Mud brick

Mud and other earth-based materials are among the oldest building materials on earth. The oldest surviving examples in ancient Mesopotamia and Turkey, are many thousands of years old. Mud brick construction is often referred to as ‘adobe’ which is an Arabic and Berber word brought by Spaniards to the Americas, where it was adopted into English.

Basic mud bricks are made by mixing earth with water, placing the mixture into moulds and drying the bricks in the open air. Moulds can be made from timber or metal — anything that can be shaped to provide the desired size for the bricks. Straw or other fibres that are strong in tension are often added to the bricks to help reduce cracking. Mud bricks are joined with a mud mortar and can be used to build walls, vaults and domes.

If used in a good passive design and in conjunction with other appropriate materials, mud bricks come close to the ideal of minimal environmental impact. The material is ‘borrowed’ from the environment and can be replaced after use. There is little or no processing of the raw material and all the energy inputs are directly, or indirectly, from the sun. Mud bricks are cheap and perform well thermally and acoustically. The most effective use of mud bricks in building healthy, environmentally responsible houses comes from understanding their merits and limitations.

Photo: Nillumbik Mudbrick Association

Buildability, availability and cost

Materials for making mud bricks are readily available in most areas, and in some cases may be sourced directly from the building site.

Mud bricks are a forgiving medium well suited to owner–builder construction as well as experienced building professionals. Several proprietary mud brick makers, builders and other groups in Australia provide information about making and building with mud bricks. The Earth Building Association of Australia (EBAA), is a not-for-profit group formed to promote the use of unfired earth as a building medium throughout Australia. The Nillumbick Mudbrick Association has design and construction guidelines to assist in achieving a 6 star energy rating (see References and additional reading).

Low costs in construction can only be effectively achieved by self-build, reducing the labour costs associated with the manufacture and laying of bricks. Commercially produced mud brick construction may be the same cost as brick veneer.

Appearance

The appearance of mud bricks reflects the materials they are made from. They are earthy, with their colour determined by the colour of clays and sands in the mix. Finished walls can range from a strong expression of the brick patterns to a smoothly continuous surface.

Photo: Nillumbik Mudbrick Association

Structural capability

With thick enough walls, mud brick can create load-bearing structures up to several storeys high. Vaults and domes in mud brick prove that it can be used for many applications other than vertical walls. It may be used as infill in a timber frame building or for load-bearing walls. Although its compressive strength is low compared to fired clay bricks or concrete, it is more than adequate for the task if designed properly and assessed by a suitably qualified person. Typically, Australian mud brick structures are single or double storey.

Durability and moisture resistance

Mud brick walls can provide structural support, but they need protection from extreme weather (for example, with deep eaves) or continuous maintenance: the 8-storey ancient mudbrick structures of Yemen have been repaired continuously for centuries. Although some soils are very resistant to weathering, mud brick generally needs protection from driving rain and should not be exposed to continuously damp conditions.

Thermal mass and insulation

Mud brick walls can provide moderate to high thermal mass. For most Australian climatic conditions, the common practice is for walls to be a minimum of 240mm thick, for both post and beam and load-bearing construction. This provides effective thermal mass, with extra insulation as required in seasonally cool or hot climates.

To achieve the levels of insulation needed for sustainable house construction and to achieve National Construction Code compliance across most of Australia, it is often necessary to add insulation to mud brick constructions. This can be achieved by constructing one of the external walls in mud brick veneer or insulating a stud-framed wall on the inside, thus creating an internal timber feature wall. You may also externally insulate the mud brick wall so the thermal mass of mudbrick is exposed to the interior. In some milder climate zones, where thermal insulation is less critical to the overall building performance, mud brick walls may not need additional insulation.

One way of dealing with mud brick’s limited insulation properties is to construct some or all of the outer walls with framed construction, and use mud brick for partition walls or as an internal ‘reverse brick veneer’ on some external walls. This approach also allows the building to reach ‘lock-up’ stage very quickly and provides a protected space to make and dry the bricks.

Traditional earth buildings often used walls up to a metre thick: these would provide reasonable insulation and enormous mass to stabilise internal temperatures. This is not usually cost effective or the most efficient use of space on all but the largest sites. The more common 240mm thick walls are more practical for most urban sites.

Sound insulation

A well-built mud brick wall has very good sound insulation properties. It can be equivalent to a monolithic masonry structure (heavy mass walls with no cavities) in its capacity for sound attenuation. Some modern mud brick homes use mud brick for external walls and light partition walls internally. However, it is more effective for thermal and acoustic performance to use mud brick for the partition walls and lightweight, well-insulated external walls.

