How do you fix a deck or porch post that is rotting at the bottom?

Why posts rot at the bottom

A wood post is end grain at both ends, and end grain soaks up water like a bundle of straws. When the bottom sits directly on concrete, pavers, or soil, every rain and every sprinkler cycle wets it and the surface underneath holds that moisture against the wood. The top of the post dries in an afternoon; the bottom inch stays damp for days. That is where decay fungi set up, and often where subterranean termites arrive.

Many older North County porches and patio covers were built this way, with the post toe-nailed to a slab or set in a footing with the concrete flush or even cupped around the base. They worked for years because the wood was good and the climate is mild. Eventually the bottom goes soft while the rest of the post is fine.

Deciding how much to replace

Probe up the post from the bottom. If the soft wood stops within the first several inches and the rest is sound, the post can be shortened and re-set on a raised base. If the rot reaches a foot or more, or the post is checked and split through, a new post is the better repair. Posts hidden inside a wrap or column cover need the cover removed to know, because the cover holds moisture and the damage is usually worse than it looks from outside.

Shoring first

A porch post holds up a roof. A deck post holds up a beam and everything on it. Before the saw comes out, the load gets transferred to a temporary post or a shoring jack set on solid bearing a foot or two away. For one porch column we handled recently, that meant temporary shoring, one careful afternoon, and the roof never knew the difference. This is the step a quick fix skips, and it is the reason a quick fix sometimes cracks the stucco above.

The detail that stops it happening again

The goal is to get wood off of wet surfaces. There are two good ways. A metal standoff base anchors to the concrete and holds the post bottom roughly an inch above it, so water drains away and air reaches the end grain. The residential code generally expects posts on exterior concrete to be preservative-treated or naturally durable, or to sit on a pedestal or standoff above the slab, so this is the detail an inspector would expect to see as well.

The other way is a masonry pedestal. On the porch column project, Levi built a brick pedestal with a cap, shortened the post, and set it on a standoff atop the cap. The post never sits in water again, and the pedestal gave a thin entry some visual weight it was missing. That is the architect in him: fix the rot and improve the proportion in the same move.

• Cut back to sound wood or replace
• Shore the load before cutting
• Standoff base or masonry pedestal under the new end
• Seal the cut end grain
• Slope concrete away so water does not pool at the base

Material choices

For posts that stay exposed, pressure-treated lumber rated for ground or near-ground contact is the durable choice, and the cut end gets a brush-on preservative before it is set. Where appearance matters, a naturally durable species or a treated core with a finished wrap works, as long as the wrap is held off the base so it can drain. Hardware should be hot-dip galvanized or stainless; treated lumber eats ordinary zinc plating.

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Sources and further reading