Sod homes were inexpensive to construct, robust, warm in the winter, and cool in the summer. They were, however, vulnerable to pest and mouse infestations and might be badly harmed by weather. To create sod dwellings, settlers cut and piled bricks from prairie sod. Build sod houses in September, when the grass roots are the thickest. The walls will be strongest then, before the plants begin to grow again in the spring. Sod houses have been used for thousands of years in parts of Europe, Asia, and North America. Early Americans built them as shelter during the cold winter months. They used whatever material was available - dandelions, straw, or wood - and so varied in style that some were almost tent-like while others were more like houses. A sod house should be well constructed with straight, smooth walls and a door and window that open out rather than in. The roof should be made of green timber, which will not rot if you need to rebuild your house later, and should be covered with clay or gravel to keep out rain and snow. Americans continued to use sod houses into the 20th century. Some farmers still use them today. The government also used them during World Wars I and II when they needed housing quickly; these "fortified camps" were usually only temporary until better buildings could be built.