By Ronald W. Leigh, Ph.D.
May 4, 2017
Copyright © 2016 Ronald W. Leigh
Introduction
Have you ever noticed how many mailboxes are crooked?
One very common method of installing a mailbox is to attach it to a treated 4x4 post placed about 2 feet into the ground, usually surrounded by concrete. When first installed the mailbox post may be perfectly plumb, but there are two problems:
- in time, the ground moves
- in time, the 4x4 post warps
Then, if you want to fix it, you have to deal with that post stuck in concrete.
Here we will describe an entirely different method of installing a mailbox. We illustrate using a MailMaster Plus Step 2, shown below.
Installing a mailbox according to the method described here has the following advantages:
- The "post" will not warp because it is a section of 3-inch PVC pipe.
- By using ground anchors instead of a post in concrete, the mailbox can be (relatively) easily relocated.
- When the ground moves, both major and minor adjustments can be made to return the mailbox to plumb.
- Major adjustment — the ground anchors can be turned, with each half turn effecting an adjustment toward vertical amounting to between 2 and 3 degrees.
- Minor adjustments — the bolts mounting the base to the anchors allow for continuous small adjustments so the mailbox can be fine tuned, returning it to "dead on" vertical.
By the way, the U.S. Postal Service suggests that Mailboxes should be placed 6 - 8 inches back from the point where the curb raises, and the "floor" of the mailbox should be 41 - 45 inches above the road surface. Further guidelines are found here.
Anchoring the mail box
Rather than bury a 4x4 post in concrete, anchor the mail box using three "ground anchors," also called "earth screws." These anchors come in various lengths; we used 16-inch anchors. Here is an above ground view of the anchors attached to the base.
Of course, the anchors are first screwed into the ground in a triangular pattern, then three modified 5/16 bolts (explained below) are placed in the inside holes, then the base is attached to those bolts.
The anchors will be spaced approx. 18 inches apart, center to center, as shown. (The precise spacing between anchors will depend on the exact shape of your base.)
We have drilled 2 holes in the top of each anchor. Eash hole should be 3/8 dia. (to accept a 5/16 bolt). Each corner of the base will be bolted to one of these holes. If you need to make a major adjustment to the vertical alignment of the post, you may want to use that second hole. After being screwed into the ground, each anchor can be turned in half-turn or whole-turn increments, thus the need for two holes. A half-turn adjustment of the anchor makes a difference of approx. 2 or 3 degrees in the vertical alignment of the post. You can also make minor adjustments as explained below, under "Base."
Post, flanges, and base
Here is a picture of the assembled post, flanges, and base.
Post
The post is a section of 3-inch PVC pipe (schedule 40), 28 inches long. This pipe has a 3.5 inch outside diameter, which matches the actual size of a 4x4 post.
Flanges
At the bottom of the post are two flanges. They are "closet" flanges (typically used to attach the bottom of a toilet to soil pipe). The post is simply cemented into the flanges using the standard cleaner and solvent typically used for cementing PVC pipe and fittings. The flanges are bolted to the base as shown. (They are shown mounted to the bottom of the base, but could also be mounted to the top of the base.)
The flanges shown below are called "3x4" since they will attach to the outside of a 3-inch PVC pipe or to the inside of a 4-inch PVC pipe. A 3x4 flange is typically stronger than a flange made just for 3-inch pipe. You should use these stronger flanges so they will stand up to the heavy snow that will be thrown against the mailbox by the snow plow.
When horizontal forces are applied to the post, bending stress is concentrated in the flanges. So if your mailbox is hit by a car, this is the "weak point" that will break and the mailbox itself may be spared major damage. Fortunately, the post and flanges are the least expensive parts of the assembly, and they are relatively easy to replace if the flanges are broken.
Base
The base is made of six sections of 1.5 inch x .125 inch aluminum angle – three sections for the larger outer triangle, three for the inner triangle.
The base will be attached to the ground anchors with 5/16-18 stainless steel bolts, 4.00 inches long. It would be difficult to insert the bolts from the bottom up after the anchors are turned into the ground, so we have cut the head off each bolt and replaced it with a nut at the "head" of each bolt. (Of course, if you are able to find stainless steel threaded rod, you can simply cut three 4-inch lengths and end up with the same thing.)
You will be able to make minor adjustments to the vertical alignment of the post by raising or lowering the nuts at one or two of the three corners of the base, as shown below.
Attaching mailbox to post
The mailbox is shaped to accept a 4x4 post (actual 3.5 x 3.5), which would fasten a flat surface to a flat surface. Since we are using a 3-inch PVC pipe (schedule 40) for the post, we would have a flat surface joined to a curved surface which would allow unwanted movement. Thus we have added the corner pipes shown below (black) so there are more points of contact with the post and more stability. The corner pipes are approx. 5/8 inch outside diameter, or large enough to firmly contact the 3-inch PVC pipe when assembled. They can simply be taped in place before assembly. You could easily use a couple strips of treated wood to accomplish the same thing.
The MailMaster mailbox comes with two 5/16 diameter lag screws for attaching it to a 4x4 post. Since we are using a PVC pipe in place of a 4x4, we need a different method of attaching the mailbox. There are several ways to do this. One good method is to use two 5/16 carriage bolts (5.5 inches long) running inside spacers made from 1/2 inch PVC pipe as shown below.
If you are using carriage bolts, you will need to use a small file to square the holes that accept the square shoulders under the heads.
Finished project
Once the post-flange-base assembly is attached to the anchors, and the mailbox is attached to the post, you can adjust the mailbox to make it perfectly plumb, as explained above.
We have covered the base with crushed rock which allows water to drain.
