One of the most annoying yard projects we can think of is dealing with a sagging wood retaining wall. Now we agree, compared with other types, wood retaining walls are easy to build and inexpensive. After earth preparation a 100-foot-long by 3-foot-high retaining wall can be completed in a weekend. And, wood looks great too... for at least five years. And it holds up for 20 years or longer. Unfortunately, it doesn't hold up forever. Wood retaining walls are temporary. At the other extreme, and for not much more than $50,000, you can easily build a poured-in-place concrete retaining wall that is guaranteed to last longer than you will. However, for most of us somewhere in the middle would be better.

When it comes to retaining walls, the latest and greatest are those that consist of interlocking concrete blocks. They are relatively easy to install and are reasonably priced. But there is a drawback. Interlocking block walls don't end up straight and level. In fact, most block retaining walls traverse at quite a slope. But then designers of these relatively new and unusual systems have deemed the sloped configuration to be architecturally pleasing... and we agree.

Best of all, block retaining walls are do-it-yourselfer friendly. You definitely can do this by yourself. Block retaining walls are similar in complexity to the wood blocks you played with as a kid. However, keep in mind that the lifting part of this task is not for everyone. Block wall construction requires strenuous digging and backfilling, the movement of tons of rock and gravel and the movement and placement of hundreds of concrete blocks.

There are two basic types of interlocking retaining walls... those that have interlocking flanges (sort of "L" shaped) and those that connect with pins or dowels. We like both. For our example we will use the L-shaped style.

Building a DIY retaining wall involves preparation. Once you have determined approximately how many blocks you will need, you determine how to get them to the place where they will be needed. The less handling, the less work. The blocks will come to you stacked on wooden pallets. If the work is planned to occur in an existing rear yard, forklift access must be considered. In the past we have removed sections of fence for this purpose... in the long run a lot less work.

With the block, rock and gravel delivered, the next step is the foundation. This one is made of gravel. Dig a trench about a foot deep. Cover it and the hill to be retained with landscape fabric and backfill the trench with about 6 inches of gravel. Place the first course of blocks upside down and use the gravel to adjust the first row so that it is perfectly level in both directions. Each of the following rows of block must be installed flange down. It is also important to ensure that each course (row) remains level. Before the wall gets too high, install the perforated drain pipe (perforations down) and cover it with gravel. Several inches of coverage is best.

In a bathroom the only good flush is a full flush. With a retaining wall the only good interlock is a full interlock. And a full interlock cannot be achieved unless each course of block is completely clean. Even the smallest piece of gravel can throw the wall out of level, reduce its strength and render a crooked unappealing finished product. Bottom line: sweep each course thoroughly.

You'll need to sweep because after each course is installed you will need to shovel in rock and compact it solidly with a tamp. You can purchase or rent a tamp or you can use a 4x4 post, if you have the strength. In any event, the rock must be installed and compacted as you go.

As you near the top of the retaining wall, lay the landscape fabric over the rock and cover with soil. The fabric is used to keep dirt from clogging the drain rock. Part of the strength of this type of retaining wall is derived from its ability not to have to withstand hydronic pressure. The drain rock and pipe act to shed surface and subsurface watershed, reducing hydronic pressure and keeping the face of the wall looking better longer.

The final step is to cap the wall to give it a finished look. Since the top of the wall isn't designed to be structural, the cap can be affixed with an adhesive. We suggest that you look for an adhesive designed specifically for use with concrete.