Screens help keep a system clean. They do this by separating debris from the rainwater before storage, and often even before conveyance. Screens require maintenance, yet should serve to make that maintenance easier than if the debris was allowed to enter the rest of the system. Typical debris includes leaf litter and trash that has blown onto or been thrown on top of the roof. Screens can be bought commercially (Figure 2-13) or made to order from available materials (Figure 2-14 through Figure 2-17).