Denver Pergola Footings

Late March is when backyard structures get tested here. A foot of that heavy, soaked spring snow lands on a pergola overnight and doubles what the posts are carrying, and it stays up there for a day because the sun hasn't got to the underside yet. Then in June a downslope gust comes through in the evening and tries to do the opposite: pick the whole thing up and set it somewhere else.

Those two moments, months apart, are why we build pergola footings the way we do. We set them across Washington Park, Berkeley, and University Hills, usually for shade structures going onto yards that already have a patio, and the difference between a structure that shrugs off both events and one that racks, leans, or walks is entirely below the surface.

Down is the easy direction

Holding weight up is the part everybody plans for. A pergola with a slatted top carries very little on a normal day, and even loaded with snow the downward load per post is modest compared to what a house footing handles. If down were the only direction that mattered, you could set these things on a paver and walk away.

Up is the hard direction, and it's the one that gets ignored. A pergola is a frame with a lot of surface and not much weight, which makes it good at catching wind and bad at resisting it. A gust hitting the roof at an angle lifts the whole structure and tries to rotate it, so one row of posts gets pushed down and the other gets pulled straight up. Concrete only resists that if the post is tied to it. A post sitting on a base plate with two short screws is just resting nearby.

What we set before the concrete goes in

• Holes taken below frost depth so ice can't jack a post out over a winter
• Steel cage or vertical bar tied in, not a single rod pushed into wet concrete
• Post bases set wet, at the right elevation, squared to the layout and each other
• Standoff bases used wherever the post sits in a spot that stays wet
• Uplift capacity of the base checked against the hardware schedule, not assumed
• Tops of piers finished sloped so water runs off instead of sitting on the base plate

Depth, and the winter that finds your shortcut

Frost heave is not subtle when it gets hold of a shallow pier. Water in the soil freezes, expands, grips the sides of the concrete, and lifts. Then it thaws and the pier comes down, but not all the way and not evenly with the others. Do that thirty or forty times a winter and after a few seasons the structure is out of square, the beam has twisted, and the posts are visibly out of plumb. Nothing broke. It just wandered.

The other depth question is what the hole bottoms out on. Plenty of older yards have a foot or two of fill somebody spread to level things decades ago. Setting a pier in that fill puts your structure on the least reliable soil on the property. We look at what comes out and keep going when it isn't right.

Post connections, compared

Best months to dig, and what to do about winter

Spring through fall is the working season, and early fall is the sweetest part. The soil is dry, holes stand open cleanly, and the concrete gets weeks of mild weather before the first hard freeze. Spring works as soon as holes stop filling with meltwater. If we find water in a hole, we pump it, look at the bottom again, and place the same day.

Winter installs happen and we take them. Ground gets thawed instead of chipped, mix water comes warm, and we blanket the pier tops overnight. Slower, and it costs more. The rule we don't bend is that nothing gets placed against frozen soil, because when that soil thaws it shrinks away and leaves your pier with a gap on every side. Want the pergola up for summer? Book the footings for early spring.