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Asphalt Composition Roofing Shingles

 

Problems with Laminated Asphalt Composition Roofing Shingles

(March 2013)

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The most popular roofing shingles in Southern British Columbia in the last 10-15 years have been Laminated Fibreglass Composition Shingles, also referred to as "Architectural High Definition" and "Sculpted Tab Composition Shingles".

Traditional composition shingles such as the standard 3-tab shingles have a single layer of fibreglass mat dipped in asphalt and coated with granules. The laminate shingles have two (some have more) layers of shingle material, with the outer surface layer having large cut-outs or spaces between the tabs. This gives the shingles a deeper 3-D look.

The net effect is that these shingles are heavier and thicker than traditional shingles and generally have longer service life ratings . . . typically 25 years or longer.

Asphalt Composition Roofing Shingles

In the last five years or so, the inspectors at Metro Home Inspections have noticed that a large percentage of roofs older than 5 years old (and some as little as 2 years) have areas where the roofing nails are severely rusted both at the nail heads (concealed under the overlapping shingles) and along the shank of the nail where they go through sheathing and are visible in the attic. Usually there is a mildewed stain halo around the nail. When the stained areas are tested with moisture meters the plywood or chipboard sheathing invariable tests wet.

The mouldy-wet areas gradually expand until there is significant mould coverage on the underside of some parts of the sheathing well before the end of the expected service life of the shingles and before the shingles have the appearance of significant aging. (See our comments on Moulds in a separate technical bulletin.)

Asphalt Composition Roofing Shingles

Clearly the nails are getting wet and water is penetrating the roof assembly. The problem is that we are not sure why, and there appears to be no consensus within the roofing contractors community.

We have observed:

How is water getting under the shingles to the nails? We have read some who speculate that:

  1. Water migrates from the side edge of the shingle
  2. Water seeps upwards from the lower "butt" edge by capillary action.
  3. Condensation causes water to condense and some how water is drawn between the shingles.
  4. Water migrates directly through the shingle material during prolonged periods of rain . . . which is all to frequent in the Pacific Northwest.
  5. Some sources are blaming the nails themselves.

Traditionally, roofing shingles were hand nailed using hot-dipped galavanized nails. Most of the composition shingle roofing is now applied with power nailers that use electroplated zinc nails. The zinc coating is much thinner so that nail rusts more easily. This has never been considered a problem since the nails are not suppose to get wet.

Some sources speculate that the adhesive tar strip which makes the shingles self-sealing traps water. Some manufacturers have continuous tar strips and some have gaps in the strip that are suppose to let water drain out. But we haven't noticed a correlation between the type of adhesive strip and rusting nails.

What we think is that the nails should be driven so that the nail heads are embedded in the tar strip under the overlapping shingle. We have not noticed any rusting of the nails when this is the case.

However, none of the shingle manufacturers are currently specifying this installation procedure and trying to drive the nails in a precise line on the shingles slows down the productivity of the roofer.

Currently a lot of home inspectors are assuming the rusted nails are due to condensation except that condensation tends to act in a more uniform way over larger areas. We think that they are wrong.

At this point we can only caution and advise our clieints as to the current situation.


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