A retainer wall should be built when the weather is nice and there aren't any storms or other issues coming. Trying to install a retainer wall in the rain or when a storm is coming will not be effective. It will also take longer and cost you more in labor fees. You may need to install a retainer wall for several purposes, the main ones being: To manage water runoff Provide usable land Provide extra seating You need to control downhill erosion Your home is downhill from soil fault lines Your foundation is threatened by a sliding hill Consult with a contractor and an engineer any time you are building a retainer wall.