HOW TO: `Rock-it' science for the beginner
Building a stone wall may not be rocket science, but it can definitely be a challenge. The following guidelines will help you build a beautiful and functional wall that will stand the test of time.
From the pyramids of Egypt to the meandering stone walls of New England, stone is the most basic of building materials and has an element of permanence unlike any other. The use of stone has increased in popularity in recent years - with both residential and commercial properties using stone walls to add a unique feature to their landscape.
Types of walls Stone walls can be categorized by their function, their method of construction or by the type of stone used to build them. Stone walls are either freestanding or retaining. They are mortared or "dry-laid" (mortarless) and can be built with any type of stone.
Freestanding walls are "finished" on three sides: the top and both sides. Often, they assume the role of a fence in the landscape, delineating space and controlling traffic flow. Retaining walls are finished on two sides: the top and one side - the front of the wall. They function to retain material on a slope, in an elevated patio or in a raised planter.
In regions where the ground freezes, walls held together with mortar require footings that extend below the frost line to prevent movement. This can be costly. But, since colonial times, people have built dry-laid (mortarless) walls that flex with the expansion and contraction of freezing and thawing soils. These walls depend on gravity and the friction of stone upon stone to hold them in place.
You can use any type of stone to build walls. In our area of upstate New York, limestone is readily available. Bluestone and granite are also frequently used. You can either use stone that is collected (fieldstone) or quarried. Become familiar with local materials and suppliers to get a feel for your options.
The dry-construction process Stone-wall construction is learned by doing - like most things, you get better and faster at it with time. The first thing you learn when constructing stone walls is that, in addition to a big pile of rocks, you must also have a big pile of patience. Building a rock wall is a lot like trying to complete a puzzle for which there is no single end result. Some of the pieces never quite seem to match up the right way and others are missing and must be chiseled.
We teach dry-laid stone-wall construction to students in our Landscape Construction class at Cobleskill College. A few years ago, we built a 40-ton bluestone wall. The students went to the quarry to hand pick the stone. It was the best thing we could have done because they couldn't "blame the stone." Dry-wall construction requires a good eye and trial and error to find the right piece for the right fit. In addition to experience and patience, there are some guidelines for stonewall construction that, if you follow them, will minimize frustration and make the process more efficient and enjoyable.
The 3 Bs: base, batter, backfill When constructing stone walls, you should start with the 3 Bs. They are basic rules that you must follow to successfully construct stone walls.
q Base. One of the greatest benefits of dry-wall construction is that you don't need a footing. Dry walls are constructed to "float" with the freezing and thawing soil. The primary purpose of the base for a dry-laid stone wall is to not restrict movement of the wall. Construct the base to physically attach the wall to the soil so it doesn't sit on top of the ground. Proper base preparation also helps provide adequate drainage and makes it easier to place the first course of stone.
Construct walls with a minimum base of 6 inches (larger walls may require a base of 12 inches or more). Excavate the existing soil to the desired depth. Be careful not to over-excavate; the bottom of the trench must consist entirely of undisturbed soil. Remove all loose material so that the wall does not settle during construction. Use crushed stone or material that is well-drained and easy to work with. Some builders use a base of "crusher run" - a mixture of stonedust and #1A, #1 and #2 stone.
Depending on the size of the stone you're using, you can also use the base material as backfill - we use either #1 or #2 crushed stone. While a base of crushed stone would never work for a patio, it is suitable for a dry-laid wall, especially when large stones are laid.
q Batter. Wall batter refers to the degree of slope the wall face will have. Batter is extremely important when constructing a large, freestanding wall and necessary for any retaining wall. Batter allows the weight of the wall to exert force against the earth being retained. If you construct a retaining wall with a vertical face that has no batter, your wall is not exerting any force on the soil. The earth being retained will push against the wall and eventually cause it to fail. Construct your walls with a minimum batter of 0.5 inch per foot of wall height. For larger walls, increase the batter to 2 inches or more. A 4-foot-high retaining wall with a 2-inch batter will pitch or be set back a total of 8 inches at the top.
