What has caused this slate problem?

[Kenton H. Shepard, CMI]

What has cause this slate to fail?

[dhelm]

I don't see slate roofing in this area so my answer will be a guess. Looks like spalling caused by moisture infiltration and freeze thaw cycle.

[Larry D. Kage]

Originally Posted by kshepard

What has cause this slate to fail?

We don't see slate around here either but I wold guess rusting iron content, in the slate, expanding and staining downward.

[David P. Valley]

Kent,

I see tree branches.

I'm willing to bet that there was something hanging over this roof that was rusting away.

[Kenton H. Shepard, CMI]

Larry was close... it's called a "mineral inclusion" and it's a mineral impurity of some kind. Since this one is bleeding redish, iron is probably right.

[Frank Albert]

Originally Posted by kshepard

Larry was close... it's called a "mineral inclusion" and it's a mineral impurity of some kind. Since this one is bleeding redish, iron is probably right.

Yep.