Solid post-setting takes some hard labour, but makes for longer-lasting projects
With plenty of trees, both living and dead, the question of how best to anchor one end of a clothesline or mount a colossal purple martin condominium is most often a no-brainer — the clothesline gets stretched between that giant hemlock and the cottage eaves and the birdhouse gets screwed to the white pine snag.
Job done. In a perfect world, that would be it. But in the actual cosmos we are forced to populate, things are rarely so simple (the clothesline tree dribbles noxious sap onto the tea towels and the martin pole is so punky its innards look like sponge toffee). Time to switch to Plan B and set some posts the old-fashioned way — with lots of back-breaking labour.
Whether they’re used for lightweight jobs like holding up a section of fence or more demanding duties like elevating a slam-dunkable basketball net to regulation height, the traditional way to set posts of all sizes — and arguably the sturdiest — is to sink them into solid terra firma. Because direct contact with soil is the surest way to rot any piece of wood, only -pressure-treated posts should be used below grade and, even then, applying a cautionary coating of copper-naphthenate end-cut preservative to the underground bits isn’t a bad idea.
Start with a good foundation
The key to a solid post-setting experience starts with a good foundation. Whether you use a rented gas-powered auger, a clamshell post-hole digger, or a spade, the hole should be about one-third the overall post length in depth — plus an extra 6″ for a gravel drainage base — and three times its diameter. So for a 6 x 6 post that protrudes 8′ out of the ground to support, say, a clothesline, you’ll need to set a 12′ post in a hole that’s approximately 18″ wide and 54″ deep. Better get digging.
Tamp down the bottom and sides of the hole, pour in at least 6″ of crushed stone or gravel, then place the post in the hole, making sure it is plumb and the face is properly aligned. Have a helper hold the post steady or build some rough bracing to keep it from moving around. Now for the fun part. Rather than mixing up batches of concrete and pouring them into the hole, use fast-setting concrete mix, sometimes called “post mix,” which you simply pour into the hole straight from the bag. Then add water and allow it to soak in to make your concrete. (Just follow the directions.) If the hole is really deep, you’ll want to work in stages, filling and soaking the first couple of feet, checking for plumb, then adding more post mix. Don’t fill the hole right to the top — you’ll want some room to hide the footing with topsoil. Finally, plumb your posts again and in something like 20–40 minutes, the base will be solid enough to stand on its own — but don’t start jamming dunks for a good 24 hours (even if the bag says four hours is all you need).
Some post-setting practitioners maintain that because concrete acts like a sponge that holds moisture in contact with wood, it’s better to secure posts using tamped limestone screenings instead of concrete. The hole is prepared the same way and layer after layer of the screenings, available in bag or bulk at garden centres, are meticulously wet down and tamped to create a solid base that allows water to freely drain away from the wooden post.
If digging isn’t your thing, and high strength isn’t really an issue, post spikes are a convenient option. Usually 24″ or 30″ long, these spikes have a cup on top designed to receive a standard 4 x 4 post. With a scrap piece of post set in the cup to protect it from damage, the spike is driven into the ground with a sledgehammer. The post is then secured in the cup with screws or bolts. Post spikes are handy, especially if the post might be removed one day, but they really can’t compete with the dug-in version for overall rigidity. Some have an adjustable cup, but even then, it’s important to drive the spike straight — no easy task when pesky rocks get in the way — or the finished post will look like that tower in Pisa.
Attach the posts
Attaching posts to solid rock can be difficult. One solution is a post base, a metal bracket designed to affix posts to concrete piers. These can be attached to the rock base with expansion bolts, although depending on the design of the base, it may have to be modified to work in this fashion. A more elaborate post-setting solution would involve pouring a concrete monolith, anchored into the rock with rebar, and fastening the post to it with expansion bolts. If you go to this much trouble, it had better be worth it, so save this type of installation for those really important items, like that woodcutter whirligig your great-uncle constructed using only a penknife. And think about planting some new trees so future generations won’t have to set so many posts.
This article was originally published on August 12, 2005