Fire and pest resistance

Since earth does not burn, and earth walls do not readily provide habitat for pests, mud brick walls have excellent fire and pest resistance. Note that a bushfire attack level (BAL) rating may still be required if located in a bushfire-prone area. Your local council can assist with this. Consideration may also need to be given for termite control in the design and construction.

Toxicity and breathability

Mud bricks make ‘breathable’ or vapour-permeable walls, but some mud brick recipes include bitumen, which potentially results in some outgassing of hydrocarbons. Ideally, earth should be used in, or as near as possible to, its natural state.

Environmental impacts

Mud bricks could have the lowest impact of all construction materials. They require very little generated energy to manufacture; however, they require large amounts of water. Their embodied energy content is potentially the lowest of all building materials but the use of additives such as cement, excessive transport and other mechanical energy use can increase the embodied energy.

The greenhouse gas emissions associated with unfired mud bricks can be very low. However, even adding a small amount of cement to a thick mud brick wall can change the embodied energy from extremely low to high.

Photo: Nillumbik Mudbrick Association

Cement-stabilised soil bricks

Cement can be added in small quantities to soil that would otherwise not be strong enough or stable enough to use in construction. Base soil types that require cement for stabilisation include sandy or loamy soils, and soils with too much clay content. Cement also hastens the drying and curing time, making these bricks more appropriate for commercial off-site manufacturing, and especially those buildings on a tight timeframe (manufacturing and air-drying pure mud bricks for a whole house may take a year).

Cement also has the advantage of chemically stabilising the earth material against weathering and water penetration, which could potentially make it useful in highly exposed environments. However, it should be noted that due to its relatively poor insulating properties, external insulation of cement stabilised soil bricks will be required in most climates. In this case, its protective properties would be of no benefit.

All other construction considerations remain the same for cement-stabilised bricks as for other earth building materials.

Design and detailing

Load-bearing walls

Load-bearing mud brick wall construction requires particular attention to good bonding (avoiding continuous vertical joints) and ensuring stability by having returns on the walls that buttress them against sideways forces. Normal traditional masonry practice applies to the pattern in which bricks should be laid. It is possible to create architectural features such as arches, squinches and domes in mud brick; they feature in older adobe structures but are rare in modern buildings of this type.

Openings

Lintels can be in any structurally appropriate material, although timber is typically used. Beams and lintels can be formed from quite ‘rough and ready’ timber and readily blended into the mud brick construction. Mud bricks can also be laid to form arches, particularly over small spans (less than a metre), and even domes, although this requires high levels of bricklaying skills as well as more stringent demands from engineering and approvals processes.

Finishes

Finishes can range from rustic to smooth, with this flexibility of approach being one of the material’s many appealing qualities. After brushing to get a fairly even surface, the final finish is usually a mud slurry, typically completed by hand. This slurry may also be the final waterproofing coat or it may have a further clear coat of proprietary waterproofing material.

Linseed oil and turpentine can also be used as a final finish, and can be a very effective method of protecting walls susceptible to erosion. There is even the option of using the natural plastic of cellulose processed by cattle (that is, cow manure), to create a mud and manure slurry with varying degrees of fibre (although this is seldom used in Australia).

Photo: Paul Downton

Cob, wattle and daub and earth render

Cob is an ancient earth building technique of mixing earth, sand, gravel or pebbles and straw with a little water. It requires minimal construction skills and may be the world’s most common building material.

Cob walls are built without formwork by placing lumps of the cob mix by hand to make massive walls, typically 450–600mm thick, built up in layers. Each layer needs to dry out sufficiently to support the next. It lends itself to making free-flowing sculptural forms.

This high thermal mass material has some insulation value. Cob building depends upon wall thickness for its structural and environmental performance. Cob is fireproof and can be used to make stoves, fireplaces and chimneys.

Cob walls require firm footings to avoid movement and cracking and to keep the base of the walls dry. They need to be whitewashed (lime and water) for weather protection.

Wattle and daub consists of panels of woven timber lattice set within timber frames. The ‘wattle’ lattice was traditionally made from wood coppiced from trees that would continually regenerate branches for future harvesting. The panels of woven timber are daubed with a mud and straw mixture up to a thickness of 50–150mm. As with all earth constructions, wattle and daub walls need sealing against the weather with a breathable finish such as whitewash.

Earth render is fundamentally a mud or clay slurry that may be reinforced with straw or cow manure. It can be applied to mud brick or cob walls and can also be used to render straw bale walls.

