What would cause this?

Yea Kenton little baby granules adhere to the momma metal shingles.

What’s a Meatal, BTW?

From a distance they can appear as cementitious shingle.

http://www.englertinc.com/roofing-shingles.aspx?Page=2

Meatal is like how meat turned out after my ex-wife got finished “cooking” it. You didn’t so much chew it as wear it thinner over time. Or vice versa.
If those were actual meatal shingles on Jeff’s roof you’d see the corners sticking up all along those ridges. Kind of like roofing a home in what had once been perfectly good pork chops.

To all:
In the first picture two of the three vent pipes are off plum. Would this have something to do with the ridges in the tile?

I have seen this on several houses over the last couple of months. These lines are typically caused by a rake wall being built a little too high and/or not allowing the rafters (or trusses) to deflect the same as the adjacent rafters (or trusses). This will typically occur at shear walls. I have found that a slight rise can been seen at the plate line perp. to the rafters. This can be the result of framing the rake wall and/or a raised area in the slab on grade. The raised slab is typically due to trying to finish around anchor bolts and the footings below.

The interesting part of this type of condition is that it is only noticed with flat tile. There will be an entire subdivision of homes but only the flat tiles roofs are an issue. The horizontal lines formed by the flat tile make for a great way to highlight the variations.

This is a non-structural issue. It is aesthetic only. Solutions have ranged from “do nothing” to jacking up the adjacent members and reinforcing them, to “remove flat tile and replace with ‘S’ tile.”

I know one truss manufacture provides a disclaimer in their contract for this very condition.

Thanks Chris… and to everyone that put in their 2 cents. I appreciate it very much.

Jeff

I agree with Cheremie, I think she called it right, it looks like a crowning problem.
Another quick way to check is to take a 78" level or other type of long straight edge (something that will span at least 3 rafters), and check for a crowning problem while in the attic. Some framers are careful to make sure that all crowns are in the same direction, others just throw it together as fast as they can.