How Install Concrete Deck Footings

Our tutorial outlines a step-by-step method for installing concrete deck footings for a low deck or wood patio.

While there are a variety of ways to install footings, we use a fast and simple technique. We’ve used this method several times with excellent results. Here’s how to install concrete deck footings.

How Install Concrete Deck Footings

How to Install Concrete Deck Footings

The key to successful footings is accurate measurement and alignment prior to post-placement. 

How Install Concrete Deck Footings - measure the area

Step 1: Measure Your Deck Area and Determine Footing Placements

Start by measuring your deck area and then determine where you want/need support.

Our deck’s dimensions are about 14’ x 25’ (with a portion of that cut out for concrete steps). Along the long side, we will use five support posts, spaced roughly every 7’.

(Note: Because we are building a pergola above the deck, two of the support posts will be larger beams than these 4×4 posts. Here’s how we installed the pergola posts.)

There is one middle support post along the shorter side and then two posts placed in the center of the deck.

How Install Concrete Deck Footings - dig holes

Step 2: Dig Your Holes

Dig holes to the depth required for your deck height. Because this will be a low deck, the holes are about 18.” For a 4×4 post, a diameter of 12” is sufficient; try not to go much wider than this since it makes for extra digging work.

How Install Concrete Deck Footings - loosen the soil

It’s helpful, depending on your soil type, to use a pry bar to loosen soil that’s deeper than a shovel can go.

How Install Concrete Deck Footings - alternate with scooping

Alternate using the pry bar with scooping out the loosened dirt with a small shovel until you reach your desired depth.

How Install Concrete Deck Footings - use string

Step 3: Install a String to Determine Straight Lines

One of the best ways to determine a straight line is via string. You want your post holes in a precise line so you can install your framing without issue.

How Install Concrete Deck Footings - measure 2 sides

Measure two sides of where your string will run (e.g., in this instance, the center string is exactly 7’ from the foundation of the house; the outer string is 14’ out), then attach the string at those points. Doing this ensures a straight line. It’s a good way to mark level, too, if you add in a level check and adjustment to the stringing.

How Install Concrete Deck Footings - fit them with concrete forms

Step 4: Fit the Holes with Concrete Forms

Once your holes are dug, it’s time to fit them with concrete forms. For installing 4×4 post footings, a 10” diameter concrete form works well. Measure, then cut the forms to your desired height.

How Install Concrete Deck Footings - use the forms

Note: Concrete forms are excellent for holding the concrete in place for footings that extend above ground. In this example, however, the concrete support needs to reach only to ground level, not above. Use a hand saw to cut the concrete forms at your desired height.

How Install Concrete Deck Footings - cut the posts

Step 5: Prepare Your 4×4 Posts

Cut your 4×4 posts a bit higher than what you’ll need. In this example, 8’ long 4×4 posts were cut into thirds because they need to extend only about 8” above ground, and we wanted to have a bit extra.

You always want a few inches more than you think you’ll need. The extra post will be sawed off later, so you don’t need to worry about making them exact at this point. 

How Install Concrete Deck Footings - posts before

Step 6: Determine the Exact Post Placement

Before you set your posts into the concrete, determine where the post should hit. Once they are set, there’s no changing them.

You used string to determine the position of your holes, but now you need to run string to determine the exact placement of your posts. 

In this example, part of the deck frame is mounted to the foundation of the house, so we made measurements off of the endpoint of that frame. Two 2x6s will serve as the outer frame, and we will mount an additional 2×8 on the outside of those boards for the finishing side piece. In total, 4-1/2” is required outside the 4×4 post to make the frame work. 

How Install Concrete Deck Footings - use nails

Pound a nail in at the point where your post must align, then run string to the post-install corner. Use a square to determine 90 degrees.

How Install Concrete Deck Footings - add a stake

Pound in a stake that corresponds to your 90-degree string, and tie the string taut onto the stake.

How Install Concrete Deck Footings - create corner

You now have a straight, 90-degree line from one definite corner of your deck to the future position of a corner of your deck.

How Install Concrete Deck Footings - perpendicular string

Do the same thing with a string running perpendicular to create the full and precise corner position. Check for 90 degrees.

How Install Concrete Deck Footings - forms in holes

Step 7: Align Your Concrete Forms

Place your concrete forms into all the holes along both strings. The string should run to the side of the center above your form. Aim to have your post placement as close to the center of your concrete forms as possible. However, if they’re a little off-center, it’s okay as long as there’s enough room for the concrete to form all the way around the post.

How Install Concrete Deck Footings - add water

Step 8: Mix Your Concrete and Set Your Post

Once your concrete form is positioned and ready, use a hose to run about 2” of water into the bottom of it.

How Install Concrete Deck Footings - fast setting concrete

Make sure you have fast-setting concrete that states it doesn’t need to be pre-mixed. The process is so quick and easy if you get the right concrete mix.

How Install Concrete Deck Footings - mix the concrete

Pour about 1/5 or 1/4 of a 50# bag of concrete mix into the water at the bottom of your concrete form.

How Install Concrete Deck Footings - place the post

Place the post down into the concrete, and immediately check for level against the string. Be sure the side of the post next to the string only grazes it and doesn’t push it out of place in order to touch. If it’s not quite right, pull the post out and wiggle it down into the concrete or slide it until you’re level, barely touching the string.

How Install Concrete Deck Footings - use a level

Once you’re level (remember, this might take a minute or two to get right, but be sure you get it right), pour about half the bag of concrete into the hole while someone holds the post in place. Add a little water, then pour the remainder of your concrete mix in. Add more water until it’s a couple of inches above your concrete mix.

How Install Concrete Deck Footings - use pry bar

Use the pointy end of your pry bar to make holes in your concrete. These won’t actually be holes, of course. Instead, the up-and-down pry bar poking is “mixing” the concrete, finding pockets of air, and giving them a place to escape before the concrete sets.

How Install Concrete Deck Footings - check level

Check for level on perpendicular sides of your post. Your post may have moved, so make adjustments before the concrete starts to set. Remember, it’s fast-setting, so you have a few minutes but not a ton of time.

How Install Concrete Deck Footings - check the side

At the end of the day, the side of your post should just graze the placement string without bending it or pushing it out of the way.

How Install Concrete Deck Footings - repeat process

Step 9: Repeat on All Footings

Repeat this process for all of your footings holes, being sure to check for level in multiple directions multiple times on each post while the concrete is fresh. Allow the concrete to cure according to weather and package instructions.

How Install Concrete Deck Footings - complete

You’re well on your way to completing your deck frame. Note that two of the holes here are for pergola posts, which we will install using brackets in the concrete