Hello, I've got a client that wants a trellis. I had originally specced the posts and some of the structure out of PTW, but the client wants cedar. So it will be cedar. My question is do I dare put the posts below grade? I live in Saskatchewan, Can so the climate is quite cold. The ground is pure sand, according to the builder the waterline is ~12' down and the frostline is 8+ feet.

I was planning to place the PTW posts 3-4' below grade by digging a hole by hand, maybe pouring a little concrete at the bottom to establish an elevation. Then lining the hole with fabric and filling around the post with crushed rock after the post is in and the frame is up. This is how the pole barn builders around here do it(with or without the concrete).

How long would cedar posts last under grade in that environment? Or should I decide on a way to build with them above grade? If so what have you done?

To help with frost, use a layer of 2" foam, 4'x4' with a hole the size of the post in the middle, place this just below grade, insulating the earth around the post.

I find it odd that we dig a hole and fill it with crushed stone, it is like a bucket to collect water. But if the soil is well drained the bucket will leak out the water before it freezes, maybe. This would not be the case in loamy or clay based soils, here a drainage ditch to light would be needed or use the same soil to back fill.

A trellis, is this in a garden? If it is anything like my gardens things change and in 20 years I suspect it will be removed and replace with another something. I would expect a life of 20 years. The bigger issue, beyond the life of the post is the frost heaving, address that first. Even if you cast a concrete pier frost could fowl it up. The post could act as a cold sink and aid in freezing the ground around the post, so keep it as low to grade as possible.

No the trellis is on the front of the house. It is designed to shade the exposed southern windows in the warm summer months but leave a clear view, and allow the sun to come in in the winter.

I've also wondered about the concept of filling a hole with crushed stone in certain environments. The site I'm working on is pure sand so I'm not concerned with making a "crushed stone bucket".

I like your idea of the 2" foam. I think it would work quite well in an environment that has many freeze thaw cycles. We don't get a lot of that here. It often takes till May for the ground to thaw and by then it is usually warm enough not to freeze again.

I'm told by a local that has been supplying cedar to this area for many years that the posts will last 50+ years if packed with crushed rock around them. Although the raising of the bents would be much simpler with a little concrete poured to elevation, I may skip that step to avoid a place for water to pool at the bottom of the post.

The frame is designed with 6 posts and 14 knee braces both because the client wanted the knee braces and because it will stiffen the frame from one post to the next. With the joinery any post that wants to heave will have resistance from its neighbouring posts.

Leslie,

I attempted a reply a couple days ago but I don't see it here. Hopefully I didn't post to the wrong thread.

I wouldn't hesitate to use cedar below grade for your purpose. I can elaborate if you wish.

I'm going to assume that your trellis is more of a pergola with substantial posts and rafters/perlins exposed to the elements.

I wouldn't use crushed rock since the soil is well-drained and the structure should be rigid. I think the pole barn builders use crushed rock to fill around posts because the material is very angular and works better than tamped earth to resist the forces on a post. I don't see how fabric would improve the lifespan of the posts either.

I also don't see how the insul described above would help in your climate (8' frost line: brutal). But I won't surmise further not knowing how the space will be maintained (shoveled or not, hardscaped patio or not, etc.).

Instead of mixing concrete to bring post holes to the same elevation I just use road gravel (easily transported in 5 gal buckets for small jobs). Dig post holes a few inches too deep and bring the bottom to elevation by by tamping gravel in the bottom.

Brook

When I did my fence, some years ago, I dug the hole with a small 12" wide bucket backhoe. This allowed me to make the post hole only 12" wide but kind of long as the hoe created a hole sort of like an arch.
I bought some crushed stone, and after placing the fence post online and on mark, held by temporary braces in two directions, I back filled part of the hole with the crushed stone.
To make the stone last till the end of the project. I used two pieces of 12" wide pine boards as forms to keep the stone from spreading out too far. The form boards were backfilled outside the post area with the dirt dug from the hole. As I backfilled by shovel, dirt on one side, stone on the other, I'd tamp it down. As the holes were filled up, I'd grab the form board and pull them up. This making them kind of a slip form.
This worked good for me. And it made my load of 3/4" crushed stone go to the end of the project.
I milled out the fence post out of regular eastern white pine, but painted the ends going into the ground, with some "fence post paint". This paint is made to preserve wood that would normally be "in direct contact" with the soil. I painted on several coats. And above ground I just painted the posts with regular outdoor white house paint.
In my area the frost only goes down 4' or so.

This idea of filling the hole with crushed stone is so that the surface water that would run into the ground along side of the post can drain away from the post and into the ground. Keeping the post dry as there is no soil there to absorb the water. Keeping the post dry prevents it from rotting.
I read all of this in a book on fence construction.

These fence posts have lasted, for us, many years. Some of the rails have seemed to fail before the post have.

Good luck with your project.

The notion that rock in a post hole will keep timbers relatively dry is a little sketchy.

Without a barrier (like filter fabric described above) soil will eventually fill the voids between the rock. Granted there will be less water-holding capacity in a rock-soil mixture than a straight soil mixture but the moisture content will basically be the same as the surrounding soil. I doubt water is draining though the rock as it would through a sieve.

Once soil is saturated water moves mechanically to areas of lower moisture content or lower pressure...even into a huge void such as my basement.