What Size Should Porch Supports Be?

Porch Construction

• There are essentially three layers to a porch. On the bottom are the posts. They stick up out of the ground vertically. The entire weight of the porch is supported by the posts. On top of the posts are the beams. Each beam lies on top of a row of posts. On top of the beams is the decking. The decking planks run at right angles to the beams. In determining the size of the porch supports, posts and beams, there are three essential figures: post height, post spacing and beam span.

Load Area

• The first step in designing a porch is to calculate the load area. This is the maximum area with no support directly beneath it. A load area has supports on its edges. To calculate the load area for a porch, multiply the space between posts by the beam span, which is the space between beams.

Beam Size

• Determining the spacing between posts and the beam span is not an exact science. It depends, in part, on the size of your building materials. Large posts and thick beams allow you to use fewer posts and wider spans. Smaller materials require smaller distances between supports. Consult a beam span/post space table. Look at the different options available for different materials and choose a combination that is right for your budget and time. A beam span of 6 feet with 4 feet between posts requires the use of two 2-by-8-inch boards for the beams. A beam span of 8 feet with 10 feet between posts requires three 2-by-12-inch boards for the beams.

Post Size

• To determine the size of your posts, consult a post height/load area table. Post heights 6 feet or less with a load area of 120 square feet or less need only 4-by-4-inch posts. Larger load areas or taller posts require 6-by-6-inch posts.

Warning

• Every city and state has its own set of building codes. Before you get too deeply involved in planning your porch, consult your local building codes to determine if there are any specific design requirements.