When trying to decide which post lengths you need for your wood pergola kit, there are a few things to consider. The most important of which would be what part of the country you live in. Do live in the northern states where the ground is frozen for many months out of the year? Or do you live in the south where the ground never freezes. If you live where it freezes you will want to be sure that you set your posts deeper than the frost line. This will ensure that frost heave will not pop your pergola out of the ground like a cork. The correct depth can be determined by calling your local building code department. We also have a map that shows average frost depths across the country. This map is to be used as a reference only. Your local building code official will know the exact depth to dig to stay in compliance with local codes. So in essence, warmer climates may be able to use 10' posts, which would be buried 2' in the ground, and colder climates will most likely use 12' posts which would be buried 4' in the ground.
If your pergola will be going on top of a patio you can order the 8 foot post option and add the post attachment hardware to your order. This will connect the posts to the patio and also hold the bottom of the post up 1" from the patio surface to keep the bottom of the posts out of water that may accumulate on the patio. Here is a picture of the post bases that would be provided.
If you will be sinking your posts into the ground you have two different options on how to pour the concrete around the posts:
Option #1. Works in warm climates where
the ground does not freeze.
Option #2 for climates where the ground freezes.
Notice option #2 has a flared bottom. When the concrete is set below the frost line and flared on the sides this prevents the frozen ground that will be pushing on the sides of the concrete from popping it out of the ground. Also the use of concrete form tubes helps this because the smooth surface of the tube will not allow the ground to grab the concrete and pull it up. If you follow these suggestions you are sure to have a shade pergola that will stay put where it was intended to be for the life span of the lumber. Take a shortcut and your bound to have a pergola that is out of level. Frost heave can push posts out of the ground 3" or more if not installed properly.
