When trying to decide the length of pergola posts needed 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 or footings 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.
Your local building code official will know the exact depth to dig to stay in compliance with local codes. So in essence, in warmer climates you 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.
The easiest and most recommended method of installation is to dig footings to the correct depth (below the frost line in your area) and use post mounting hardware to attach the posts to the concrete footings or concrete patio as follows:
Order the 8 foot or 10 foot post option and add the post attachment hardware to your order. This will connect the pergola posts to the patio or footings and also hold the bottom of the post up 1″ from the concrete surface to keep the bottom of the posts out of water that may accumulate on the patio. Make sure if you will be setting the posts on top of footings, that the footings are dug below the frostline as pictured below. Here is a picture of the post bases that would be provided:
Here’s a great video on how to pour concrete footings the correct way:
If you will be sinking your posts into the ground you have two different options on how to pour the concrete around the posts:
Works in warm climates where the ground does not freeze.
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. We also recommend if you are putting the pergola posts in ground, that you wrap the bottom part of the post that will be in contact with cement in a peel and stick roofing membrane which can be purchased at your local hardware store.
If you follow these suggestions you are sure to have a pergola that will stay put where it was intended to be for the life span of the lumber. Take a shortcut and you’re 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.
If you have any questions about post depths and options please give us a call or send us an email and we will be happy to help you.