Principles of Drystone Work
A drystone wall is built stone on stone, without use of mortar or concrete. Gravity and skill ensure that the wall stays where it’s intended. A well-crafted drystone retaining wall will outlast and have a smaller carbon footprint than a similarly sited mortared wall. In the steep terrain of western North Carolina, drystone retaining walls are an excellent solution for addressing erosion, creating flatter yards and gardens, and otherwise sculpting the earth.
In a residential setting, by code, a retaining wall is limited to 48 inches tall. Anything taller, must be signed and stamped by a licensed engineer. In general, engineers don’t love stone walls. They like things that are consistent and predictable and more easily quantified than the crazy ecology of a stacked wall. This is boring, but expected. The human elements- care, intention, and experience- that the builders put into their structures cannot be easily factored into mathematical formulae.
Drystone walls have fallen out of favor in preference for segmented block, prefabricated panels, and poured concrete structures. These entities are the same throughout their structure and therefore more easily analyzed. It’s an apt metaphor for the state of modern society, this devotion to conformity and standardization. When using prefabricated, predictable materials, it matters much less if the people building care at all about what they’re doing. And if you can subtract care from the process, you can subtract cost. It’s always cheaper if you don’t give a damn. Cheaper in the short term, at least.
What Makes it Stay Up?
I get a version of this question quite frequently, particularly when people discover my preference to avoid concrete and mortar in my work. The answer is answer is simple: gravity and skill. Gravity is generally viewed as the enemy of a stone wall- but only if the wall is poorly built. Gravity holds the whole thing together.
Care + practice = skill. It takes time and failure and more time to acquire the skill needed to make beautiful and strong walls. Understanding the basic rules, and caring enough to follow them is the starting point, and the continuing journey for a craftsperson.
The rules that govern the construction of a drystone wall are simple and robust:
Two over one and one over two
End them in
Maximum contact- minimum movement
Good hearting
Batter back
The top must sit over the bottom
Weather always wins
The principle of emergence states that simple rules lead to complex (and beautiful) results. Bird flocking is often cited as the exemplar of emergence. The strange and wonderful synchronicity demonstrated by a thousand starlings sweeping through the air, without a leader, without discussion, is built on simple rules: maintain the same distance from your neighbor, maintain same speed, avoid predators. Gorgeous and complex patterns emerge. A drystone wall is another stunning example of this principle in practice. Every wall will be different, even if created by the same builder using the same materials. These simple rules offer infinite possible outcomes. And a well-built wall is inherently beautiful.
These are the principles of drystone construction.
Two Over One: One Over Two
This is the very simple principle that you must cross your joints. Tall vertical lines in masonry construction, often called running joints, weaken the structure significantly. Crossing joints- imagine the traditional running bond of the most common brick wall- creates a network. That web of dynamic forces is fundamental to what holds a wall together. It is shocking how often builders will violate this basic principle. Many new houses are wall papered in stone, to give the illusion of strength and permanence. These veneered walls use mortar, whether visible or not- to adhere the stone to the house frame. The mortar becomes a crutch or an excuse; often this rule about crossing joints is completely ignored. To my mind, a well built wall follows the principles, whether or not mortar is used. Poorly built wall are always ugly.
End Them In
Stones should run deep into the wall. Sometimes, in an effort to conserve their rock pile, people will use the long side of a stone as the face. This is called tracing and it makes for a shallow structure and therefore a weak wall. If you have a stone that is shaped like a french fry, run it long into the wall.
Maximum Contact- Minimum Movement
The next stone you put in should make as much contact as possible with the stones that are already in the wall. And so on and so on.
One of the great attributes of a dry stone wall is its flexibility. It has the capacity to move and therefore respond to environmental factors. The day-to-day expansion and contraction that comes with being heated by sunlight and cooled by frost will cause minuscule movements in a wall but will not crack it apart. It is not supposed to be a rigid structure. Inflexibility fails in walls, as in life.
Minimal movement is a great attribute. On the other hand, stones that pivot from side to side on a single point cause trouble. Maximum contact, minimum movement is where practice becomes most important to a wall builder.
Good Hearting
A stone wall is more than its face. There needs to be structure behind the stones you see. This is called ‘hearting.’ Some traditions demand that this is as carefully considered as the face stones. Others, like myself, take a more casual approach. Hearting should be stone or gravel and well consolidated. It shouldn’t have the ability to move or settle much over time. Soil or organic matter should be avoided at all costs. I use stone chips and gravel extensively to promote drainage behind my walls. It is important that the gravel not get between the stones, but only rest behind. Bits of gravel can act like ball bearings, making it hard to get maximum contact- minimum movement. I like to place a filter fabric behind my walls, separating the cut slope from the hearting. This prevents the backfill from silting up, and in the case of red clay, staining the wall.
Batter Back
A good dry stone wall has a slight backwards slope to its face, called the batter. Imagine the wall leaning into the hillside which it is intended to retain. This strengthens the wall and gives it leverage against the slope behind it. If your wall is a free-standing structure, then both sides batter back, towards the center. Every stone worker has their own ratio for the batter of a wall. I like 1/2” of slope per foot of wall height. Some traditions are steeper than that.
The Top Must Sit Over The Bottom
Lean back, but not too far. The top of the wall, where your capstones rest, must be sitting squarely over the bottom of the wall. It is a common beginner’s mistake to lean the wall back too steeply. A very steep batter on the face will have the top of the wall sitting over soft soil backfill instead of the hearty structure of the wall. The weight of the cap will compress the soil. The path of least resistance for that soil will be forward; it will blow out the bottom of the wall, collapsing the whole structure. This is where the concept of gravity really comes into play with the construction of the wall. With practice and care, gravity is a great ally.
Weather Always Wins
Fundamentally, all construction is an attempt to resist the forces of weather. And while weather is not a singular force, in my experience, water is the prime mover. Almost all of the structures we build live their lives in the weather. We have to constantly be conscious of what happens when it rains. Where does the water go? Where does it collect? What happens when it freezes? What happens when it builds up or when it moves the earth?
I think one of the most beautiful things about stonework is the endless potential. It suits me as a lifelong pursuit because there’s no end to what I can learn, tweak, try, mess up, try again, change, misapply, realize, and make. If I get bored, it’s my fault. That means it’s time to explore.
At the heart of this craft, there are a few simple rules. Understanding them, as principles and as thoughtful actions, opens up a whole world of creative possibility.