To maintain a consistent batter throughout the wall, use a batter gauge. Cut a 2x4 or 2x6 to the desired batter and wall height and fasten it to a mason level with duct tape. While reading plumb on the level, lay the gauge against the face of the wall to ensure a consistent pitch.
q Backfill. Install a filter fabric between the backfill and the soil behind the wall. Place the fabric from the base course to where the top of the wall will be. The fabric is necessary to ensure that the soil doesn't wash into the backfill aggregate and inhibit drainage.
Use #1 or #2 crushed stone as backfill material. Also, use stone fragments generated from the material you are building with. You can clean the site as you build. Do not use soil. Installing correct backfill material ensures that water can flow through the face of the wall and prevents hydrostatic pressure from forming behind the wall. At more than 60 pounds per cubic foot, the weight of water is an enemy of stone retaining walls. Saturated earth behind any retaining wall creates incredible hydrostatic pressure that exerts force against your wall and can cause it to fail.
Place a 4-inch, perforated, PVC drainpipe along the length of the wall, behind the base course, to remove water. PVC is rigid and withstands the weight of stone and backfill material. Place a filter fabric over the drainpipe to prevent debris from washing into it. Also, make sure backfill aggregates surround the drainpipe. The aggregates allow water to flow uninhibited into the pipe.
By using #1 or #2 crushed stone behind the wall, you eliminate voids in the wall cavity that will, over time, cause extra shifting and settling. A tight well-built retaining wall may require 3 feet of crushed aggregate behind it.
Stone placement Unless you are building your wall with rubble (which tends to be round, irregular stone), you will build in courses or layers. Build courses to vary in height. Stagger the horizontal seams along the length of the wall and be sure that they are level and do not "run" with the slope. Interlock the stones so that the courses don't look obvious. Keep the height of the vertical seams to a minimum. Try not to stack stones together that will create a vertical seam longer than the thickest stone used.
Place the largest stones in the bottom of the wall to provide a firm "foundation." Use thinner stones that will not bear much weight for the top layer to "cap" the wall. Perhaps the most important pieces of the stone puzzle you create are the "key" or "bond" stones. Select large stones and place them so they extend beyond the thickness of the wall into the backfill area. They help to anchor and stabilize the wall.
Veneered walls Mortaring face-stone to the surface of an inexpensive concrete-block wall frame is a great option when the stone used is expensive or if deep footings are not necessary. Entirely different from a dry-laid wall, veneered walls are rigid and must have a footing. The majority of the wall is generally concrete block. Metal wall ties are imbedded in the mortar joints of the block to join the stone. The veneer stone can be irregular or cut like face brick in a brick wall.
Hand-chiseled walls When constructing a dry-laid stone wall, the less chiseling, the better. It is both time consuming and hard work. However, when an abundant supply of stone is available and the budget permits, hand-working individual stones results in extraordinary walls. Use carbide-tipped chisels and large blocks of stone while closely fitting the stones together. The basic principles of construction remain the same, except you should take great care to individually shape each piece of stone for a precise fit.
The demand for dry-laid, stone-wall construction continues to increase. Although more demanding and time-consuming than other types of wall construction, dry-laid stone walls represent a specialty niche for the landscape contractor. Follow the basic construction guidelines - they can go a long way toward building a beautiful wall that will last for many, many years.
- Sort through your stone pile and separate the largest stones. They will be used for the base course and as "key" or "bond" stones.
- Excavate soil for the base. Remove all loose material. Excavation depth should be a minimum of 6 inches.
- Install base material in trench. The material should be lightly tamped.
- Begin placing stones on top of the base. Work the stones into the base. Use the larger stones in your pile - this is the foundation for your wall.
- Place more stones on top of the base course. Set them back to achieve the desired batter. Use a batter gauge to attain a consistent batter throughout your wall.
- Place the drain pipe (if needed) in the bottom of the backfill area. Wrap the pipe in filter fabric to keep debris out. Do not place the drainpipe directly behind the first course of stones - this creates voids. Make sure there is backfill material between the wall stones and the drainpipe.
- Install filter fabric between backfill and soil to be retained. Use large landscape staples or pins to secure it and keep it out of your way.
- Begin backfilling. Backfill as you build your wall rather than pour the backfill behind the wall when it is complete.
- Continue stacking stones and backfilling. Place larger "key" or "bond" stones throughout the wall.
- Place the capstones. After you have built your wall to the desired height and backfilled, use the smaller, thinner stones to "cap" the top of the wall.
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