Pressure Treated Southern Yellow Pine 4x4's provide the strength for your vinyl pergola. The vinyl provides protection from the elements and nearly eliminates any maintenance concerns. Our special post brackets mount to your deck or patio, and each post mounts into the brackets. The vinyl sleeve (with or without the post trim) slides over the posts. It's as easy as that. Years, or decades later, your pergola still has 4 good legs to stand on. Actual outside dimension of the finished post is 5" x 5"