These very old techniques that date back to the earliest days of building are uncommon in the modern era. If you are contemplating using these methods, it is worth checking out the growing number of earth-building websites and networks through which people exchange information and experience.

The build process

Construction process

Mud brick wall construction has generally been the province of owner–builders. A large proportion of mud brick buildings are now constructed by, or with the help of, experienced commercial builders. The potential for sourcing the main wall construction material from one’s own site, making the bricks, and building the walls can be very appealing as both an economic and lifestyle choice. The first stage of construction may be excavating the mud from the site.

The clay content of mud brick can range between 30% and 70% and overall earth content may also include silt, gravel and stones. Mud brick should not contain any organic matter. There are tests available for suitability of the earth, and the approval process may require an erosion test. Before excavating for on-site mud, consider the site layout to minimise carrying and transport; ensure there is space to keep any topsoil separate for use on the garden.

Owner builders should recognise that mud brick making is a labour-intensive activity. A house may require around 10,000 bricks, and a working couple making bricks in their spare time may average a production rate of 200 per week. Mud brick moulds can be made from wood or metal. Bricks must dry evenly to avoid cracking and they should be covered to avoid direct sunlight and overly quick drying.

A typical standard mud brick is 300–375mm long, 240–250mm wide and 125mm high, and can weigh up to 18kg. Smaller brick sizes are recommended for owner building. Mud bricks can be made in a range of sizes and moulds and can be made in special shapes for fitting around structural elements and accommodating pipes and wires. After the footings have been placed and the bricks are ready for laying, the building process is similar to that of any other masonry construction.

Structural design can involve the walls, framing and the footings, and should be prepared by an experienced designer and requires checking by a qualified engineer. All masonry construction has to comply with the National Construction Code, and Australian Standards. For example, all masonry walls are required to have movement/expansion joints at specified intervals. The EBAA and Nillumbik Mudbrick Association have a wealth of information and support available for owner–builders and professionals.

Photo: Nillumbik Mudbrick Association

Footings

A raft concrete slab can make a clean, flat surface for making mud bricks. The usual method is to employ strip or raft concrete footings. A damp-proof course must be laid between the footings and brick wall to prevent rising damp. A ‘splash course’ of fired bricks may be advisable to prevent erosion of the lower course of mud bricks from heavy rain, unless the wall is protected by verandas or deep overhangs. It is possible to make footings from rubble, but unconventional construction may make it harder to obtain building approvals.

Frames

Although mud brick can be load bearing, it can also be built as ‘infill’ within a timber-framed wall, usually with post and beam construction. The advantages are that a roof structure can be erected for weather protection for both mud brick making and construction. Disadvantages include the need to connect with and build around frame structures. Detailing should allow for shrinkage between frame and mud bricks.

Joints and connections

Mud bricks are laid on thick mortar beds that are essentially the same mix as the brick, but in its ‘muddy’ state, such that the mortar joints are visually consistent with the bricks. Once dried, it can be difficult to distinguish between mortar bed and brick, and some builders exploit this ‘seamless’ appearance to create a monolithic effect. It is common practice in the commercial mud brick industry to lay mud bricks in a sand-cement mortar, but this will give a different visual character where the mortar joints are distinct from the bricks.

In framed mud brick construction, there may be timber or steel columns around which the mud brick walls are built. Considerable roof overhang is generally recommended to afford some protection to walls from driving rain. This requirement is less vital in more sheltered areas, but care must be taken to provide a good quality render and waterproofing finish (see Finishes in this chapter). In load-bearing mud brick construction, roof structures bear on wall plates.

Walls are laid in the traditional manner of masonry with string lines to provide a guide to vertical and horizontal alignments. The mud mortar beds are normally quite thick and need to provide complete bedding for the bricks. Perpends (vertical joints between bricks) are similarly thick (about 20–30mm). The intention is to produce a wall that is effectively monolithic – that is, as if it were a single piece of material.

Photo: Nillumbik Mudbrick Association

Fixings

Fixings into mud brick, for everything from roof structure to door frames and picture frames, need to allow for the relatively poor ‘pull-out’ strength of the material. Roof frame fixings will likely be designed and detailed by the project engineer. Strong fixings can be achieved by embedding dowels or plugs into a wall, the depth and type of which should be determined by a skilled builder or engineer if the load-carrying capacity of the fixing is critical.

Learn more

• Explore Thermal mass to see how to use the thermal mass of mud bricks most effectively.
• Visit Designing your home to see the steps to take during the design stages.
• Read Insulation to find out how to combine insulation and mud brick.

Authors

Original author: Paul Downton 2013

Updated: Dick Clarke 2020