Q & A - Rusty Roof Flashings

Noticing rust on the roof flashings, Georgia wanted to know what was causing this to happen.

Georgia from Topeka asks, "What is causing the rust on my roof flashings?"

Ted and Georgia,

I am not sure what is the case with your roof, but here is what could be the issue. A roof flashing is a metal covering that is weaved into the roofing materials and a protrusion coming through the roof to keep the roof from leaking. It is common to have pipes removing hot air from a water heater, HVAC or kitchen and bathroom exhaust fans. There are also plumbing vents from 1 1/4 ” to 4″ round that comes through the roof. All of these need what we call roof flashings, roof jacks, or roof safes just to mention the common names.

These metal flashings are made of steel that is galvanized and is a ferrous metal that can rust. They are also made of aluminum, lead and copper which are nonferrous. Nonferrous metal won’t rust but costs more money to use. Steel, even with a galvanized coating and paint, will rust with the right conditions present. The main reason I have seen for rust is when a galvanized flashing is used for a hot water heater or HVAC exhaust pipe and has not been painted, or the paint has worn off. The hot air when it meets cooler air causes condensation, as seen on the pipe in the blue circle. These droplets of moisture can cause the unprotected steel to rust over a period of time, even if it is galvanized. This roof was about twenty years old that was removed. A new aluminum flashing has been painted and ready to install on the double-wall hot pipe.

How do I tell if the pipe is hot?

The building code requires that a pipe extends higher out and away from the roof elevation if it is carrying gas fumes, like from a gas water heater. These flashings are taller and typically are made from aluminum so they resist rusting. Most folks know where the water heater is and can see the flashing on the roof.

A rusty flashing can be signs of a possible leak and needs to be looked at. I advise replacing all flashings when doing a roof replacement. Paint them well and use the correct type of flashing on the hot exhaust pipes, so you have many years of a trouble-free roof.

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