High winds, snow drifts, flooding, termites, and just plain old Father Time can all be reasons for a sagging or leaning fence. Bad neighbors, big dogs, young kids, and naked sunbathing can be the reason to install a new fence. Both situations will require you to set new fence posts for the successful completion of your goal.
If you don't do it right, not only will your new fence look like Homer Simpson built it, but it will also end up needing to be replaced far sooner than if you had done it right to start with. I will walk you though the correct way to do it. You may be tempted to cut corners or to listen to old uncle Fred and do it the way that sounds easier, but it will all end in tears that way. Trust me.
You will need:
1. Your new fence post. Buy one at least 2' taller that your fence because you will be burying it 18" deep. Also make sure you get one that is pressure treated for rot and termite resistance, redwood, or cedar. Redwood and cedar are naturally rot and termite resistant.
2. 4 wooden stakes 2'-3' long. Vampires can substitute metal stakes if they are nervous.
3. 2 2"x4"s 8' long cut in half.
4. A box of duplex nails. (They have 2 heads so you can pull them out after using them with little effort.) You can substitute a box of deck or drywall screws here as well.
5. A 50# bag of pea gravel.
6. A 1 gallon can of asphalt emulsion.
7. Enough concrete to fill 1.5 cubic feet. That is either 3 60# bags or 2 90# bags.
8. Tools- A hammer for nails or a screwdriver for screws because the other way around would just be silly. A Shovel, a manual post hole digger, or a post hole digging machine from a rental yard. A Cheap paint brush. A wheel barrow and mixing hoe (Focus you sick newbie, focus!), or a cement mixer from a rental yard.
Dig your hole. Either use the hole from the old post that you ripped out as a starter or locate you new one either at your starting point or 8' on center from the last post of the fence you are adding on to. Make sure the hole is 12" in diameter and 24" deep. You can use smaller holes, as small as just double the dimensions of your post, but then you are sacrificing strength ans stability. Not a good idea if the old one was knocked down by mother nature.
Put 6" of gravel in the bottom of the hole. This will allow any water that gets in the hole or absorbed by the post to drain out easily instead of having the bottom of the post sitting in damp concrete or in contact with wet soil for a long period of time. Water promotes rot people.
Post placement. Before it goes into the hole there are a few things to do. 1st is to use a nylon string to determine where exactly in the hole to put the post when you are replacing or adding to an existing fence. Tie it off so that when you put the new post in you can have it touch the line and therefore it will be in line with the others. Skip this step if it is the first post in a new fence. The second thing to do is paint the end of the post that will be inserted into the ground with asphalt emulsion. This will and an extra layer of waterproofing to help ensure a long and happy life for you new post. Only paint the part of the post that will be in the hole plus one more inch for good measure. Also you will need to check the post on 2 perpendicular sides for plumbness with a level. There are levels out there that you can simply drop on top of a post to check plumbness on both sides at the same time.
Secure the post while you mix and pour the concrete. Set the two stakes 3'-4' away from the post on different sides. Either nail or screw the cut 2x4 pieces to both the post and stakes to hold the post in place.
You will need concrete, water and either the mixer or your hoe and wheel barrow. At this point you WILL be tempted to buy the mix that says you only have to pour the dry mix into the hole, and then pour water on top. There is nothing special about these mixes other than their packaging and price. You can do this with ANY concrete mix, but don't fall for it. This method leads to a sub par base for your post. To give you an idea of how poorly it will mix and set, go get some instant oatmeal, pour the hot hot water on it, but don't stir it. Come back in a few minutes and see how well it has turned out. Pretty bad right? Now is that the result we want for our shiny new post? No it is not. Instead, mix it according to the instructions on the package and pour into the hole and let it set. The concrete package will tell you how long it will have to sit (This is the CURE time, and not the SET time. Set time just tells you when you can no longer pour or work the concrete. Cure time is when it has reached it's full strength.) before you can start banging on the post to finish installing the fence. If you start too soon the concrete will still be soft, and you can knock your post out of alignment and throw the whole project out of whack!
One thing you can do to speed this along is to use a product called Rapid Set cement. This stuff sets in 15 minutes, and after and hour you can start nailing, or screwing on the rest of the fence. Just don't mix too much at one time or you will end up with a large rock sitting in the mixer or wheelbarrow after 15 minutes.
Follow these steps and you will end up with a strong foundation for your new or replacement fence.