When replacing your roof because of damage and/or age, most building codes allow you to put a fresh layer of shingles on the old one. It may seem like this would save you money. However, after further analysis, maybe not.
Many building codes allow only two layers of shingles on your roof. However, this limit only applies if your roof has a 4:12 (1:3) pitch. This means that for each 3 feet of roof, the rise of the roof can be only 1 foot.
When you have multiple layers on your roof, the added weight puts stress on your roof decking. An average roof of 30 squares adds about 9000 pounds to your roof. That is equivalent to 9 one-half ton vehicles on your roof.
In addition, having multiple layers on your roof makes the roof hotter because the heat is trapped in the layers. This can cause inflated air-conditioning costs and the premature aging of the shingles.
Having multiple layers on your roof can also affect any inspection reports that you get. If you try to sell your home, the home inspector will probably negatively view the layers on your home. Layers can also void your warranty.
Having multiple layers on your roof can prevent the detection of leaks or other roof damage. Whether the problem is your flashing or rotted and damaged wood, you cannot prevent or fix a problem if you cannot see it.
Since shingles are made for flat surfaces, they are less than effective when covering gaps, homes, or dips. Overlapping shingles creates an uneven texture, and if any of the existing shingles are misshapen, that flaw multiplies with layers.
Layering of shingles might look attractive to you, especially if you will do the work yourself. Having multiple layers of roofing does not make your home more waterproof. In fact, layering often creates problems instead of solving them.
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