Infiltration Trenches are usually seen at drip-lines of houses and camps. Roofs are considered an impervious surface. Impervious surfaces can’t accept water and as a result all the water from your roof is moved. This focused beam of water is called the drip-line. Drip-line trench pulls water underground so roots can filter excess nutrients. The ¾” rock filled trench encourages water to drain into the soil rather than over it.