Adding a Drain Line for a Dry-Stack Retaining Wall
Make an easy path for groundwater to drain away if you want your stone wall to last.
Walls built without mortar naturally let water that accumulates in the soil escape between the stones. However, water can also collect behind the footing. To ensure good drainage, you can install a perforated pipe behind the wall and cover it with filter fabric to prevent it (and the gravel beneath it) from clogging with silt.
Locate the drain as close as possible to the bottom of the footing and embed it in the gravel behind the wall stones 1. Here, we are using a 4-in. single-wall perforated pipe. Surround the drain line with enough gravel to prevent the wall stones from crimping or crushing it as the wall settles 2. Take care not to damage the pipe with a shovel as you spread gravel, and avoid setting stones directly over the pipe as you continue building the wall 3. If the lowest point is at one end of the wall, direct the pipe to daylight where the wall ends, surround it with wall stones, and cut it flush to the wall 4.
Adding a Drain-Line T
If the lowest point is between the ends of the wall, add a drain-line T and run your drain line from it. First, use a transit or laser level 1 to determine the lowest point along the footing. This is easy to do by sighting a common tape measure 2.
Lay drainpipe along the length of the wall, and mark the lowest spot 3. You can use a small rock as a marker instead of scratching or painting the asphalt driveway; chalk is a good alternative. Use the T-fitting to determine where to cut the drain 4. Remember to allow for overlap where the T-fitting and the drain line couple together. Cut the pipe and fasten it to the two sides of the drain line 5.
Build the wall around the T-section. To make the drain outlet less obtrusive, recess the T-opening a few inches 6. Position a flat rock under the outlet to direct the water away from the wall footing. Stack rocks on each side of the pipe, and bridge them with a horizontal stone to carry the weight of the wall 7. Then continue building the wall over and around the pipe 8.
Excerpted from Masonry Complete (The Taunton Press, 2012) by Cody Macfie
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