Rainwater tanks are the simplest systems for homes, and the easiest to install. In most cases, the precipitation is used for irrigating plants: the tank is already located outside. Commonly used rainwater tanks are not overly large (a common size is 224 litres), meaning that they require an overflow if the roof surface to which they are connected is too large. These tanks can be fitted with a pump and if necessary an automatic top-up system. Freezing temperatures present a risk, and may damage wooden tanks beyond repair. The limited retention capacity means that the yield is low. These tanks will quickly overflow during heavy rainfall. Besides the traditional wooden barrels, various plastic models are also available that are less susceptible to freezing damage.