Shingle Overlays—Just Say No Way

Eventually, each owner will need to replace the shingles on his/her residence. At that time, the owner will need to decide whether to install new shingles over the existing shingles, known as overlaying, or to strip all of the shingles off to the roof deck before installing the new shingles. For the former, the existing flashings are also normally kept or supplemented. For the latter, flashings are normally replaced, as well as the underlayment. I will state up front that my firm, Criterium-Cincinnati Engineers, does not support overlaying and this blog lays out our reasons.

Building owners usually only give one reason FOR overlaying shingles and that is the cost is less than fully replacing the existing roof components. On the other hand, roofing professionals and engineers have a number of reasons NOT to overlay. These reasons include the following:

• Total removal of all of the old roof materials allows inspection of the roof deck, valleys, joints between the roof and walls or chimneys, and areas around roof penetrations, such as sanitary or roof vents. This inspection could discover rotted wood, insects, holes and a number of other issues that could compromise the roof. Some might argue that these issues are visible from within the attic. Having been in more than a few attics, I can assure that these issues are not always visible. For some residences, the attic cannot even be entered and in others no attic exists, such as in residences with cathedral ceilings. We believe that a residence’s first roof replacement should particularly not be an overlayment because of the poor quality of some builders.
  

• Total removal of the old shingles can assure that an underlayment is installed. Underlayments perform two main functions: an additional water barrier against leaks in the shingles and separation membrane between the shingles and the roof deck. Shingles installed directly on the roof deck sometimes melt or stick to the deck. But, shingles expand and contract on the roof deck as they heat up and cool down. Shingles stuck to the deck are restricted in their movement, which could result in the shingles buckling or tearing, shortening their life or creating leaks.
  

• One of the most vulnerable parts of the roofing system is valleys. Valley flashings can fail and more than a few of these flashings have also been installed incorrectly. Poorer quality builders may not have installed valley flashings at all, trusting that the underlayment will protect the valley. Instead, shingles are weaved across the valley. (By the way, Criterium-Cincinnati Engineers does not support weaving shingles over valleys because of the increased chance for leaks.) Replacement, rather than reuse, of the valley flashings helps assure that valleys will be adequately protected. If the roof is older, new techniques have been implemented since the old roof was installed that adds better protection. One such technique used by better quality roofers is to install an elastomeric membrane beneath the flashing. Not only does the membrane protect against rain leaks, it protects against leaks due to ice dams.
  

• In some areas of the country, roof areas above eaves are another area that may need extra protection because they are where ice dams tend to develop. These areas are also vulnerable to shingle lifting due to wind. Better quality roofers install an elastomeric membrane as underlayment over this area in addition to the underlayment used on the rest of the roof. Most original roofs do not have this degree of protection.
  

• Rakes, the roof area over the gable, are other areas that are exposed to potential lifting. During overlayments, another layer of shingles are laid along the rake, which raises the edge of the shingles where they are better exposed to the wind and lifting.
  

• Roof penetrations, such as vent pipes, also need to be flashed to prevent water leaking through the gap between the roof deck and penetrating object. But, these flashings can also fail and are sometimes damaged by high winds. When a roof is overlaid, these flashings are usually not replaced. So, what happens if these flashings fails before the second (or third) roof is replaced? Removal of the flashing could damage the roof shingles. The usual patch is to smear the flashing seams with tar or roof caulk. Tar or caulk does not normally last as long as the roof shingles and will have to be periodically inspected and repaired. If a leak develops in the tar or caulk, the usual (wrong) solution is to apply more tar or caulk, and the new material commonly fails quicker than the previous material. Even if the flashing is replaced, the best place for it is in the first layer of shingles, which is virtually impossible when more than one layer of shingles is installed.
  

• Many two-story homes have second-story exterior walls that meet first story roofs. Flashing is supposed to be installed between the wall and the roof to prevent water entering through the gap between the roof deck and wall sheathing. As other internet posters on the subject of overlayments point out, roofers installing overlayments try to reuse the installed flashing by bending and weaving it with the new shingles. Alternatively, they may also pry out the siding, shove new flashing up under the siding and weave that flashing with the new shingles, leaving the old flashing in place. We have also seen where a roofer installed a continuous flashing up under the siding and ran a bead of roofing caulk between that flashing and the new shingles. And we have also seen where new flashing was not installed at all, but the roofer relied on roofing tar to stick the edge of the new shingles to the old ones. None of these alternatives are acceptable, especially if the old flashing leaks.
  

