So how deep should you make the post holes?
Simple you want around a third of the post in the hole. So for a 6' (1.8m) fence you would need at least 8' (2.5m) posts. With a third of the post in the hole your fence will be very sturdy, any less and your posts can work free and lean over. If you use a solid concrete base you may be able to get away with only a quarter of the post in the ground but remember the leverage at the top of the post is now allot greater.
So once you have used your auger to dig a 3' hole what next?. Well in my grandfathers day he would have placed the post in the hole and earth rammed the dirt around the post. This is still a great way to hold a post in the ground and earth ramming can be as sturdy as any method perhaps even more so. However it is time consuming and hard work.
Personally I love rapid set concrete. For a standard post hole use 11/2 20kg bags of the stuff. The instructions on the bag says to use two bags but I mix in some of the soil removed from the hole and is works very well.
First place the post in the hole and make sure it is the right height. If it needs to be a little higher lift the post out and put some soil back in the hole. Of course if it needs to go lower you have two choices, make the hole deeper or shorten the post :-).
For the end post place the post in the hole so that the outside surface of the post forms the line of the fence in at least one direction. If its a corner post then make sure it lines up in two directions. Pour some water in the hole, about 2.5 litres and 3/4 of a bag of rapid set concrete. Using a crowbar mix the water and concrete by plunging the bar in and out of the hole without moving the post. Next put some soil in the hole all around the post and mix again. We are not looking for a perfect mixture just make sure the soil and concrete are moist. Now level the post, make sure it is still in position and perfectly vertical in both directions.
Use a spirit level or plumb bob they give the best results.
Keep the post vertical
Pour in some more water and another 3/4 of a bag of concrete. Mix a little with a crowbar and top with soil, check the post is still vertical and press the soil down around the post with your boot or post hole rammer (piece of 4"x2" wood). That's the first post done.
Do the same with the other end posts and any bend posts. The concrete is still going to be wet so either call it quits for the day or brace the post in the vertical position so you can string a very tight string line between the posts. A diagonal brace from a stake hammered into the ground to 2/3 of the way up the post along the line of the fence works best.
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