Check out the photo below. Do you see the problem?
The white pipe sticking up out of the roof is a plumbing vent. The rubber boot at the base of the plumbing vent is part of the flashing kit for this vent, and the boot has dried out and split. Here's another photo of a split rubber boot, taken from the same roof at a different location.
In preparation for this blog post, I hopped up on my own roof and took a look at the flashing on my own plumbing vents. I bet you can guess what I found.
I ought to inspect my own roof more often. This is what happens to those cheap rubber boots over time. They seem to last about 10-15 years here in Minnesota. Here's water damage at a bathroom ceiling (not mine), which resulted from this defect.
I've always recommended replacing the plumbing vent flashing when I find this defect, but I recently came across a plumbing vent flashing repair device that looked like a cross between a two-piece plumbing vent cap and Darth Vader's helmet. It's called a Perma-Boot®.
It's a simple product that can be easily installed in minutes, and comes with a lifetime warranty. Home Depot sells them for $18.84, which is more than twice the price of the traditional flashing pictured at the beginning of this post, but I think it's worth the price. They come in 1-½-, 2-, 3- and 4-inch sizes, but for the record, the smallest vent that you're supposed to have sticking up through your roof is 2 inches. The concern with smaller vents is that they could get blocked with frost too easily.
The only potential installation issues that I can foresee with this product are with plumbing vents that are too short or too tall. Plumbing vents should ideally stick up out of the roof about one foot, and this is what is required by code here in Minnesota (see 4715.2530). The taller the vent, the greater potential to get blocked with frost. The shorter the vent, the greater the potential to get blocked by snow. If you have a plumbing vent that's too tall for a Perma-Boot, simply cut it down. If you have a plumbing vent that's too short, make it taller.
p.s. - I think "Darth Vader helmet" is a better name than "Perma-Boot".