Shingle Roofing System Myths

by Charles McGarvey

If you are a steep slope roofing contractor, who installs asphalt-fiberglass shingles, you may or may not have heard of a shingle “Brittle Test.” Some contractors use this test to convince a potential customer that their existing shingle roofing system is so aged and beyond repair that it must be replaced.

While there are variations of the test, it consists of a roofing contractor lifting the bottom edge of several shingles on an existing roof and bending them upward several times. The contractor then inspects the shingles for cracking, creasing, deformation, or splitting. He then makes an arbitrary and subjective decision as to whether the shingles are so brittle that repairs are impossible and the only solution is to replace the roofing system. They blame the loss of material flexibility from aging, poor manufacturing, mechanical damage, poor attic space ventilation, and even the heat from a nearby fire.

However, there are no definitions within building codes (i.e., IRC, IBC, IEBC), industry standards (i.e., ASTM, ANSI, etc.), or any shingle manufacturer definitions or specifications of a “Brittle Test” for installed shingle materials. Moreover, asphalt based shingles are not elastic materials but are classified as viscoelastic, and their flexibility is affected by temperature. Thus, they are not expected to immediately return to their original shape after a force has been removed. Furthermore, the greater the upward force applied during the test, and the steeper the angle the shingle is lifted upward, the greater chance the shingle will be irreparably damaged. This is especially true as temperatures fall or the roof is shaded, as the waterproofing asphalt binder is stiffer. So, I’m not sure if any contractor performing this test, as evidence that a new shingle roofing system must be installed, has ever found any shingles which have passed the test.


What about a customer who calls a roofing contractor to replace several damaged or missing shingles because of a windstorm or mechanical damage from a tree limb that has fallen onto their roof, but after a roofing system inspection the contractor states the entire roofing system must be replaced. Why? Not because similar-looking shingles are unavailable, but because the shingles on their roof are manufactured as smaller english-sized units and only larger metric-sized shingles are available at his supplier. And since the sizes don’t match up, shingle repairs cannot be properly completed, so the entire roofing system must be replaced. Could this statement be true? The answer is “no.”

Trimming a similar-looking larger metric shingle to become a smaller size english unit shingle for a repair is straightforward. First, you remove a shingle from the roofing system. Then, overlay the smaller size original shingle onto the center of the larger replacement shingle to match the weather exposure line or depending on the similarity of the shingle – the nail line. Trace the perimeter of the smaller shingle onto all four sides of the underlying larger shingle. If the tracing is correctly trimmed, the larger shingle will now match the size of the original smaller shingle, and the nail placement line will be in the correct position for attachment. When confronted with this technique, a contractor may continue to protest that the roof must still be totally replaced because trimming a shingle shortens the life of the material. However, has the contractor forgotten that shingles must be trimmed as starter courses, first courses during recovery projects, around soil piles, against walls, in valley construtions, or along ridges and hips?


Another myth regarding the need to totally replace a shingle roofing system is when one hip section needs replacement, but other areas of the roof range from fair to good condition.

For example, what if one hip section of an existing shingle roofing system needs to be replaced because it has been damaged from winds, an impact from tree limbs, or is weathered with excess granule loss because it is south facing*. Does this mean the whole roofing system needs to be replaced? Well, according to some roofing contractors, the answer is “yes,” because, in their opinion, the installation of a new shingle underlayment on one existing hip section cannot be properly joined into the other side of the existing hip shingle roofing system without total removal of the entire shingle roofing system. And because building codes require roofing systems to be installed to manufacturer specifications, including the installation of the underlayment to join (overlap) hip roof sections together, it would thus be a violation of building codes to only replace one hip section of a roof, so the entire shingle roofing system must be replaced.

*Note – While the sun rises in the east and sets in the west, the sun is always south-facing for any building north of the equator. So, the southern facing exposure of a shingle roofing system receives full sunlight with the harmful aging effects of heat and U.V. rays to shingle roofing materials from sun-up to sun-down. This is why southern facing exposures of shingle roofing systems may age faster than the east, west, or northern sides. Furthermore, northern roof exposures do not receive direct sunlight, which is one of the reasons why moss, fungus, and lichen do grow on a north-facing surface, as there is no direct sunlight with its destructive U.V. rays that would inhibit their growth.

The reason shingle underlayments are needed includes fire code requirements, a slip sheet between the roof deck and shingles, and the ability to act as temporary roofing before shingles are installed or lessen the effects of interior leakage if any shingles on a completed roofing system are damaged or missing before repairs are made. For these reasons, shingle manufacturers specify that an underlayment must be placed over the roof deck or an approved substrate before the shingles are installed.

However, it should be understood that building codes and shingle manufacturer underlayment requirements are detailed for new building construction or total removal of all shingles from a complete roofing system. Neither address a shingle hip roof replacement that ties into other roof sections not being replaced, but both allow repairs or partial replacements of shingle roofing systems. And any experienced and knowledgeable shingle roofing contractor knows several techniques that can be used to maintain the underlayment hip integrity without having to remove other adjoining roof areas.

Furthermore, several roofing material manufacturers now supply attic space exhaust hip vents that are similar to continuous ridge vents. They can be installed during a new shingle roofing system installation; a complete reroofing project; when there is replacement of one side of a hip roofing system; or added to an existing shingle roofing system. With a hip ventilation system there is no hip underlayment integrity, as there will be a continuous slot or numerous holes cut through the hip roof sheathing and underlayment, so attic space exhaust ventilation can occur through the vent.


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