Was on my roof doing some minor tar patching around the plumbing vents and noticed that I could clearly see water standing inside the vent pipe no more than a foot from the top. I'm no plumber but I am sure this can't be right. This is about a 3 inch size vent coming up from a full bathroom. Other than noticing the water in the pipe, I have not had any problems inside the house. Any advice on this and is there a do-it-yourself fix?