What Causes That Fish Mouth Appearance in a Shingle Roof?

Posted by in Buying/Maintaining a Home, Construction & Shortcuts, Roof, Shingles | July 10, 2017

Sometimes a shingle roof will exhibit a pattern of shingle tabs raised at their center, such as seen in the featured photograph.  That is, the shingle tab is partially sealed at each end, but raised in the middle (Figure 1).

Figure 1 Tab raised at center and partially sealed at ends.

Hand lifting of the tab shows that it is raised because of a raised nail head.  In Figure 2, the nail was driven at an angle and the raised edge was bent over.  The raised edge of the nail head does not allow the tab to lay flat and seal properly to the underlying shingle strip.

Figure 2 Raised edge on nail head due to nail being driven at an angle.

Industry standards for asphalt shingles recommend that the nail be driven perpendicular to the surface such that the nail heads lay flat on the shingle.  A nail driven at an angle does not comply with industry standards.

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