been used. Wood Wooden tables are most commonly constructed using lumber boards. Protection for the wood (stain, paint, or wood protectant that repels water) is necessary to protect it from cracking, warping, or rotting due to moisture. The table-top and bench-top boards are attached to the trusses or beams using wood screws or nails. The legs can be secured with carriage bolts fastened by nuts and washers. In the context of public parks, there have traditionally been different schools of thought as to whether local or commercial timber should be used for picnic tables. In some cases, rough-hewn local timber was used for the structural supports and commercial boards were used for the benches and platform. In California in the 1930s, cross-sections of redwood and fir trees were sometimes used for picnic tables, but these proved insufficiently durable. Stone or concrete Stone or concrete picnic tables are durable but expensive. They are difficult or impossible to move, which may be a drawback in some contexts and an advantage in others. Such tables first came into widespread use in the United States in the 1930s, as part of Civilian Conservation Corps projects. However, stone tables proved unsatisfactory because they could not be moved even when the entire picnic site needed to be shifted from one location to another. Plastic Plastic picnic tables have grown in popularity because they are lighter, more durable, and less expensive than wooden tables, and require less maintenance. A common source for plastic "lumber" in picnic tables is recycled HDPE, which may be mixed with other materials such as wood flour for improved strength. Metal Metal picnic tables are becoming more popular in public parks because they are heavy and durable, and require little maintenance. Metal tables are sometimes attached onto concrete pads when theft is a concern. Thermoplastic-coated steel is often used for improved durability in outdoor applications. In addition, heavy-duty metal picnic tables are often used for indoor applications in prisons. Accessibility Picnic tables pose a number of challenges for accessibility, particularly for users in wheelchairs. In the United States, federal recreational facilities are required to provide picnic tables that are accessible for disabled users. At least 20% of picnic tables must be accessible, and if only one or two picnic tables are present, they must all be accessible. Under the Americans with Disabilities Act (ADA), at least 5% and no less than one of