The Basics of Building Stone Retaining Walls

A retaining wall must be strong enough to hold the pressure of a large weight of soil, but it must be porous enough to allow drainage. The most popular types of walls are made of stone. When using stone to build a retaining wall, there are two basic types of construction: plaster, which uses earth as a filler between the stones, and mortar type, which uses cement as a bonding agent.

First, the base of any retaining wall must sink below the frost line. This is around 6 to 12 inches in the northern half of the United States, but can be more in some areas. For a flat wall (no buttresses or overhangs), the width of the base should be equal to one quarter of the height of the wall. The wall can taper to a width of about a quarter of the width of the base.

For buttressed walls, the base should be about a quarter of the width of the wall to be tall. This refers to the widest points, when buttresses are to be used. In narrower areas, the base can have slimmer proportions

Drain pipes should be embedded in the wall at approximately 24-inch intervals and approximately 6 inches from the lower grade of the retaining wall. In some walls, it is possible to eliminate these drains, if the wall itself is porous enough, but any construction that uses mortar as a bonding agent makes drain pipes essential.

In drywall construction, it is possible (but not recommended) to start the wall at grade and not sink below the frost line. The most economical way to build a dry wall is to choose local stone, choosing large stones for the main ones and smaller stones for the crevices. The larger stones should be used to form the base of the wall with the smaller ones for the top.

The outward facing side of the wall should be as level as possible. Obstructions and outer stone edges must face inward. This gives the wall a better footing on the ground that retains and ensures a good appearance. Stones with round surfaces do not make a good wall and should be discarded.

The stones should be laid with a good joint, which simply means that the edges of the stones in one course should overlap the gaps in the courses below. When a stone in a top course is crooked or does not fit tightly, dirt and small stones should be packed to improve the bond and vertical cracks should not be left.

The wall itself should lean back against the soil it is holding back. This gives you more strength. As a general rule, the width of the base should be one third of the height. Although this degree of slope is not essential, it is practical in many areas to slope the wall as much as five or six inches for each vertical foot. Soil should be firmly compacted in all wall voids and continued back into the retained soil.

Both the strength and attractiveness of a dry stone wall can be enhanced by using it as a wall garden. You can get a mossy, aged appearance simply by planting it green in the cracks. However, more color can be obtained by planting any of several flowering plants, the strong roots of which will do the added job of holding the wall together.

Plants that can be used to good effect include: flower types such as phlox, garden roses, sedum, snow cress, azaleas, alyssum, evergreen candytuft, heather, and creeping speedwells; spread plants like moss, phlox, lavender, and hardy verbena; small rosettes as well as small tufts that need sun and root space such as sempervivium, yarrow, dwarf iris and dwarf roses; and plants you can grow from seed sown among the rocks, such as some ivy, bleeding hearts, and varieties of poppies and phlox. Semperviviums, prostrate, junipers, azaleas, and dwarf azaleas could keep a rock wall green all winter long.

Mortar and Concrete Walls

Mortar walls are simpler than dry walls. The mortar serves as a binder so matching the stones is not as essential. For a masonry wall, a cement mix of one part Portland cement and two parts sand is a good bonding agent.

Apply the mortar liberally to form a bed for each stone as it is added. The cracks between the stones should be well filled with smaller pebbles or gravel. Mortar wall is more permanent than dry wall and actually easier to build.

The top of every stone wall, whether plaster or masonry, needs protection. This is accomplished by using wide, flat stones as keystones. These may be slate or other flat stones acquired in the course of collecting the material for the wall.