Setting Gate Posts and Footings on a Slope

A twelve inch round footing three feet deep takes about two and a third cubic feet of concrete, so a pair takes nearly five, and one extra foot of depth on the downhill post adds three quarters of a cubic foot more. The footing depths given in the notes at carlsbad iron gates new trouble run deeper than most post catalogs suggest, because a gate in Carlsbad, CA often stands on a grade. Soil that slopes gives a heavy leaf less to push against.

Measure depth from the low side

Footing depth is counted from the downhill ground line, not the uphill one. A post that is three feet deep on the high side may only be two feet deep on the low side, and the low side is where the load tries to rotate the post. Around La Costa and Agua Hedionda Lagoon the topsoil is thin over dense clay, so dig until the hole walls are firm and the depth is the full three feet on the low side.

Why diameter matters as much as depth

A footing resists overturning with its width as well as its length. Doubling the diameter does far more than adding six inches of depth. For a heavy wrought iron leaf, a footing at least twelve inches wide per post, and sixteen for a dual swing pair, keeps the hinge post steady across a decade of daily use.

Drainage at the base

Water follows a slope and collects at the bottom of a post hole. Put four inches of crushed rock at the bottom, then slope the top of the concrete away from the post so rain runs off instead of pooling at the steel. This matters in San Diego County, where the marine layer keeps steel wet through most mornings and a trapped puddle at the post base begins corroding from the inside.

Keeping the leaf plumb

Set the hinge post first and check it plumb in both directions with a four foot level. Clamp a bracing board to each post and leave the concrete to cure for at least three days before hanging the gate. A heavy leaf hung on green concrete leans the post by a quarter inch in the first week, and a quarter inch at the hinge becomes an inch at the latch.

Posts for slide and cantilever work

A slide gate puts its load in line with the fence rather than across it, so the receiving post and the roller posts need footings sized to the leaf weight plus the push of a gate opener. A cantilever gate loads the support posts with a lever, so those footings are the deepest on the property. Near Oceanside and Encinitas the sandy ground calls for a wider base than clay would, since sand grips less per square foot.