Setting fence posts into the ground isn't always possible; you may be erecting a fence on a concrete pad, the ground may be too hard for digging or you may want a temporary structure that you can easily remove. If you don't dig holes, you have to mount the fence on the surface of the ground, and there are at least three ways to do this. If you don't want to attach the posts to post holders fastened to concrete, you can use one of two ways to make them freestanding.
Setting Posts With Post Holders
You can securely attach metal brackets called post holders to a concrete slab by setting each holder in place, marking the positions of the screw holes and drilling a pilot hole for a metal or plastic screw anchor using a hammer drill. You don't need to use concrete screws to hold the brackets -- wood screws will do the job. Attach the posts to the brackets with screws instead of nails -- pounding nails into the posts is likely to chip the concrete.
As an alternative to attaching brackets to a concrete pad, you can simply attach them to concrete pier blocks with wood screws and mount the posts on the blocks. This strategy makes the position of the fence adjustable, but it won't support a high or heavy fence securely. Another option that works for mounting fences on dirt is to hammer specially designed post holders into the ground, using a sledgehammer, and fasten the posts to those.
The Basket-and-Gravel Method
A large container filled with gravel can hold a post, and a container that splays out at the bottom is particularly secure. Construct a container using 1/2-inch chicken wire:
Make a circle for the bottom of the basket by cutting it out of a roll of chicken wire, using tin snips. A typical diameter would be about 24 inches, but you can make it larger or smaller, depending on the size of the fence.
Cut out a 1- to 2-foot strip that extends all the way around the perimeter of the circle -- depending on the height of the fence. Make the bottom of this strip about 2 inches longer than the top so its ends are angled.
Secure the strip to the base and join the ends of the strip together, using wire ties ; because the ends are angled, the top of the basket will be narrower than the bottom.
Place a basket at the position of each post, set the post inside it and fill the basket with 3/4-inch drain rock. Level the post as you backfill, using a spirit level.
The Wooden Brace Method
Constructing a wood cross brace that you can attach to the bottom of each post is another way to hold fence posts, as long as the ground around each post is level.
Construct a cross with two lengths of weather-resistant two-by-four lumber. Use redwood, cedar or pressure-treated fir or pine. Cut notches in the center of each board with a circular saw before joining the boards so that all four surfaces of the cross are flush with each other.
Nail or screw a cross to the bottom of each post with 3-inch fasteners.
Set each post upright in the position in which you want it, then brace the post with a pair of 2-foot lengths of two-by-four lumber. Place them diagonally from opposite faces of the post to the base so they extend in a direction perpendicular to the fence line. The fence itself supports the posts in the other two directions.