J-Channel: Around windows and doors a piece of trim called j-channel is installed. This helps finish the edge of the siding where it abuts the siding courses and also help shed water. The termination of the vertical j-channel on the sides of windows is often a place that allows for water intrusion. Many, many times this is done incorrectly which can lead to issues down the road. The best way in my opinion to terminate this trim is to have a slit in the siding that will force water back to the exterior should it run down the length of the j-channel. In most cases I see the j-channel is terminated at the lower edge of the window frame which allows for a small gap which water can run down behind the lower courses of siding. Most homes these days are wrapped with weather barrier which will help protect the sheathing but not all homes have this. Many times on homes without weather wrap when the siding is removed there is substantial rot at the corners. This can all be avoided by taking an extra 5 minutes at installation and doing it right.
As for the top of the j-channel this should be installed with a “Dog Ear” at the corners. This will force any water into the vertical j-channel and prevent it from getting behind the siding here. A considerable amount of water can run down the siding above this horizontal trim so this is an extremely important detail that should not be done. By combining this installation with the one above it will lead water around the window and back onto the exterior of the siding at the base of the window.
Vinyl siding should not be nailed to the sheathing behind it. There should remain approximately a 1/16″ to 1/8″ gap so the siding can expand and contract without warping. Often times you will see vinyl siding with waves going down the side of a house. This can be caused by tight nails. If too much of a gap is left this can chatter in the wind which can be annoying. Another thing you can do is pull apart the overlaps to ensure that weather barrier was installed below the siding. It can usually be seen if you look between the two courses.
Another area that can lead to rot behind the siding is where the roof edge terminates on the vertical wall. Many contractors will just trim around the shingle profile and call it good. This is a very common place where water will get in behind the siding. The best practice is to cut in some kickout flashing to force water to the exterior. Usually when you see this you will see the other methods insuring a quality install.