• The addition of a new layer of shingles adds considerable weight to the roof structure. A new layer of shingles weighs about 2 pounds per square foot. Although that amount of weight does not seem like a lot, think in terms of 200 pounds for ONE layer of shingles for a 10 foot X 10 foot area (one square). Add the weight of the original layer of shingles and the total weight of the shingles is now at least 400 pounds per square. Over time, this weight can cause rafters to bow or the roof to sag, particularly for roofs with long rafter spans (distances between supports), rafters that lack ties between opposing rafters, or homes with balloon framing. Likewise, some builders use the maximum spacing between rafters with the minimum thickness of roof sheathing allowed by codes. The result is that the deck sags between the rafters causing the waviness sometimes seen in some roofs. If the roof has developed sags between rafters with only one layer of shingles, what do you think the roof will do with two or more layers? Add an abnormal snow load and the roof could collapse, as some folks found out during the 2010-11 winter.
  

• As stated, shingles expand and contract as they heat and cool. The degree of shingle expansion and contraction with change in temperature further varies with thickness, material composition, manufacturing method, lot and a number of other factors. If one layer of shingles is expanding and contracting differently than the second layer of shingles, shear stresses can set up in the layer that expands or contracts less, which eventually will cause tears in the shingle. If these tears are exposed to weathering, the shingles life can be shortened. We have observed that many builders install thinner, cheaper shingles as the original roof on many homes. Homeowners, on the other hand, tend to install thicker sculptured shingles as the overlayment because homeowners believe they will get longer life from them. Guess what that usually means? Yep, two different types of shingles with likely different expansion and contraction properties that are also were likely not even made by the same manufacturer. Since the lower layer of shingles are thinner, they can expand and contract more than the overlayment shingles, which results in tears in the overlayment shingles.
  

• Multiple layers of shingles add insulation to the roof, which leads to a hotter attic. In turn, a hotter attic leads to the shingles getting hotter. Heat is a major factor in breaking down shingle materials. Further, a hotter attic means greater expansion and movement of the shingles. The result is a shorter shingle life. A hotter attic also means that a homeowner could be paying more for cooling in the summer when the attic heat is transferred into the living space.
  

• We have noted many cases of vertical stacking installation of the shingles. Vertical stacking is where the shingles are installed in eave to peak columns rather than installing them diagonally starting at one of the corners over and eave. Although shingles on properly installed roofs can start curling over time, particularly in overly hot attics, the shingles along the sides of the columns of vertically stacked shingles begin to curl more often and quicker than other shingles. The curled shingles make an uneven base for the new shingles, which creates uneven pressure on the new shingles. In turn, the uneven surface can cause tears in the new shingles exposing the inner part of the shingles to weathering, leading to shortened life.
  

• Overlayment shingles are very often not aligned horizontally with lower layers. This misalignment results in the mid-area of upper shingles arching over the tab ends of lower shingles—again with the uneven base thing, causing the tears, weathering, etc.
  

• Many shingle manufacturers offer a guarantee. However, that guarantee may not apply to overlays.
  

• Dark streaking on roofs is usually caused by a very, very hardy blue-green algae. What happens when these shingles are overlaid with new shingles? That question has not been answered; but, the possibility exists for the algae to spread from the old shingles to the new ones.
  

• Shingles that are failing begin to hold moisture. One sign of excess moisture is moss growing on the roof. When new shingles are laid over shingles that are holding moisture, that moisture has to go somewhere. If a moisture barrier exists below the roof deck, such as closed cell foam and insulation with a vapor barrier, the moisture could become trapped between the moisture barrier and the shingles, potentially creating a wood rot condition. Speaking of the moss, how sure are you that the roofer has bully removed the moss before installing new shingles? Exactly what happens to the organic matter trapped under the new shingles? Organic matter is also going to be in the old shingles from the moss rhizoids (root-like structures) that have grown into and between the shingles.
  

Most roofers estimate that useful life of overlayment shingles can be as much as 25% less than their claimed life, which means that overlayment may not cost less in the long run. For sure, if the roof leaks, the cost of repairing the roof and damaged interior finishes could also vaporize any cost savings from overlayment. The bottom line is shingles overlays just might not be worth the expected savings.

Tags: building construction, home construction, moisture, Moisture intrusion, overlays, roofs, shingles