater productivity in the mutirão. Also more
precise tasks, such as fixing window frames on the walls
or anything that required more reflection and care, were
performed by the regular staff during the week.
Part of the green roof was also built by mutirão
(Figure 5). It consisted in several layers: wooden boards
base over the timber structure, cardboard, vinyl canvas
(for waterproofing), cardboard again and sand (2 cm).
Gravel placed along the edges work as drains, as well
holes in the baseboard at the ends, to allow the water to
flow. The volunteers helped to get pieces of topsoil with
native grass cover (7 cm to 10 cm thick) from the
neighboring field and raise them to the roof.
Figure 5. The last mutirão, for the green roof. The author and
his wife in first plan. From the author.
In all, from November 2014, when the walls began,
to April 2015, when the green roof was completed,
fourteen mutirões were held, gathering 85 different
volunteers. Many of them were friends, co-workers or
family members, but about 25 percent of the volunteers
were unknown people who found the invitation online
and decided to help and share experiences.
Figure 6. The cob-timber house, a few months after mooving
in. From the author.
Acknowledgments
Some conclusions emerge from the experience with
mutirão in natural building. Firstly, it is questioned the
"democratization" of the cob, defended by some authors
referred in this paper. The main demand for labor in such
buildings is to make the walls. However, other steps
essential to complete the house (see Table 1) are not
suitable for the help of inexperienced volunteers.
It is remarkable how easily volunteers at the building
site can begin to help raise a cob wall. Yet, it is
important to emphasize that the scope of construction is
much larger than just the walls. Those who think of
building their house with similar techniques should
anticipate the cost with specialized workmanship for the
other steps.
Besides the ease of learning, which allowed the
expressive number of attendees (85 people in 14
mutirões), it is believed that the natural building itself
and the curiosity it arouses were decisive in attracting
unknow vollunteers, and would hardly happen in a
conventional building — in the Brazilian context,
industrialized bricks and structure in reinforced concrete,
for example.
In addition to being in touch with a new technique,
many volunteers have shown interest in participating to
exchange ideas and information on issues beyond natural
building — there were many conversations about food,
education, lifestyle, and other topics related to integral
sustainability, traditional lifestyle and contact with
nature.
It is evident that the mutirão sessions required a lot
of work: planning, disclosure, motivation, logistics
(rides, meals), etc. Also during the process, there was
significant effort to instruct the team, to distribute the
tasks and to keep everybody motivated and comfortable.
However, such an effort was offset by the considerable
productivity and the resultant work environment as well,
more playful and lighther, without the pressure often
seen in building sites.
Finally, it is concluded that the making of cob walls
in natural buildings is an appropriate task for voluntary
help through mutirões, although it is a more laborious

SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
activity than some industrialized techniques. However, it
is indicated the hiring forecast of skilled labor to other of
the building that require technical experience. The
mutirão in natural building is a recomended strategy due
to its potential to attract people interested in the
different, for offering savings, optimize the work and for
collaborate with exchanges of experiences among the
participants, resulting in practical gains of social
sustainability.
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