Flashing for a cedar roof

Choosing the right flashing for a cedar roof can be tricky. Here's advice on making a wise choice.

I am installing chimney flashing on a roof with cedar shingles. Since I can’t use copper due to the extracts in the cedar, and aluminum will corrode in contact with wet mortar, it looks like the best option is galvanized steel. Would you agree?

That depends.

Flashing repairs are common, that much is clear. I’m sure we’ve all seen chimneys whose flashing has been repaired with gobs of tar, roofing cement, caulk or some other nondescript goop the homeowner happened to get his hands on.

Using the right metal in the first place is probably the simplest and most economical line of defense against flashing that springs a leak prematurely.

Copper may be fine after all

There’s something charming about copper, especially as it develops its characteristic blue-green patina. But you’re wise to wonder about it.

Although it’s a robust metal with a long history of exterior use it’s not bullet proof. If you’ve ever seen copper plumbing develop pinhole leaks from acidic well water you’ll know what I mean.

The Cedar Shake and Shingle Bureau, an industry trade group, says copper can be used as flashing on a cedar-shingle roof – depending on where you live.

If the house is in an area exposed to acid rain—that’s east of the Great Lakes—then copper may not be the best choice. In those situations, the bureau says copper reacts to both atmospheric acidity and the extracts in cedar and may corrode.

You’re better off with painted aluminum or stainless steel flashing.

Unpainted aluminum will probably react unfavorably with the mortar in your chimney, which is alkaline. So use painted aluminum, or even better aluminum that has been coated with Kynar.

As to stainless steel flashing, it may be wonderful stuff. But I’ve never seen it in Home Depot, Lowe’s or the lumber yard down the road.

There are other choices, too

You have at least three other choices – galvanized steel, lead-coated copper and plain old lead.

Of the three, galvanized steel is probably the most responsible choice. Protected by a layer of zinc, it will offer good corrosion resistance under a variety of conditions.

More important, it doesn’t pose the potential health hazards shared by any building material containing lead.

And that’s too bad, because there’s a lot to like about lead flashing. It’s not available everywhere (the Roofing Contractors Association doesn’t even list it in its four-volume manual on roofing).

Lead flashing is very soft, so it’s easy to form around the complex angles and shapes of a chimney base. But it stands up to weather beautifully and it has a nice, mellow look.

I can buy it locally, but I suppose there will come a time when lead and lead-coated copper flashing goes the way of lead solder, lead plumbing and the dodo bird. Maybe that’s a good thing. Lead is a genuine health threat, especially to children.

Make sure the flashing is thick enough

Whatever you use, observe these minimums on thickness, courtesy Joan Crowe, the director of technical services for the roofing contractors group:

• Galvanized and stainless steel: 26 gauge.
• Aluminum: .032 in.
• Copper and lead coated copper: 16 oz.

Those are minimum numbers. If you live in a harsh climate, think about buying thicker material.

Observing these cautions should keep your chimney leak-free and prevent another humiliating trip up the ladder with a can of roofing cement.

Scott Gibson is the former editor of Fine Woodworking magazine, and a former editor at Today's Homeowner and Fine Homebuilding magazines. He is an avid and accomplished woodworker and carpenter.