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McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What would cause a new asphalt shingle roof to have loose tab adhesion (sealant strips)?
Saturday, June 29, 2019
When the sealant strip under the leading edge of a shingle fails to adhere properly, the shingles become vulnerable to wind damage from flapping up and down or blowing off. Several warm days with direct sun on the roof are necessary for the initial setting. Sometimes it takes up to a month; so, if the roof was recently installed, you should wait until after that before deciding there’s an adhesion problem.
But, if it has been a while and the shingles still lift up easily, then one of these issues may be causing it:
- Fasteners that are underdriven or driven at an angle create a pocket between shingles that limits the contact area and may keep the overlaying shingle from laying flat. An underdriven fastener can also keep the bottom shingle from sitting flush on the roof deck, which is necessary for proper adhesion.
- Any windblown debris that collects on the sealant strip, such as dust, pollen, or leaves, will reduce the areas of full adhesion.
- There could also be “cohesive failure,” which is defined as failure of the adhesive properties of the sealant after it has set. The defining characteristic of this problem is adhesive goo residue on both the underlying and overlying surfaces of the shingle mats at the adhesive strips, but lack of adhesion and no visible damage.
This may be caused by a manufacturing defect. But excessive heat can be the cause, or a contributing factor, in deteriorating the asphaltic adhesive material. This is usually the result of an unvented or poorly vented attic. An attic with inadequate ventilation reaches 150º F or more on a summer afternoon in our part of Florida. The repeated day/night, hot/cool thermal cycles also cause expansion and contraction of the shingles and the underlying decking, which is detrimental to adhesion. Some thermal cycling is normal on any roof, but it is greatly enhanced by poor attic ventilation.
We recently inspected a roof with cohesive failure where the soffit vents were covered over with insulation, effectively blocking air flow into the attic. The shingles had zero adhesion during the hot part of the day, but re-adhered somewhat by evening. The recommended repair was manually adhering the shingles with roofing cement after providing adequate attic ventilation.
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