Layout and Installation of Concrete Piers and Pergola Post Bases

The first step in building the pergola is to lay out the locations of the post bases. This is one step where a bit of attention to detail can save you headaches down the road, so take your time to lay things out correctly.

The steps for laying out your pergola post bases depends on whether you intend to install them on new concrete piers or on an existing concrete slab. Many local building codes require that pergolas be founded on concrete piers that extend below the frost line, so our plans provide details for this as a default. However, we have also provided instructions for installing on a concrete slab for those that choose that route.

The instructions below are also presented in video format on our YouTube channel.

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Installation with New Concrete Piers
Installation on an Existing Concrete Slab
Install Post Bases

Installation with New Concrete Piers

Laying Out Pier Centers

Our plans specify the centerline dimensions between posts, which are the same as the centerlines of the concrete piers that support them. The easiest way to lay these out is using string lines (link to a great video tutorial on using them).

The materials required for this are a number of wooden stakes (two per post), a mallet, and a string line that you can get at any hardware store.

Start by determining a reference line for laying out your pergola footprint. This can be anything, but usually it’s something like the outer wall of your house or the edge of a landscaping feature. Say, for example, that you want your first row of posts to be 5ft from the outside of the house. Starting outside the footprint of the posts, drive in two stakes at 5ft from the house. Wrap the string line around one stake, making sure that it won’t come undone when tension is applied. Stretch the string to the other stake and give it a few wraps. Then check the string for level with a line level.

This string line is now your baseline for measuring the locations of the rest of the posts. Next, determine where you want the first corner post to go and make a mark on the string line with a permanent marker. Use two more stakes and establish a string line that is approximately perpendicular to the first line. Using the concept of a 3-4-5 triangle, measure 3ft from the corner mark on one string and 4ft on the other string. The diagonal distance between these will be exactly 5ft when the two lines are perpendicular. Adjust the second string line until the lines are perpendicular.

On the baseline, measure to the next post over and marks its center on the string line. Repeat the steps above for squaring the new string line to the baseline.

Next, measure the distance to the next row of posts on the two perpendicular string lines and mark them. Add a new string line to complete the square that connects these two lines. You should now have four string lines set up that look like a large, spread-out # symbol, with the intersections marking the centers of the new concrete piers. It will probably be helpful to mark the centers on the ground with spray paint.

Installing Pier Forms

**Note: prior to digging, contact your local call-before-you-dig hotline (phone number 811 in the USA) to locate any buried utilities in the area.

With the locations of the pier centers determined, now is time to dig down to the local frost line. Likely the easiest way to do this is to rent an auger with a bit matching the required pier diameter (given on our design drawings). If you have access to a skid loader with an auger attachment, even better. Otherwise, a shovel and post hole digger will do the job, just more slowly.

We recommend over-excavating by about 6″ and placing gravel fill at the bottom of the holes prior to putting piers in to provide a nice, stable base. Use a tamper, 4x4 board, or something else similar to compress the gravel into a dense, flat layer at the bottom of the hole.

Use of a pier form such as a Sonotube is recommended for forming the circular pier. These are basically heavy cardboard tubes that keep the soil from sloughing into the hole and also hold the concrete into shape while it cures. Cut the pier form to length with a circular saw prior to inserting into the hole and center it as much as possible on the string lines.

Next cut the reinforcing bars (quantity of each and length specified on the design drawings) and arrange them in the form per the installation detail. It may be helpful to use chicken wire or hardware cloth and tie wire to keep these in the orientation needed. Mix the concrete for the pier and shovel it into the pier form, using a spare piece of reinforcing bar to rod the concrete every so often so that it consolidates. Once filled, trowel the top of the pier smooth, with a slight dome upwards so that water can run off.

Repeat for the other posts, and allow the concrete to cure for at least one day before continuing. After the concrete has hardened for a day, the perimeter of the posts can be laid out, which is a similar process to that used for installing on a concrete patio. If you used a fast-setting concrete, the post bases can be installed after laying out. Otherwise, wait for the time specified by the concrete manufacturer before continuing.

Installation on an Existing Concrete Patio

Installing on an existing concrete patio is a bit easier since you already have a base to start from.

Start by determining a reference line for laying out your pergola footprint. This can be anything, but usually it’s something like the outer wall of your house or the edge of the patio. Say, for example, that you want your first row of posts to be 5ft from the outside of the house. Mark two points 5ft from the house on the patio, and use a chalk line to mark the line connecting these two points. This line will be the basis for your measurements from here on in.

Next, pick one side to start marking a line perpendicular to your first line. The easiest way to do this is with a couple of tape measures and establish a 3-4-5 right triangle. The 3- and 4-length sides will be the two perpendicular lines, and the 5-length is the diagonal distance between them. Thanks to geometry, we know that this forms a perfectly square corner.

Pick a point on your first line to be the Outer Corner of the post, measure 3 feet away from it, and place Mark #1. Next, take your second tape measure and extend it to show the 4-foot mark, and put the leading end of it on the Outer Corner mark. Use the other tape measure, extended to show 5 feet, and put the leading end on Mark #1. Where the 4-foot and 5-foot marks intersect, place Mark #2.

Using your chalk line again. stretch it so that it intersects both the Outer Corner mark and Mark #2. Snap the line, which is now perpendicular to your first line.

Repeat these steps on the other side. Satisfied that you now have three lines, two of which are perpendicular to the first, measure back from the two Outer Corner marks to establish the overall perimeter of your posts.

Install Post Bases

With the perimeter of the posts marked, now it’s time to install the post bases. Starting at one corner, place the corner of the base at the previously marked Outer Corner. Mark the center of the post base with a permanent marker and move the base to the side.

Using the hammer drill and a carbide-tipped bit sized for the post anchor, drill on the center mark to a depth 1/4″ to 1/2″ greater than the length of the anchor. If installing on a concrete patio, it's possible that you will drill entirely through since concrete thicknesses 4″ to 6″ are not uncommon for patios. Use a broom (or air compressor blow gun) to clear the dust.

Replace the post base to its previous location in the corner. With a wrench or ratchet and appropriately-sized socket (or impact gun), tighten the post anchor, fastening the post base to the concrete surface. There should be no play in the post base when you wiggle it.

Repeat these steps for all post bases.