Posts for a deck, fence, or other structure must be absolutely plumb and firmly rooted in the earth or bolted to a concrete footing. To ward off rot, posts should be of heartwood or pressure-treated lumber, and steps should be taken to assure that water drains away from the posts. To prevent the posts from heaving, post-holes must extend below the frost line.
1. Stake the locations of the holes.
2. Choose depth based partly on how hard or rocky your soil is, and partly on how deep you must go to reach your area's frost line. For shallow holes in soft earth, use a clamshell digger; for hard soil or holes deeper than about 30 inches, you'll need a hand- or motor-powered auger. Most rental outlets offer all three of these tools.
3. To ensure posts are an even height, you can dig holes to a uniform depth or cut posts after concrete cures. To check depths, make a square and stake a level string. Cut the square's longer arm to reflect desired depth; shorter one rests on string when you reach that depth.
4. Determine whether you want to set the posts in the ground or on top of separate footings. Separate footings require more concrete because you must completely fill the postholes. You'll also need to insert anchors into the tops of the wet footings. (Local codes may dictate one method or the other.)
5. Good draining begins at the bottom of each posthole you dig. Wet down the hole, then pour in 2 to 3 inches of gravel. This prevents groundwater from collecting at the base of the post.
6. Put tubular forms in the holes to extend the concrete above ground. This helps keep surface water away from the posts.
7. After setting each post in place, adjust until it is plumb in two directions, then secure the post with two diagonal braces nailed to the post. If necessary, use string tied to end posts to check the alignment of intermediate posts.
8. Recheck that the posts are plumb, then backfill around any forms. Pack holes or forms with concrete, poking with a rod to remove air pockets. After the concrete has set for about 20 minutes, check the posts for plumb again and adjust if necessary. At the top of the hole or form, round off the concrete. Leave the braces in place until you're sure the posts are fixed.
9. Let the concrete cure 24 hours before doing any other work.