Fact: Roofing is expensive.
Therefore, shouldn’t we try to save a little money by adding one more layer of shingles on top of the old shingles? If we double the amount of shingles, we have double the amount of roof protection…Right?
Wrong! There are many very important reasons why layering asphalt shingles is not a good idea.
1. Structure. Roofs are designed and built to carry a certain weight load (typically, one layer of asphalt shingles). Asphalt (or “composition”) shingles are very heavy. For instance, modern laminated shingles weigh around 350 to 450 pounds per 100 square feet (one square) on average. If you start adding additional layers of asphalt shingles, you will dramatically increase the weight load bearing down on your roof decking and joists. Imagine the excessive weight of several layers of asphalt shingles and then add on to that a good amount of wet snow. This can, and has, lead to catastrophic roof and building damage. If you drive through a neighborhood with houses in the 100 year old plus range, you will be able to find roofs with roof decking that is visibly bowed in toward the attic. The bowing is a result of excess weight from multiple layers of shingles. Roof decking simply cannot handle that much extra weight, and it will become concave over time.
2. Warranty. Shingle manufacturers will not provide a warranty if their product is applied over old roofing material. New shingles will not seal properly over old shingles. Shingles are manufactured with an adhesive that is engineered to glue itself to specific underlayment after being exposed to warm summer temperatures. Also, roofing material will fail quicker and have a reduced life if installed over less than ideal underlayment. Do not look to the manufacturer to stand by their product even if the material has a legitimate defect.
3. Existing Roof Problems. If your roof is leaking now, chances are that it will continue leaking if another layer of shingles is installed on top of the old shingles. Again, the adhesive on the shingles will not create a watertight seal. In addition, roof leaks will damage roof decking and joists. Damaged roof decking or joists cannot be identified and properly repaired unless the old roofing material is removed prior to installing new shingles. Covering up damaged roof decking will only create bigger, more expensive problems down the road.
4. Cost. If one were to add several layers of shingles on their roof, eventually there will be a time when all the layers will have to be removed. The older shingles are, the more difficult it is to remove them. The harder they are to remove, the more you will pay for labor. It will not only cost more to remove the shingles in the future, it will also cost more to repair the damage caused by the excess weight. Roof deck damage is almost certain to occur with layered shingles. Most likely, the entire roof deck will have to be replaced. This is not cheap by any means.
5. Building Code. Minimum building code, enforced by your local municipality, has been changing over time to help eliminate the problems and dangers caused by layering roofing shingles. Most municipalities in Wisconsin (and other states) will no longer allow more than two layers of shingles on a roof. Check with your local building inspector prior to adding layers of shingles to your roof.
When weighing the pros and cons of purchasing a new roof, take extreme caution if you are considering adding multiple layers of asphalt shingles to your roof. More than likely, you will compound problems, create an unsafe weight load on your roof, and it will end up costing you more in the long run. My recommendation is that, under no circumstances, should multiple layers of asphalt shingles exist on a roof.
About the Author:
Pro Roofing, LLC is a Licensed and Insured Roofing Contractor Located in Madison, WI. Pro Roofing, LLC has been Wisconsin’s Premier Residential and Commercial Roofer Since 1999.