The Drip Lands Further Out Than the Trench: Width, Wind, and Impact Energy at an Unguttered Eave

Buildings without gutters shed their roof water from the eave edge, and the standard remedy for the erosion and splashing that follow is a gravel-filled trench along the wall. Trenches are almost always dug to a conventional width — 300 mm is common — without any calculation of where the water actually lands. This paper computes the landing point from the fall trajectory. In still air, water leaving a 3.5 m eave at a stated edge velocity of 0.30 m/s lands 0.253 m from the wall face, and once a splash allowance of 0.15 m is added the trench must be 0.45 m wide to contain it; the conventional 300 mm trench therefore misses the drip line even with no wind at all, at every eave height examined from 2.5 m upward. A light wind of 2 m/s pushes the required width at a 3.5 m eave to 0.70 m, and a 600 mm trench — twice the conventional width — tolerates only about 1.55 m/s of wind at that height before the impact point passes its outer edge. The paper gives a width table over eave heights of 2.5 to 7.0 m and winds of 0 to 4 m/s, the gravel volume each width implies, and the impact energy the gravel must absorb, which rises from 34.4 J per litre at 3.5 m to 68.7 J per litre at 7.0 m. All results are simulated, not measured.