If you're looking to build a trampoline, it is best not to place it on the ground on top of concrete or gravel to ensure safety. There is a possibility that a child, an adult as well as a dog, could be thrown off and seriously hurt themselves or others on the hard surface. However, there are ways to protect the area and make it child-proof to ensure safety if you really want to install trampolines in your garage or the ground where there is concrete.