Home work: Building a stone wall

CHOOSE THE TYPE

There are two types of stone walls. One is freestanding and creates a fence or boundary. The other is a stone retaining wall. The method of construction is similar, but a stone retaining wall requires some engineering if you want it to last hundreds of years.

THICKER IS BETTER

A freestanding stone wall can be made by stacking a single row of stones that interlock with one another. You'll also find walls that are much wider and have multiple rows of interlocking rock. Both will work, but the thicker walls often will last longer, especially if they are wider at the bottom than at the top.

STABILITY TIP

Rocks that are more angular than rounded stack more easily and are more stable. Angular rocks resist the movement induced by gravity and frost heaving. Severe frost heaving, which causes the stone walls to move up in the winter then back down in spring when the ground thaws, often doesn't cause damage to dry stone wall construction (that is, walls that don't use mortar or cement to bond the rocks together). Friction and gravity are all that keep the stones from tumbling down.

SMALL ON TOP

Use larger stones as often as possible for the lower courses. As you build higher on the wall, use progressively smaller stones.

TILT BACKWARD

Retaining walls higher than 3 feet should tilt into the hillside. This is commonly referred to as battering. It's not uncommon to see 6 inches of backward tilt for every 4 vertical feet of height. Some structural engineers may call for even more tilt.

DRAIN PROPERLY

Drainage is very important. Fortunately, stone walls that are laid dry allow water to pass right through them. But you need to be sure the back side of the rock wall is filled with smaller stones, rounded gravel or crushed gravel that allows any groundwater to easily flow and not get trapped behind the wall. Trapped water just adds to the overall weight of the soil that the retaining wall is trying to hold up.

BE CAREFUL

You can lift the heavy stone any number of ways. Many tool rental businesses will rent you an excavator that has a thumb as part of the bucket. This feature works like your own thumbs, allowing it to grab and hold onto a large stone while it's placed. Working with heavy rock and stone can be hazardous. You can quickly crush hands and feet and bones. Use extreme caution.

STUDY BEFORE YOU WORK

If you've never installed a stone wall, talk to an installer, watch him work or study existing walls to see how they were constructed. Take photos of old walls and copy the pattern and method as best you can.

INTERLOCK THE ROCKS

Be aware that some stones are set in the wall with the long axis pointing back into the wall. Doing this frequently allows you to interlock those rocks with ones behind the wall you can't see from the front. Realize that taller stone retaining walls may be as thick as three or four feet at their base. There are hidden rows of rock and stone that are integral to the structure and success of the wall.