A Better Site-Built Fence
It’s safe to say that the ratio of straight, plumb, and not rotting fences to curved, falling over, and half rotten fences is pretty low. The seeming simplicity of fence building often leads to under-designed and poorly constructed fences that suffer common durability issues.
The basic problem is how to keep a fence upright. Unlike a wall in a house, a fence has nothing at the top to connect to and brace it. Fences often fail due to a poorly designed connection of fence post to ground. The worst (and probably most common) solution is to bury the wooden fence posts directly in the ground. The dirt eventually rots the wooden post and the fence falls over. A slight improvement on this solution is to pour concrete footings that the wooden fence posts are set in. This provides a solid anchoring for the fence posts, but the moisture in the concrete still rots the wood over time, leading to the same outcome. Another issue with this method is the time sensitivity. Fence posts must be set very accurately - plumb in both directions, at the correct height, and the correct interval from one another - which can be a highly stressful process as the concrete cures. Another approach is to temporarily brace the posts in the correct position, but this is also tricky and time consuming. Metal post bases (often used in deck construction) could be used to avoid having the post set in concrete or dirt, but this creates a pivot point at the base with no way to hold the post plumb.
We solve these problems by setting heavy-duty metal posts (U-channels) into concrete and then attaching the wood fence posts to these metal posts (illustrated below). Note the bottom of the post sits a few inches above grade to prevent moisture wicking. We use a jig to locate the metal post within the concrete form tube. The footing/metal post assembly can be made anywhere, then set in the ground where desired, enabling more accurate placement. Since the wood fence posts are not set directly in the footings, we preassemble entire fence sections (posts, frame, and slats) and install them as panels instead of putting up boards one at a time.
These solutions anchor the fence posts securely and ensure that the wood posts will not rot because they have no contact with the ground or with concrete.
On this project, there were two different site conditions where the fence was to be located. In one area, we needed to set the posts into the ground, and in the other area we wanted to attach the posts to an existing concrete retaining wall to maximize yard space. In both cases, these steel U-channel posts provide a solid connection to the ground and a stiff structure for the wood posts to attach to. I will explain how we used these posts and built our fence in each condition.
Metal Posts with Concrete Footings
Instead of pouring concrete into a form and then trying to set the post, I made a simple jig to set the metal post in the exact same place in each form before pouring the concrete.
I cut blocks with a bevel on each end so they would fit into the curve of the form. I attached the blocks to the metal post exactly 3’ apart so one would be at the top and one would be at the bottom of the form, keeping the metal post in place.
The length of these blocks determines how close to the edge/center of the form the metal post sits. In this case, I wanted the front of the metal posts to be 3 1/2” from the front of the form, so when I attached my 2x4 wood post to the metal post, it would be in the same plane as the front of the form. This allowed me to cover the concrete footings with the finish fence material.
This method has two benefits:
1) It allows you to pour footings in a more convenient location. Instead of digging holes, placing the forms, pouring the concrete, and setting the posts in place, we lined the forms up in the front yard, so when the concrete truck came to pour the concrete for the upper curb, we also poured the fence footings directly from the truck.
2) It makes setting the footings easier and more accurate. After pouring them in the front yard, we rolled them around to the back yard, set string lines, dug the holes, and set the footings where we wanted them. We then fine tuned as we backfilled with dirt.
Attaching the posts to a retaining wall:
At one end of the yard, there was an existing concrete retaining wall in good shape, so we wanted to sit the fence on top of it, instead of losing yard space by digging footings inside of the retaining wall.
This method is even easier than setting the posts in footings. We drilled two holes into the concrete and attached the metal posts right to the wall. The wall was out of plumb in some places, so I cut tapered PT shims to plumb the post.
In some cases, like the above right, the wall was so out of plumb that we had to first attach a 2x4 block directly to the concrete, then screw the metal post and the tapered shim into the block.
Building the Fence panels and attaching them to the posts
Instead of putting up the fence slats piece by piece I wanted to prefabricate finished fence panels and set them into place on the posts. I built a jig that attached to the fence frames and spaced all of the slats. Once it was made, all I had to do for each section was put a frame on the work table (built to the exact size of the fence sections), attach the jig, throw all the boards in place between the spacer blocks, and nail them. This method removes any fussing with spacers or thinking about layout on individual panels.
Attaching the wood frames to the metal posts
The width of the flange of these metal posts is exactly 3”, so if you build fence sections out of 2x4s, you can join two sections on a metal post and have the joint flush with the edges of the 2x4 frames. With long screws (and washers- not pictured here) you can attach the metal post to both frames, and screw the two frames together for a very solid connection.
