How to set and align fence posts
Sooner or later most homeowners face the prospect of building a fence. Probably the most involved part of this task is setting and aligning the fence posts. For long fence life, use posts of Western red cedar or California redwood. These woods inhibit the decay-causing action of fungus and insects. Specify heartwood, the reddish brown wood from the central part of the tree; sapwood, the lighter wood close to the bark, is much less durable. Mountain cypress, sometimes sold in the Southwest for fence posts, seems to endure as well as cedar and redwood.
DIGGING THE HOLES
For low fences, sink the posts at least 18 inches into the ground. For fences 4 to 6 feet high, sink the posts at least 2 feet; the deeper the better, especially if you want the completed fence to present a solid barrier to winds.
Moisture should be able to drain past the bottom of the posts. For this reason dig holes 4 to 6 inches deeper than the posts will be set, and fill the bottom with rocks and gravel. In clay soil, it’s wise to go down even farther, to provide a foot-deep drainage basin of gravel beneath each post.
Locate the fence’s corner points, if not self-evident, by using a string and guides called batter boards.
SETTING POSTS IN EARTH AND GRAVEL FILL
Where the soil is stable (not subject to sliding, cracking, frost heaving), backfilling with earth or earth-and-gravel works fine for most fence posts.
Dump a big base stone into each post hole, or use a few smaller stones, or several inches of gravel. Tamp well, using a good-sized length of 2 by 4 lumber. Set in the post, and shovel in gravel a little at a time while you adjust the post until it’s aligned and vertical. Continue filling with earth, earth-and-gravel, or gravel, tamping firmly every 2 or 3 inches. If the hole desire. (Although either spacing allows for the most economical use of standard lumber lengths, other spacing may often be dictated by the type of fence, the appearance desired, or the slope of the ground.) Over uneven ground, drop a plumb line from each string mark to pinpoint post locations, and drive in marker stakes. Then begin digging.
Digging tools. Of the two most popular hole-digging tools, the auger type is good to use in rock-free earth, the clamshell type better in rocky soil. In soil that is extremely rocky, a digging bar and a spoon-bladed shovel may be the only combination that will work. Digging is still tough work even with these tools, but the resulting hole (especially with the auger) is much slimmer than with pick and shovel digging, and gives better support for posts and backfill. And less fill is required -an important consideration if you are setting the posts in concrete.
If you have more than six holes to dig, and the earth is not too rocky, power diggers are certainly worth investigation. One-man and two-man power augers are often available at tool rental shops. A rented jackhammer equipped with a spading tip could also do the job. The jackhammer is often the tool of is wide, big rocks jammed around the post near the surface will minimize side movement. Slope the top of the fill so that water will run away from the post. In light, sandy soil -which offers easy shoveling but poor stability for fence posts -nail 1 by 4 cleats of heartwood cedar or redwood across the fence posts near ground level.
SETTING POSTS IN CONCRETE
Concrete fill can use up a surprising amount of cement, sand, and gravel -but it gives the strongest setting by far. The concrete should be angled down at ground line to divert water away from the post. Don’t let concrete get under the post, where it could hold in moisture and speed decay. Above all, never set fence post ends completely in concrete.
For fence post setting in concrete, you can use a lean mix, with only a third the cement needed for a walkway mix. A mix of 1 part (by volume) cement, 3 parts sand, and 5 parts gravel is good. Keep the mix rather dry. To extend the mix, keep a supply of washed rocks on hand, and place them around the perimeter of the holes as you pour.
Using dry concrete mixes (cement, sand, and gravel all in one bag) saves ordering time and trouble, and means you won’t have leftover sand or gravel to dispose of. You will need about a bag of mix to pour around a 4-inch post, sunk 2 feet in about a 10-inch diameter hole.
Posts freshly set in concrete can be forced into a new position for perhaps 20 minutes after the pour; they should then be left alone for two days before boards or stringers are nailed on. During a spell of dry weather, fill the small crack between post and concrete with tar or caulking compound.
Frost heaving. Heavy frosts bring two problems: frost heaving, and concrete cracking. To minimize damage from heaving, dig post holes down to a foot below normal frost line; shovel in gravel; drive nails into the sides of each post near its bottom end, and place this end in gravel; pour concrete around nail area; complete the fill by using gravel or gravelly soil.
To prevent concrete collars from cracking when wet posts freeze and expand, cut shingles to width of posts, oil them, and place alongside each post before you pour. When concrete has set, remove the shingles and fill the open spaces with tar or sand.
ALIGNING THE POSTS
Many people consider the aligning of fence posts to be one of the knottiest problems associated with building a fence. Among the many procedures in use, three of the most workable are described below.
Comer post method. This involves setting corner posts first: firmly, permanently, exactly vertical, and with their faces in flat alignment. When they are solidly in place, stretch aligning strings between corner posts, top and bottom. Mark points on the top line to indicate where the centers of the intermediate posts will be, and transfer the marks, using a plumb bob, to the lower line. Set each intermediate post in gravel, with its face brushing (but not distorting) the aligning strings. Backfill carefully, checking to see that each post remains vertical as you work.
If rough posts snag the aligning strings, slip a piece of ¼-inch material between each corner post and the strings, then nail it on. With batter boards , move both ends of the cord 1/2-inch. Then keep each intermediate post the same Vmnch distance from the strings. Once all posts are in, make a final check by eye. (You can correct any misalignment at this time by pushing the post into the correct position and retamping the concrete or earth fill.)
Batter board method. With this method you lay out fence post locations first, then set the posts successively. First, build batter boards and mark the intended post centers on the aligning string. Continue as in the corner post method-but this time you will have only one string to guide you, and you’ll need to check verticals frequently with a level or plumb line.
Here’s a tip for plumb line users: Wrap the line around a piece of scrap wood half as thick as the bob is wide. Hold this block against any top corner of the post. When the plumb line lines up with the corner edge, and the bob is brushing the wood, the post is vertical.
One-man method. The ‘stake-out’ method makes it possible for a man working alone to align a fence. Drive two stakes into firm ground near each post, and nail an arm to each stake. Set posts onto rock or gravel at the bottom of the hole, checking alignment against the string. Use a level or a plumb bob to true one face of the post. Then tack its support arm in place. Do the same for the adjacent face. Check both verticals again, adjust the arms if necessary, and drive in the nails.