What's causing this discoloration of a shake roof?

[Kenton H. Shepard, CMI]

Most of this slope looked like this, and it was just this slope. Dark discoloration of the shingles just below the butts of overlapping shingles. No evidence of fire anywhere and it didn't come off on your fingers like soot.

I'm stumped. Anyone know?

[Peter C. Russell]

My guess would be a reaction to the flashing.

What type of cedar is it and what was the flashing material.


Iron Stains
Iron stains may appear in two forms: the reddish brown discoloration caused by rust, and blue-black discoloration caused by the reaction of iron from nails and other metal objects with extractives from the cedar. To prevent these stains, use only hot-dipped galvanized, aluminum or stainless steel nails when fastening cedar.

[Jeffrey R. Pope]

It looks to be pretty standard "aging" to me. Most wood-shake roofing material I see looks like this.

How old is the roof?

[Chuck Evans]

I would have thought it was staining due to fungal growth in the areas that don't dry as fast, but I don't see many cedar roofs around here.

[bjones5]

shakes under the flashing......moisture, combined with no sun turns them black

[Larry D. Kage]

Originally Posted by kshepard

Most of this slope looked like this, and it was just this slope. Dark discoloration of the shingles just below the butts of overlapping shingles. No evidence of fire anywhere and it didn't come off on your fingers like soot.

I'm stumped. Anyone know?

We had that type of leaching from the cedar in the log homes we built. It comes out of the wood and is left when the water evaporates...Tannin, I think. It's been a while.

[Scott Gilligan, CMI]

Almost looks like creosote that they used to put on telephone poles as a wood preservative. It caused a dark staining in cracks and crevices, but would fade due to UV deterioration.

[Steve Dobson]

Originally Posted by lkage

We had that type of leaching from the cedar in the log homes we built. It comes out of the wood and is left when the water evaporates...Tannin, I think. It's been a while.

I agree, I think it is the tannin's in the cedar, leaching down

[James F. McKee]

North side of the roof??

[ROBERT V. YOUNG]

Flashing material.