The starting point for most custom-built planters is the basic box; the designer's personal style is reflected in the surface decoration. However, regardless of your design, there are certain rules of thumb that you should follow in building a planter: Decay-resistant lumber, such as cedar or redwood heartwood, is best. Pressure-treated wood can be used instead, but avoid any treated with creosote, which is toxic to plants. For any plywood in the planter, use the exterior-rated pressure-treated type. If the planter will be made entirely of dimension lumber, use boards that are nominally 2 inches thick —actually 1/2 inches thick—for all but the smallest planters.
We can build a box quickly and easily by screwing the corners together; the joints can later be hidden under trim. We always use corrosion-resistant fasteners; for additional strength, we apply waterproof glue. Should we waterproof a deck planter? To give your custom-crafted planter the longest possible life, line its interior with a waterproof barrier to separate the soil from the wood. Two easy-to-use materials are heavy-duty plastic sheeting and roofing felt (tar paper). We completely cover the bottom and sides, staple the material in place around the top margin—the soil will hold the rest snug to the sides—then make slits over the planter's drainage holes. Or, we could paint the planter's interior with a waterproof, bituminous roof coating or roofing cement. For longest-lasting protection, we use a more solid liner fabricated to fit the planter's inside dimensions.