Wet Rot

[Kenta]

Hi,
My house is 3 years old and I am starting to see wet rot at the bottom of vertical beams near the front door. The pieces make contact with the door, and they have weather stripping on them. The wood turned dark under the paint, and screwdriver goes right through them. It has some wet rot. Also, the threshold is starting to warp upward. A friend told me that most likely the water barrier under the door was not properly installed. Wood under the water barrier is absorbing moisture and expanding, causing the threshold to warp. Threshold pieces are not securely put in place and water can seep in. They put nails in the threshold to keep it down, but this punctured the water barrier and made the problem worse. Warpage is creating gaps causing the wood to remain wet.

Now my home warranty does not cover wood rot, but I think my case should be, since the problem is caused by improper installation or imcompetence of the workers. Has anyone had to fight a similar battle? I will contact the builders but I want to be reasonable also. What are your thoughts?
Thanks
K