When our camera was out in the renovation centre, one viewer posed the problem of having a cedar fence with the top foot rotten but the rest in good shape. Of course this was probably caused by a square top cut not being protected by a fence post cap. Look around and you will see that most fence posts either have bevelled tops or caps to prevent standing water and ice from getting easily into the top of the posts.
Now what to do without removing the whole post?
First cut off the rotten part. The secret to cutting a post square when it is out in the yard and you are working on a ladder rather than having a 4x4 conveniently on a shop bench, is to mark the cut line off all the way around the post. If the end of the line doesn't match up with the beginning, keep working at it until it does. If you can't get a pencil straight around the post, don't expect to get a saw to go squarely through the post.
Now cut about 1/2 inch into all four cut lines, right around the post. Then keep working around your post cutting in about 1/2 inch at a time on each successive face. The cuts in opposite side of the post will guide your saw and keep it going level across the post. If you don't understand this, ask the cook of the house how they cut a cake into two level layers. Same technique!
I have tried to design a splicing technique that would be easy for a novice wood worker to rebuild the top of this fence post. The first thought is of course to put a dowel in the top and then simply stand another 4x4 over the dowel, but standing on the top of a ladder it will be almost impossible to drill a dowel dead centre and straight into both pieces of wood.
I opted for a modified mortise and tennon. Tennons are easier to make than mortises, so we will cut a tennon into the solid part of the post and then build up a new top. You will need some 1x4 cedar and some 2x4 cedar to do this job but I planned it for someone that has no more than a saw and maybe a hand plane to work with.
Mark and cut a shoulder on one face of the post that will perfectly match the thickness of the 1x4. Measure your 1x4, don't count on it being either 1 inch or 3/4 of an inch, they vary from mill to mill. Use basically the same "cutting around" technique to get a good straight cut down the post. Start a cut into the layout line across the top of the post, then cut down the line on one side, rock up and over to the other side and cut along the pencil line. Keep rocking from one side, over the top and down the other side as you cut into the post, and your shoulder will end up exactly like your layout line. If you just start at the top and work your way down, I can almost promise you it will veer off into or away from the post rather than following your line.
Now put a 2x4 standing up on the top of the post alongside the 1x4 that is sitting in the shoulder cut. That is where you will mark off and cut the second shoulder. The tenon is not symmetrical nor in the middle of the post. That is because I don't want to have to plane down the centre piece of wood to some precise measurement, too much trouble.
When the second shoulder is cut off, you can put another 2x4 down into this shoulder and glue (waterproof glue or construction adhesive please) and screw the whole assembly together. Countersink your screws a bit on the final side because this 2x4 will be about 1/4 of an inch too thick to match the post. Now you can easily either plane or sand off this excess until it ends up flush with the side of the post. Don't forget to put a fence cap on the top of this new assembly.