Good neighbors make good fences and good fences require good planning, materials and construction. The Fence Bible (Storey Publishing, 2005) acts as a reference to construct any fence that might be right for your property with an explanation of project options and detailed step-by-step instructions from fence-building and home-improvement expert Jeff Beneke. The following is excerpted from chapter three “Wood Fences.”
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Installing Concrete Footing
If you plan to bury your fence posts in concrete, one of the big decisions that confronts you is whether you want to mix your own concrete and, if you do, which products to use and how to do it.
There are circumstances under which it makes sense to have ready-mixed concrete delivered by truck. Under ideal conditions, it will save time and not prove much more costly than the alternatives. Ready-mixed concrete is ordered by the cubic yard. One cubic yard equals 27 cubic feet (or 46,656 cubic inches, if you need to do the math) and is typically the minimal amount you must order. Assuming that you dig 36-inch-deep holes that are 12 inches in diameter, dump 6 inches of gravel in the bottom of each hole, and plan to use 4 x 4 posts, you ought to be able to fill 15 1/2 holes with 1 cubic yard of concrete. You should plan to overfill each hole, however, and you will almost certainly spill a bit, so a safer bet is that you could handle 13 or 14 holes with a minimal order.
However, before the concrete truck pulls up to your house, you need to have all the holes dug and all the posts placed and braced. You need to make sure that the truck can get fairly close to the holes (although you do not want a concrete truck driving through your yard), and you should have a small crew available as soon as the truck arrives, equipped with shovels and a couple of wheelbarrows, so that you can work quickly. If you keep the truck waiting to unload too long, you may have to pay extra.
The process normally involves sliding a wheelbarrow under the truck’s chute, filling the wheelbarrow with no more concrete than you can safely handle, wheeling the concrete to the hole, shoveling the concrete into the hole, then returning to the truck for another load. Helpers can be handling another wheelbarrow as well as working on the filled holes to create a smooth surface that slopes away from the post (I usually use a margin trowel for this job; see the patting concrete around pole photo in Slideshow).
If you have a large number of holes to fill but would rather work at a more leisurely pace, bracing and filling one or two holes at a time, then you would be wise to rent an electric or gas-powered concrete mixer. You can throw bags of concrete mix (premixed concrete) into the power mixer, but it would be more cost effective (and only slightly more time-consuming) to mix your own dry ingredients: 1 part Portland cement, 2 parts sand, and 3 parts course gravel. All of these dry ingredients should be available at a good lumberyard or home center, but you may have to go to a rental store for the mixer itself. I have seen small power mixers that can be wheeled right up to and emptied into the hole. Use a shovel to stir the concrete mix (or the individual ingredients) in the mixer, then slowly add water until you reach the right consistency. When you can form a small pile of concrete that holds its shape, the mixture is ready to use. If water starts pooling on the surface of the mix, it is too wet.
For fewer holes, it is usually easiest to buy bags of concrete mix, empty the contents into a mortar tub or wheelbarrow, add a little water, and mix the ingredients with a hoe (a mortar hoe, which has two holes in its blade, is the best tool for this, but a standard garden hoe will work nearly as well). You can usually find bags of concrete mix (make sure that’s what you buy; you don’t want mortar mix or cement) in sizes ranging from 40 to 90 pounds. One large bag will yield about 2/3 cubic feet.
Concrete mix contains the proper proportions of Portland cement, sand, and gravel, but they aren’t always mixed well in the bag. For that reason, I suggest you do not try to empty and mix only part of a bag. Use full bags only, and mix up the dry ingredients well before you start adding water. Read the instructions on the bag to determine how much water to add. Measure this amount of water, then mix about 90 percent of it with the dry mix. Add small additional amounts until you reach the right consistency (able to hold its own shape, without water pooling on the surface).
Keep in mind that the clock starts ticking as soon as you add water to the mix. Concrete will start hardening in 45 minutes or less in warm weather (in up to 90 minutes in cool weather). But even though the hardening (technically known as curing) process begins quickly, you should leave the posts undisturbed for at least two days after pouring the concrete. Once the concrete cures, apply a bead of clear silicone caulk at the joint between the post and concrete, and renew the caulk anytime you start to see a gap developing at the joint.
Yet another option is to use a fast-setting concrete mix. This stuff is sold primarily for setting posts and flagpoles and the like. It comes in 50-pound bags, and the manufacturers suggest that you dump the dry ingredients straight into the hole until they reach about 3 inches from the top. Then pour in the recommended amount of water and let it soak in. Under normal weather conditions, the concrete will cure in about 30 minutes. Top off the hole with some soil and tamp it to encourage water runoff. This type of concrete is a little weaker than normal mix, and from what I’ve seen a bit more expensive. For these reasons, I haven’t used it, but I know of fence builders who swear by it. The “fast-setting” appeal of this product strikes me as more likely to appeal to a contractor trying to finish a job as quickly as possible, and I’m not sure most do-it-yourselfers would be too impressed by the feature. Some builders take the same approach with normal concrete mix (dump it dry in the hole, then add water). I have even heard of people dumping dry mix into a hole and letting the moisture in the soil take care of matters. With some luck and the right kind of soil, this may work. But I am not going to recommend it as a dependable technique for setting fence posts.
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Excerpted from The Fence Bible © Jeff Beneke, photographs of hardware, materials, and tools © 2005 Storey Publishing, Illustrations by Melanie Powell used with permission from Storey Publishing. Buy this book from our store: The Fence Bible.