Bulging asphalt roof shingles

In March 2019 we had a new CertainTeed Landmark Pro roof installed. They replaced the OSB decking with new plywood and installed a CertainTeed synthetic underlayment.

We live in the Seattle area and have noticed - 2 months later- several areas where a shingle bulge has developed and is going up coursing towards the second story. It is not exactly a straight line as you can see from the photos. The bulging in the shingles seems connected - that is, it isn’t an isolated shingle here and there showing a bulge.

Is this due to a recent couple of weeks of very high heat we experienced here, or, is this due to poor install issues? We had a Tamko roof for 20 years installed with roofing felt as the underlayment and had not seen this bulging in the old roof.


I’d say the most likely cause is a piece of plywood isn’t nailed down properly. What happens when you step on his bulge? Does it go down? Why was all the OSB replaced anyway?

Also plywood can swell a bit with temperature changes and if the sheets are butted too tight together can on rare occasions cause buckles to occure.

I dont think they used synthetic here.
Is this section the same pitch as the rest of the house?

I think They used Ice and water shield underlayment ( self-adhered)
because of the low pitch on this section.
2 or 3/12 pitch.
This is a vertical wrinkle.
A big vertical wrinkle in the underlayment due to either not pulling it straight or it just wrinkling because of missed debris or sticking up sheeting nails underneath it.

If you renail the deck and dont make sure every nail is flush, then these vertical wrinkles will happen before and after the roof is complete using most brands of self-adhered’s.

They will come and go also
But always in the same spots.
Above a rafter where the sheeting nails were not nailed flush and usually where two pieces of sheeting are butted together at a rafter.
The auto nailer shot his nails at an angle to hit the rafter. This leaves the side of the nail head sticking up.
Many laborers do not take the extra effort to bring out their hammer to make sure all the new and old sheeting nails are flush.
This is what causes this wrinkle.

Hi IslandRoofing,

Thanks for the feedback. We replaced all the OSB due to the large amounts of rainfall we have in this area. We believed that the OSB soaks up water and can cause mold issues. Felt better having plywood.

When we step on the bulge it flattens but bulges back up when we release it.

While OSB may not be a top of the line construction material it’s perfectly suitable for your roof sheeting and the idea it could cause mold issue is complete hogwash.


Hi roof_lover,

Thank you for your thoughts on this.

Our roofer used a synthetic underlayment and the pitch of the lower roof is the same as the upper roof (4/12). The wrinkles or bulging were not there when the roof was first installed. Below is a picture of the work in progress where the red ovals indicate where the bulging is occurring now.

Island is correct. OSB is a great product for Roofs. If the issue wasn’t there when the roof was finished, I’m going with a nailing issue, caused by the buildup of heat. As the attic heated up, the decking had some movement at the rafter, causing a little heat bubble in the underlayment. Now why doesn’t it do it between the rafter joints, I don’t know. I’m just thinking what I would do if you ask me to come look at it.
One last thing I forgot about. Good job climbing on the roof with those slip on dress shoes. Hee, Hee… That part was joke.

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I am suspecting a ventilation issue being as there is a head wall with no vents under it. This is typically the least vented section on a roof. There is intake at the eaves and nothing at the head wall. I’d be curious to know if the bulge roughly followed a plywood seam.

The pics are great.
Ok, i see no ice and water shield was used.

Personally, i think installing synthetic
On a 4/12 should be a crime
But i understand it is accepted by just about everyone.
I also think that two story needs a gutter.
The drip line under the two story will be one of the first place to have a problem.
This section should have been two-ply or ice and water shield due to the second story roofs water crashing into it.

All this said, the issue is sheeting nails not flush at a rafter and most likely where two sheets butt together.