Often the phrase "roofing begins at the soffit" has been uttered here but it is a really low place where roof's roots dwell the furthest: the sewer.
When in high school student near the Casa Linda/Casa View area of Dallas, the phrase "stink stack" found its way to my vocabulary. The littlest of pipes protruding from the roof are these methane risers. The poop left to drain to the septic systems of Dallas make their final assault, I never noticed that in the word, up through the roof sewer vents before their gravitational descent to the water reclamation site.
From the foundation to the roof, these pipes serve as clean outs and methane gas vents. When cutting in the roofing around one, the roofer is nauseated to the point of either hurrying or taking a break.
This gas more than smells bad, it is corrosive and just any flashing won't do. Galvanized will be destroyed by it. The designers of septic systems wanted the methane out so quickly they made the system work without a top. They let the rain go right in so the gas will not be impeded by a cap.
The plumbers love it because they can stick their snake right in without any trouble. Ha! there is trouble. The snake tears up the lead sewer flashing.
So the rain water doesn't fall between the pipe and the lead flashing, the lead is crimped into the pipe and the rain enters into the septic system without any harm done, unless the plumbers ran the pipe at an angle and didn't glue the fittings together properly. Because the pipes are intended to carry away gases and not water under pressure, the plumbers often hastily glue these pipes together without water in mind. Then the builder has the great idea to make the vent from the bathroom on the front of the house come up on the rear slope or he has several pipes joined into one. Pipes running level sometimes end up with a negative slope because the ground moves, the home sags, or the pipes just bend.
The lead must be crimped into the pipes with some slack to compensate for this movement even, especially, when the pipe is straight down to the foundation.
The roof sagging a little and the lead tight to the top of the pipe is what the roof repairman needs. The result is a torn base on the lead flashing base.
In the recent past, 4 lb. lead was used but today, if lead is used, it is 2 1/2 lb. The old thick lead didn't crimp over but just held snug like the autocaulks builders use today. The ones the builders plumbers use are the cheapest money can buy. Money is an object. These neoprene flashing are totally destroyed in a few years if they are not kept painted. That's why many roofers went to the metal base plate with a gasket. These two type of flashings are also called 3n1's because they can be modified to fit 1.5", 2", and 3" pipes.
In Irving I learned a lesson to not buy these at Home Depot because the gasket wears out in a few years. The ones I get from my roofing wholesalers in Dallas don't.
I never knew why squirrels ate the lead sewer flashings on the roof until I took the lead removal and containment course required by the EPA for people who work on homes built before 1978. Lead tastes sweet. So if you paint the lead stacks the squirrels will not find they as readily.
Now you know why the squirrels turn around after making it three quarters of the way across the street.
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