[header] How to build a deck railing [title] Count how many posts you will need. [step] Your deck might already be supported by posts, especially if it is covered. If not, you'll need to install these so that there will be something to support the railings. 
Plan to evenly space posts around the perimeter of your deck, such as every 6 feet (1.8 m). [substeps] Keep in mind that some posts may be larger if they're going to have a gate attached or support stair handrails.