Most projects that require
posts come with the same important decisions that accompay almost every
construction project – foundations and roofs.
Wherever you need a post,
you need to place it on or in a foundation.. Most of the time this means digging a hole (or lots of
holes) and lowering the post into the hole. The foundation is made by packing material around the post
to refill the hole. Sometimes,
especially for more temporary structures, the material packed in the hole is
the same native soil that was removed to make the hole. Other times, you will want to make the
foundation out of concrete. If you
are working on a concrete or masonry surface, you may be
anchoring a base plate to the surface with expansion bolts.
Let’s start by considering placing a post in a hole and refilling the hole with native soil. If you are in an area with sandy or loose loam soil this may not be an option. Most soils have enough clay or rock that they will pack well and make a good foundation. There are a number of advantages to refilling the hole with the soil that was removed. First, you have no additional cost of materials. In fact, it saves you from having to haul off all the material that was excavated from the hole. Second, if you have to remove the post, it is a simple matter to just pull the post, not an easy task with concrete foundations. You may have to dig a deeper hole than is needed for posts set in concrete. This also means you may need to use a longer post. The post will also be in constant contact with the soil. Such contact encourages rot, bugs, etc. Even with pressure treated or high-tannin (redwood, cedar) posts, over time, the posts will fail.
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The most common way of
anchoring posts is with a concrete foundation. Depending on the number of posts, how deep they are, how
much they are supporting, you may use anywhere from a sack of premix to a truck
load of readymix. Premix sacks are
commonly available in 40#, 60# and 80# sacks. These are the perfect solution for very small jobs and where
the load on the post is not great.
Typically, people set the post, pour in the premix and then just fill
the hole with water to activate and eventually cure the premix into concrete. This creates a foundation that is one
step stronger than packing native soil.
If the post will see a significant load (wind, support of an arbor,
etc.) it is common to properly mix the premix (usually in a wheelbarrow) and
then pour the concrete into the hole.
This makes a much better concrete and therefore a much stronger base for
the post. This is hard work and
very time consuming if you have a number of posts to set. This is a perfect excuse to get your
concrete in a trailer (hopefully from your friendly local Cart-Away Supply store or a rental or
landscape yard that uses Cart-Away® trailers). Concrete made and transported in a
drum-style mixing trailer should be a strong as any concrete you can buy. A four sack mix is usually good enough
for posts but some uses will call for a five sack mix. A one cubic yard concrete trailer will
hold the equivalent of about 50 of the 40# sacks of premix. If you have a very big job, you may
even need to get a readymix truck to deliver the concrete. Remember that the readymix trucks are
rarely dispatched for less than about three cubic yards of concrete. That means you would need to have about
100+ posts to set to make it worthwhile to use a truck.
To make sure that your posts
last the longest possible, two things should be considered: water coming from above and water in
the soil around the post. Capping
a post will extend its life by preventing water from causing rot from the
top. You can accomplish much the
same thing at the base of the post by setting the post on a base of gravel in
the hole before pouring the concrete and by tapering the concrete away from the
post at the top of the hole. If
you go check out failed fence posts you will often discover that water was
puddle against the post for years just because no care was given to the
concrete poured around the base.
Various hardware
manufacturers offer post bases for setting into your concrete patio or for
anchoring onto the concrete surface.
Check your local codes as many now require that the post be set with a
gap above the grade to prevent ground contact which invites rot and bugs.