Deck Building 9 - Digging Foundation Holes and Setting Pier Blocks
Digging the Foundation Holes
Margin of Error: 1”
Most Common Mistakes
- Not digging deep enough for local codes and frost line
- Not digging into stable, undisturbed soil
- Not squaring off walls at the bottom of the hole
I won’t insult you by trying to tell you how to dig a hole, but there are a few things you need to note before taking a shovel to the soil. First, size is important; your diameter should be about 16 inches, but always check your local codes to be sure. The depth of the hole is also often regulated by code, as proper depths can range from 12 inches to 60 inches, depending on the frost level in your area (the colder the climate, the deeper the hole).
Once you have determined the diameter and depth, dig your holes to those specifications, being sure to dig good, straight (not sloping) walls. Dig until you hit stable undisturbed soil that will not settle. Be sure not to go too far down because you should never backfill a hole with loose dirt before pouring concrete. This compacts and causes settling which could crack your concrete.
Finally, you’ll want to take a moment to square off the walls at the bottom of your hole before you continue.
Pouring the Footings and Setting the Pier Blocks
Margin of Error: 1/2”
Most Common Mistakes
- Top of pier blocks not level
- Pier blocks not properly aligned
- Neglecting to use the metal connector where required
Now you are ready to proceed with pouring the concrete for your footings, but first a few words on mixing concrete. On small deck jobs I recommend that you simply buy the ready-mixed concrete with all needed ingredients in a bag. This is an easy solution; just mix it with some water, and pour it in the hole. However, before you start, be sure to buy enough material to fill all of your holes. You don’t want to run short and have to rush back to the store to finish off a hole.
Mix the concrete in a wheelbarrow or pan with a cement hoe, and be sure to follow the instructions as to the proper mix. Pour the concrete in the hole within an inch or so of the top and smooth it out with a piece of 2x4 or a trowel until it is relatively level. Place your pier blocks while the concrete is still wet.
Pier blocks serve as a transition from the posts supporting the girder to the concrete foundation footings. Although they can be built at the site, they are available at all home centers and hardware stores in a range of styles and sizes and are therefore not worth the hassle of pouring them yourself. The most common type is simply a small truncated concrete pyramid on top of which the wooden post sits. The weight of the deck keeps the post in contact with the pier block. A version of this is a pier block has a small piece of redwood or pressure-treated lumber embedded in the top so that the post can be toenailed to the block. Often, metal fasteners are used in areas where there are earthquakes. They are embedded in the fresh concrete and the posts are bolted to them. This prevents the posts from shaking off the pier blocks.
After the hole has been filled with concrete, and the concrete leveled and smoothed, drop the pier block into the fresh concrete and work it down until at least three to four inches of the base of the block are embedded. As you set the blocks, be sure of two things, that they are properly aligned and that they are level.
To check alignment, simply drop the plumb bob from the marks on your string to be sure the tip of is in the center of each pier block. To check if it’s level, use a small torpedo level, placing it in both directions as well as diagonally across the top of the pier blocks until the tops are correct. Tap and move the blocks around to make any needed adjustments.
After your pier blocks are properly set, allow the concrete to harden (this can take anywhere from two to 24 hours). Then you can begin building the girders and posts. We recommend keeping the concrete damp while hardening for a stronger pour. You can do this by sprinkling water on the pour as it dries or by laying wet cloths across the top.