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How-To

How to Make Wooden Porch Posts Last

Control moisture and avoid rotting wood by providing a specific path for water to follow.

By Chris Ermides
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Whether solid or hollow, porch columns are highly susceptible to moisture damage. On a pitched floor, water can build up behind the column and eventually penetrate the end grain. Sealing the end grain with a high-quality primer is key, but providing a path for water to travel is an often-overlooked detail. When the porch design calls for a wide column, wrapping a structural post with 1x trim is a common practice. But building a hollow column that is both structural and aesthetic makes venting the post easier. On solid columns, rout drainage channels on post bottoms to provide a tunnel for moisture to travel through and to allow air movement. Cutting a shallow rabbet around the bottom and then filling it with caulk serves a similar purpose.

Stout and hollow

Butt joints create the most stable joint, especially when assembled with epoxy, biscuits, and screws. Restoration carpenter Mike Davis hides the joint with a V-groove followed by polyurethane caulk. He sets the column over a stack of 2xs set in metal pan flashing and bolted to the frame below. The column’s end grain is kept 1⁄4 in. off the floor by a 2x block screwed inside the column.

Illustration of a column with text showing a v-groove, metal pan flashing, caulk, backer rod, and other details

Wick-free base

Unprotected post bases can wick moisture up like a dry sponge. A rabbet cut around the post base provides a channel for backer rod and caulk. Seal the end grain with Primkote or an oil-based primer. Fill the uphill and sides with backer rod and polyurethane caulk. Leave the downhill side open to allow moisture to escape. A V-groove channels water away from the house.

Illustration of a bottom of a porch post with labels of the v-groove, rabbet, backer rod, caulk, and downhill side

Hang the top and bottom rail

Mike Davis devised a system that is sturdy and slick. He locks the top and bottom rail to a stainless-steel lag bolt left 1 in. proud of the column. Using a plunge router and jigs, he routs two slots in the bottom of each rail: one for the lag bolt’s shank, the other for its head. The railing slips over the lags. He leaves a 1⁄4-in. gap between the rail and the column. Using backer rod and polyurethane caulk, he seals the top and sides of the rail-to-column joint. The bottom again remains open to allow for drainage.

Illustration of a rail agaist a solid column

Check out The Rotproof Porch for more tips for building durable porches.

Drawings by Dan Thornton. Photo by Roe Osborn.


RELATED LINKS

  • How to Make a Rot-Resitant Deck and Porch Railings
  • Porch-Post Rescue
  • Two Ways to Detail Durable Porch Piers

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