Dry Stone Walls – Principles of structurally sound construction

Words: Dry stonewalls, or stonewalls built without the use of mortar, have been around for millennia. There are examples of dry stonewalls and structures that date back 5000 years and are still standing today. Nearly anywhere in the world where stone of suitable sizes was plentiful, walls were built. Dry stonework’s durability and strength is wholly dependent on how it is stacked. Dry stonewalls are an ideal form of building whenever you have a ready supply of stone. They are long lasting (100 – 200 years is common) when compared to other forms of fencing, and often outlast mortared masonry construction. When a dry stonewall does finally fail, it can be rebuilt using the same stone. Dry stonewalls are usually built with local stone, traditionally right from the ground nearby. By building without mortar, you are simplifying the needed tools, materials and supplies. Traditionally dry stonewalls were constructed with undressed stone, and the varying styles reflect the best use of the stone available.

Fundamentally, a dry stonewall is a structure that can flex and move. As the ground settles or heaves a well built dry stonewall will simply move with the ground. Because of this, a dry stonewall typically needs minimal foundation work as compared to mortared work. A dry stonewall also allows water to pass through it with ease, whereas a mortared wall will trap moisture, often leading to failure when the water freezes. In many cases, a well built dry stone wall will last longer than a mortared wall. In many cases building without mortar is also historically accurate. Traditionally in many areas dry stonewall building was a separate trade from masonry and those in the profession were referred to as ‘wallers’. For most of the 20th century dry stonewalling was in decline, but in the last several decades it has been seeing a strong resurgence as historic preservation, longevity, aesthetics, and a reducing C02 emissions have become driving factors in many projects. A dry stonewall’s strength depends on how it is stacked, and to a lesser degree the stone used, it is difficult to have a engineered strength assigned to it. However there is tremendous precedence of dry stonewalls throughout much of the US and the world. When combined with plans and specifications detailing correct stacking, this can often be used to overcome strength concerns. Becoming proficient in dry stone walling takes practice and training, and mastery often takes years if not decades to achieve. However the concepts are not complex and with only a few days of training many people can build reasonably structurally sound walls.

The Stone Trust, a non-profit organization, was founded in 2010 with a mission to preserve and advance the art and craft of dry stonewalling. It was created by a group of dedicated dry stone wallers concerned that the lack of knowledge about dry stone construction was leading to poor quality work that frequently failed. The Stone Trust has quickly grown to become instrumental in providing training, education, certification, and resources to stone wall builders and designers throughout North America. Since 2010, over 1400 people have attended workshops or gained certification. Hands on, practical training is the backbone of The Stone Trust’s programing. With over 40 workshops in 2018, ranging from 1 day to 5 days in length. It offers everything from introductory workshops focused on the basic principles to advanced specialized courses in shaping stones, building arches, steps, and other features. This training is then complemented by dry stone certification from the Dry Stone Walling Association of Great Britain (DSWA-GB). The DSWA-GB is the oldest and most widely recognized certification program in dry stonewalling. Becoming certified is the best way to quickly improve one’s skills and credentials. Increasingly projects are specifying requirements for certified wallers, and employers are looking for the credential. DSWA-GB certification has 4 levels: Initial, Intermediate, Advanced, and Master Craftsmen. Each test is a practical hands-on rebuild of a section of wall in a 7-hour time limit. The quality of passing work, volume of wall, and complexity of features, all increase with each level. Structural integrity is stressed and working efficiently is critical, as the needed rate of build is high. Information is available at www.thestonetrust.org Basic Wall Design: Traditional dry stone walls were most commonly built as livestock fences. With a hight range of 3 to 6 ft. Walls lower than 3 ft high often look unfinished and tend to not have enough face area for the unevenness of the individual stones to visually blend into an smooth and even wall face. All dry stonewalls should have a batter (the top is narrower than the bottom). This adds to the stability, strength, efficiency of building and use of stone. Batter is described as a ratio of run to rise. A 1:6 batter means that for every 6” of height the wall gets narrower 1” on each side. The typical range for batter is from 1:6 to 1:10.

If you are using flatter stones a steeper batter like 1:10 may be appropriate. If you are using more irregular stone or if you have a wide variety of sizes, than a more sloping batter of 1:6 is generally appropriate. The top width of a wall is typically 14” to 18” wide. Narrower walls make more efficient use of stone, while wider walls make it easier to use larger stone can tend to be a bit sturdier. A base width of 28” to 34” is fairly typical. Sourcing stone: Palletized stone can work great, but tends to be quite expensive, and is often sorted by size/selection for masonry applications or veneer. To have all the sizes needed for dry stone wall construction often multiple grades or sizes will need to be purchased. Waste piles from stone quarries or construction sites, crushed stone quarries producing an 18” minus size, and gravel pit tailings can all be good sources of stone. With these sources, there can be a lot of unusable stone, but it is often available for as little as $20/ton. Even with the land owner’s permission, it is generally frowned upon to take down old stone walls or structures, and in many areas it is illegal. It is possible to build a wall out of nearly any type of stone, but not any look or style can be achieved with any stone. What is ‘Good stone’ may be defined by the astatic desired. Flat stone is not necessarily good, and is often slower to build with than blocky or rounded stone. In general look for stones that tend to have 1 longer dimension (cubes and spheres are hard to build with, but rectangles and cylinders work well). If a more refined look is desired a smaller flatter stone will achieve that best. Tools: While professional wallers can use quite a range of tools, it does not take much to do a good job. Here are the basics:

- Mason string (the braided 216 lb test line is best, avoid the light weight twisted line)

Batter frames to hold the string are key for building efficiently. These can be simply  made from wood and/or 5/8” rebar.

