Yes, a pergola needs footings in Colorado, and they need to go below the frost line unless the design protects the posts another way. In Denver that line is 36 inches. The footing also has to hold the posts down in wind, which is the load people forget.
This guide is for a new pergola on a new patio, deck or yard, where the footings can be planned before anything is built.
Does a pergola need footings?
A pergola is a post-and-beam frame with an open top, and it’s easy to treat it as furniture. It isn’t. It’s a structure standing in the wind with its posts anchored in soil that freezes and swells.
The IRC’s general rule for frost applies to it as it does to other permanent supports. R403.1.4.1 says piers and other permanent supports of buildings and structures must be protected from frost, either by extending below the frost line in Table R301.2, by a frost-protected shallow foundation, by design to ASCE 32, or by bearing on solid rock. The code exempts small free-standing accessory buildings with no living space, light-frame, 600 square feet or less, with walls 10 feet or lower. Whether your building department reads a pergola that way is a local call, so ask it before you rely on the exemption.
Which rules apply and whether you need a permit depends on size, height, whether the pergola attaches to the house, and the city. Check with your building department. Don’t assume that an open-topped structure is exempt.
How deep do pergola footings need to be?
In Denver, at least 36 inches. That’s the frost line depth in Table R301.2 of the Denver Residential Code, and other Front Range jurisdictions set their own. Other Front Range and mountain jurisdictions set their own values.
The Department of Energy’s winter-weather guidance for ground-mounted solar shows why the depth matters. It says that as ground in cold climates freezes and thaws, foundations can work their way up out of the ground, and that the pile depth should extend significantly below the frost depth. That guidance is written for solar arrays, but the mechanism is the same for a pergola post. Freezing soil grips the post and lifts it a little each winter, and it doesn’t all come back down.
Depth for frost is a minimum. Wind uplift and the soil under the footing can both ask for more.
What pulls a pergola post up
A pergola’s own weight is small. That changes what the footing is for. A house foundation mostly carries weight down. A pergola footing mostly holds the frame down.
Denver’s Table R301.2 lists wind design speeds of 115/125/140 mph and a ground snow load of 43 psf. A pergola with a solid or fabric cover catches wind like a sail. Every post needs a footing that resists being pulled out of the ground as well as pressing down.
A shallow concrete plug or a bolted-on block can hold the weight of the frame and still pull free in the wrong gust. This is also where the “no-dig” shallow ground anchors sold for pergolas run into trouble. The Australian product page that ranks first in the SERP for this keyword sells screw footings of 500 to 1,000 millimeters (about 20 to 40 inches) in length and says nothing about frost depth, wind speeds or building codes. Check any product against your local frost line before you buy it.
Clay is the third lifter. The Colorado Geological Survey says expansive soils can expand 10 percent or more when the clay gets wet, with pressures of 20,000 pounds per square foot or more on foundations and slabs. A light pergola post doesn’t have the weight to resist that. Wet clay near the post, from a sprinkler head or a downspout, can lift one post and not the next, and the frame twists.
Pergola footing types compared
| Footing | How it works | Fits | Watch for |
|---|---|---|---|
| Surface blocks or pavers | Post sits on top, held by weight | Nothing in Colorado | Won’t resist uplift or frost movement |
| Concrete pier (formed tube) to below frost | Concrete cast to depth, post bolted on a bracket | Most new pergolas on a patio or lawn | Needs digging, cure time, and cleanup. Concrete work is a concrete contractor’s job. |
| Post set in a concrete plug | Post buried in the pier | Rustic builds | Wood in contact with the ground decays. Check treatment and local rules. |
| Shallow no-dig ground anchor | Short screw or plate with a bracket | Light structures where the frost line is shallow | Check the length against your frost line. Get the manufacturer’s engineering for your frost and wind. |
| Helical pile with a post bracket | Steel shaft with plates, turned to the engineer’s depth and torque | New patios and pergolas on clay or fill, tight access, no-cure schedules | Needs a design. Cobbles and shallow rock can stop it. |
ICC-ES AC358, the criteria that manufacturers’ helical pile evaluation reports are issued under, covers helical piles for compression, tension and lateral loads. A pergola needs the tension and lateral capacity, and that’s where a helical pile is a good fit. The plates below the frost line resist being pulled up.
Don’t bolt posts to a patio slab
The common mistake is to build the pergola on an existing patio. A patio slab sits on top of the ground, usually without footings below frost. Bolt a tall frame to it and the slab and the posts move together, or the slab cracks around the anchors.
On new construction you can do better. Plan the pergola’s footings with the patio, pass the posts through block-outs in the slab, and keep the slab from touching the posts. The patio can move with the soil and the pergola stays put.
If the pergola attaches to the house, the house’s frost-protected foundation and the pergola footings can move differently. A freestanding frame placed a short distance off the wall avoids tying two structures to each other. If you do attach it, have an engineer detail the connection.
What it takes on a new build
Site Safe Helical Piers installs helical piles for new decks, additions, homes and commercial projects across Denver and the Front Range. We install to your engineer’s depth and torque, and we leave the concrete to your concrete contractor.
For a pergola, a typical sequence runs like this:
- Call Colorado 811 and mark private lines.
- Lay out the post locations with the patio or deck plan.
- Get the footing design: type, depth below the frost line, and the uplift each post sees.
- Install the footings and set the post brackets to line.
- Frame the pergola and have the building department inspect where required.
Pile and pier work is quick once the design exists. See our services or contact us, or call 970-673-5615. Our article on helical piles for decks covers the footing details if the pergola sits on a deck, and the Colorado soil guide explains the clay.
