Diy Garden Bridge . Use 4×4 lumber for the posts. Dig down or build up in order to ensure that the blocks are level with each other. How to Build a Garden Bridge DIY projects for everyone! Garden from www.pinterest.com.au This garden bridge plan provides an arched walkway, making it somewhat more complex to construct. Materials i used sawn treated pine, very rough cut pieces but they're cheap and treated to make them more durable. Dig one trench roughly 10 inches deep, 54 inches long and 12 inches wide and pour in 3 to 4 inches of pea gravel.