Question
patio drainage
I had a very old brick patio that I removed because I wanted to replace the bricks with new ones. The patio has a slight drainage problem because the backyard slopes toward the house. The old patio was sloped slightly away from the house, but after a hard rain, water tended to puddle in the middle of the patio for a while before it got absorbed. Finally, the patio is covered with a trellis supporting a large wisteria that's planted at the outside edge of the patio--about 20 feet from the house. Over the years, the wisteria's root system has raised that outside corner of the patio so that the lowest spot is no longer that outside corner of the patio. In effect, there is a slight "bowl" effect caused by the wisteria on one side of the patio.
I have been thinking about installing some type of french drain to help absorb any water that might pool toward that bowl. Most of what I read about french drains says that they are installed at perimeters of patios. Is there any reason I couldn't install some type of french drain at the low spot? My picture is not very good, but you can probably see how the wisteria on the left has raised that corner about an inch higher than the low spot--about 7 feet to the right.
Finally, I have clay soil, so drainage through the soil is bad. Are there any rules of thumb about how large or deep to make a french drain based on the size of the area to be drained? I realize the relevant fact is how much rain is going to fall, but I can't answer that. What I'm trying to do is improve a situation that's not terrible to begin with. But as long as I'm at the spot where I might improve the situation, I think it might be a good idea to do something.
Any advice would be appreciated. Thank you.
AnswerYou could certainly do a French drain but the wisteria roots would likely clogg it in short order. Instead install an small flat topped area drain maybe a 6x6 or 12x12 at the largest and pipe it around the corner of the hose to a lower elevation. You will need a small drain box, pipe, and a flat grate to cover it. The key it to ensure the top is level or just below the surface of the brick when you lay it. Since the brick will almost certainly settle a tiny bit it is a goo diea to set the drain about 1/4" lower than the top of the bricks.
See www.ndspro.com for ideas for drains.
Best of luck
Sean J Murphy
www.seanjmurphy.com
www.seengineering.com
