5 Reasons To Remove Your Asphalt Shingles When You Have A Metal Roof Installed

One of the benefits of metal roofing is that on some occasions, it can be installed directly over your current roof. This reduces the cost of installation because you no longer have to pay to have your old roof removed. However, it is not always a good idea to install a metal roof over an old asphalt roof. Below are some of the reasons that you should consider having your old roof removed before installing a new metal roof. 

Your Local Building Code May Not Allow Multiple Layers Of Roofing 

In many areas, depending on the slope of your roof, you are only allowed to place two layers of roofing on a home. This is because each layer of roofing adds more weight to your roofing deck, which can eventually cause your roof to fail, especially if you live in a snowy area. If you already have two layers of asphalt shingles, it is likely that you will not be allowed to add another layer of roofing material, even a lighter weight metal roof. However, in some rural areas, if you have a steeper roof, you may be allowed to have three layers of roofing installed. You should check your local building code if you are unsure how many layers of roofing you are allowed to add.  

Asphalt Shingles Can Cause Telegraphing Beneath Metal Standing Seam Roofs 

Over time, asphalt shingles can cause telegraphing in metal standing seam roofs. Basically, dents following the horizontal lines of the shingles will begin to form as the weight from the metal roof presses against the shingles. Structurally, this should not cause too many problems, but it can be visually unpleasant and may require some minor metal roof repairs.

To avoid telegraphing, you can opt for metal shingles, which have horizontal lines in them. If there is telegraphing under a metal shingle system, it is much less likely to be noticeable. 

Asphalt Shingles Can Rub the Protective Coating Off of Steel Roofs 

Metal roofs expand and contract slightly based on the outside temperature. If a metal roof is installed on top of an asphalt shingle roof, the asphalt, along with any nails or staples holding it to your roof, can scratch the underside of your roof. This can wear your roof down over time, eventually causing leaks or holes. Furthermore, if you have a steel roof with a specialized anti-rust coating installed, the scratching can compromise your roof material and lead to eventual rust on your roof. 

Your Contractor Will Not Be Able to Visibly Inspect Your Decking 

If any of the decking supporting your roof is weak or rotting, it should be removed and replaced before your new roof is installed. In some homes, you may be able to check the roof decking from an attic or crawl space. You can also do a touch inspection on top of your asphalt shingles, looking for soft, spongy patches of roof. However, the most thorough inspection method is a full visual inspection after the old shingles have been removed. 

Since metal roofing tends to last significantly longer than asphalt shingles, it is a good idea to make sure your decking is in tact and can support the roof for its lifetime. 

Removing Old Shingles Ensures that Mold and Mildew Will Not Be Trapped Beneath Your Roof 

Sometimes, mold or mildew grows between your roof shingles and is not visible without removing or lifting up your shingles. If you install metal roofing over asphalt shingles with mold or mildew problems, it is likely that the mold or mildew will continue to spread beneath your metal roofing. This can eventually compromise your roof. 

In some cases, it may be safe to install a metal roofing over an old asphalt roof. However, it is important to consider the possible consequences and discuss your particular project with a roofing contractor you trust. 


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