Grass and soil drainage

Asked April 25, 2017, 10:35 AM EDT

Hi, We have a large concrete patch we'd like to cover in soil and grass. The soil will be about 30cm deep before it hits a concrete bottom, but I'm wondering if this is enough for drainage, or will the grass be constantly soggy even in mildly wet conditions. We're in Ireland so we get a lot of rainfall. Is there anything we can do to help the drainage? We're reluctant to rip up the concrete base.

Outside United States

1 Response

Thank you for your question. The issue of drainage over concrete is really a combination of hydrology, physics and soil science. The concrete needs to have enough of a slope that water drains off. (Does it drain off now? If it pools now, it will continue to pool once it's covered with soil.) What type of soil will go over it? Water runs most quickly through sandy soil, because the grains are larger, and there are more air spaces. It runs most slowly--and pools most readily--in clay soil, for the opposite reason. Once water has an outlet, the molecules 'follow'each other (magnetic attraction), and you get a flow unless/until something blocks it. So, go observe your pad after a rain. If it has drained then, you'll have drainage when it's covered. If there's pooling, you might want to drill holes every metre or so. I suggest you put down a layer of pea gravel or small gravel, and get soil that's equal parts clay, silt and sand (loam). That would be the ideal basis for your new lawn as well. Hope this is helpful. Good luck!