- Shovel for preparing the foundation

- Tape measure

- 2’ level

- 6’ pry bar for moving big stones

Brick or mason hammer (about 22 oz)

- 2 to 4 lb hammers for stone trimming

6 to 10 lb sledgehammer for breaking up large stone

If you get into shaping stones, a selection of specialized stone shaping hammers and chisels can greatly improve efficiency and accuracy. The Stone Trust’s website offers a great resource with tools for sale and descriptions of the best uses. The Stone Trust also offers a wide array of demo tools to use at workshops.  Preparation  The goal of foundation preparation is to minimize settling and maximize drainage. Foundation preparation is usually quite simple: remove the turf or loose topsoil and compact. If the soil seems particularly prone to settling, holding water, or you’re looking for an added bit of assurance that the wall will last, putting in a foundation of clean crushed stone is a good solution. Depending on the site and situation a 6” to 18” depth of crushed stone about a foot wider than the wall is the typical protocol. 3⁄4” to 2 1⁄2” crushed stone without fines, is the typical size range that is suitable. 3⁄4” has the advantage of being easier to shovel, while larger sizes tend to be more stable. Use sharp angular crushed stone, not rounded gravel stone. If you are installing a crushed stone foundation in the water needs a way to get out, otherwise you just made a pond under your wall. A 4” perforated pipe that flows to daylight is the most common solution. When compacting the foundation, a jumping jack type compactor is the most effective. Driving back and forth a few times with a loaded pick up truck or piece of equipment is also fairly effective. A walk behind vibratory plate compactor is not effective. Overall there tends to be an excess of attention put on the foundation, often as a way to make up for poor dry stone wall building practices. A well built free standing wall is usually okay sitting right on the native soil. Material Management: Sorting your stone and laying it out before you begin to build will greatly increase your speed, and allow each stone is used to its maximum benefit. When building you will want an equal amount of stone on each side of the wall. Starting at the wall foundation leave 18” of clear walking space on each side. Then line up the thickest stones you have nearest the wall, with stones of decreasing thickness placed progressively farther from the wall. At a minimum sort the stones into rows of big, medium, and small. Special pieces that have the size and shape needed for through stones, cover stones, cope stones, etc., should be set farthest from wall, and be clearly distinguishable, so they wont accidently be built into the wall at the wrong time. Hearting should be placed in piles near the wall foundation every 6 ft. or so, which makes it easy to reach at all times. 5 gallon buckets are also great for moving hearting around. A skilled waller can sort through a 20 ton dump truck load in about a day. When building anytime you find yourself thinking about tracing a stone, or breaking it smaller, ask yourself if it really belongs somewhere else in the wall. Most often it does, and this will increase the strength of the wall, speed up your build, and reduce your wasted material.

The 5 basic rules of walling (inset): A dry stonewall is fundamentally held up by friction and gravity. The friction between the stones keeps them from sliding apart and the weight of the stones increases the friction. When building a wall, the aim is always to maximize the friction and use gravity to the best advantage.  This can be distilled down to 5 basic rules:  

Set all the stones so their length goes into the wall, not along it. Just like when stacking firewood, where only the ends of the pieces of wood are visible in the finished stack. By placing the stones length in you are maximizing the friction between stones, and also getting the center of mass of each stone closer to the core of the wall. This is the rule most commonly broken and one of the primary reasons walls fail. Setting stones in the wall so the long edge shows in the face is called Tracing.

Heart tightly. Hearting is absolutely key to building a strong wall. It adds many points of contact between stones, increasing friction, and keeps stones from moving independently of the entire wall. However – anything you can readily shovel is too small  to be used for hearting, and will act like ball bearings in the wall. Using the fewest, biggest pieces to fill the voids is important.

Cross the joints. Just like standard brick work, each stone should span the joint in the  course below and sit firmly on the two stones either side of that joint. Vertical joints  running up the wall through multiple courses are called Running Joints.

Build with the plane of the wall. Set the wall stones so that their faces line up with the  outside face of the wall, creating a smooth even plane.

Set stones level. Each stone set on the wall needs to be able to support stones on top of  it. The simplest way to achieve that is to set each stone so the top (or in some instances the bottom) is level. While there are certain types of construction that do not follow this rule, there always needs to be thought about planning ahead to the stones on the next course.

Conclusion Dry stone walling is a craft unto itself. The methods of design building have been developed over hundreds of years to maximize longevity and efficient building. Dry stone walling has a long history and continues to be suitable for many projects today. Today's projects tend to be more about aesthetics than function, so it is important to remember not to sacrifice structure to achieve a particular look. Nothing looks worse than a wall that has fallen down, and a strong wall will look good for generations to come. About the author: Brian Post is a DSWA-GB Certified Dry Stone Wall Master Craftsmen (one of 8 in the USA). He is also a licensed Landscape Architect. Brian is the Executive Director of The Stone Trust, a DSWA-GB certified dry stone walling instructor and examiner. He is also the proprietor of Standing Stone Landscape Architecture, a design build-firm specializing in complex dry stone projects.

Dry Stone Walls – Principles of structurally sound construction

Words: Brian Post, Executive Director, The Stone Trust Photos: The Stone Trust

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