What causes these ridges in shingle courses?

[John Kogel]

The first two pics are fiberglass impregnated shingles on shiplap. The third is similar faults on a different roof, OSB sheathing. The third pic shows damage as well, probably, I'm guessing, from scraping with a snow shovel. Anyway, that was my explanation for the damage there.

Can anyone give a reason for the ridges forming like that? The shingles on the other sides of both these roofs were laid relatively flat. If it was cold when these were laid and the tar strips became stuck, OK, this could cause them to hump up on a hot day. But ........ the pics were taken in cold weather. In hot weather, would the tar strips not let go and allow the shingles to spread?

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[Rick Cantrell]

"Can anyone give a reason for the ridges forming like that?"

Yes.
The same crew that installed the siding, also installed the roof.

[Rick Hurst]

Just looking at that siding long enough and I'd probably slid off the roof.

[H.G. Watson, Sr.]

Would call that: "Shingle buckling over the deck joints."

Movement in the decking - not installed properly (shiplap used, board or skip deck no more than 6" and lapped without spacing for expansion warping, or an initial conditioning issue/MC of the milled lumber, i.e. failure to pre-condition and sticker in advance and going ahead and shingling too soon after installation not allowing for intial conditioning and then re-smoothing felt - OSB or PLY not spaced & clips, moisture content/exposure, direction and span support can also, etc. are some examples) and/or less than flat smooth taught install of felt/tar paper or other (wrinkled underlayment, etc.).

Contraction/expansion movement - moisture & temperature environment in the under deck space (attic, etc.) a leak, etc.

Whatever changes/imperfections under the composition shingles will "broadcast" through them.

Keep in mind that even if everything was done as correctly as possible, Regions with high humidity fluctuation = vulnerability to seasonal buckling events: it is a direct consequence of climate and the nature of the sheathing materials and the nature of compo asphalt shingles. Some visable buckling or picture framing with seasonal shifts with the OSB or PLY of the compo shingles won't be completely able to be avoided (without risking wind-blown lifting/tear off) when there is high humidity fluctuation at the location.

Is why most mfgs recommend against installing asphalt shingles over dimmensional or milled lumber deck and use Plywood with proper spacing and clips as the basis standard and spec proper conditioning of the deck material and stretched smooth felt or wrinkle free underlayment and go on and on about proper conditioning/ventillaton minimizing moisture of the underdeck zone.

Buckles or buckling. You can word search the manufacturer literature or use the term on a search engine (such as: buckling shingles) you'll find the info you need (rather than "ridge" or "ridges").

Here is a link to CASMA Tech Bulletin 9: http://www.iko.com/misc/CasmaPdf/09-Buckling.PDF

[Ken Rowe]

To me it looks like the shingles were probably installed over another layers of badly cupped and curled shingles. That or wet tar paper.

[Jerry Peck]

To me it looks like the shingles were probably installed over another layers of badly cupped and curled shingles.

That's what it looks like to me, the spacing of the ridges are the same as the spacing of the shingles.

[Michael Carson]

I have seen some folks partially remove the original layer of shingles up to a certain point then install the new over the remaining old. From the ridge it would look like one layer but in reality it was 2. Don't know why folks do that.

[James Kiser]

Can you tell if it's 2 or more layers of shingles. could also be a attic ventilation problem.

[John Kogel]

Ok, best answers to date, Pics 1 and 2, Curled shiplap planks, possibly, or 2 layers.
Pic 3, probably 2 layers of shingles on OSB.

Rick, the siding is Rustic bevel cedar, very popular in 1975.

[Ken Rowe]

Ok, best answers to date, Pics 1 and 2, Curled shiplap planks, possibly, or 2 layers.
Pic 3, probably 2 layers of shingles on OSB.

Rick, the siding is Rustic bevel cedar, very popular in 1975.

If the house was built in 1975 it's probably 2 layers on plywood. Original layer of shingles was probably about 20 years old when they put the second layer on it. That layer is now 15 years old.

[H.G. Watson, Sr.]

Nail/staple pops/lifts can also cause that effect (tabs to buck).

Again whatever movement/changes/imperfections under the composition shingles can/will broadcast through them.

[Door Guy]

My guess is a two layer roof that should have had the first layer removed.

[Jim Hintz]

In the instances where I have come across this condition, it was caused by overlaying cupped/curled shingles with another course of new roofing.