Three inch pipes are used to handle most residential toilets. 1.5 inch lines are used to handle waste from kitchen, bathroom and powder room sinks. 2 inch drain lines are common pipes for laundry drains. Clogs can develop easily on horizontal runs of pipe. Friction causes the water within the pipe to slow down. To achieve good water flow, try to install horizontal drain pipes so they have a minimum of 1/4 inch per foot slope. 3/8 of an inch per foot is actually better.