As an investor who operates in dollars and cents, a roof gets replaced only when it must.
But, here are the things to consider:
1) Strength of the roof structure.
Can the rafters bear the weight of another layer of shingles? Is the roof structure made with 2×4 or 2×6 rafters? If it’s 2×4’s, are there strongbacks installed or wood trusses?
Prepare ahead of time so you’re not dealing with damage, money, and headaches later.
2) Condition of the bottom layer of shingles.
If the first layer of shingles is REALLY worn out, you won’t want to put another layer of shingles on top. It’ll accelerate the wear and tear so just tear them off.
3) Type of shingle material on the bottom layer.
If the first layer is wood shingle, you won’t want to put an asphalt shingle on top. If you ever sell the home, this may become a problem with your buyer’s homeowner’s insurance company.
4) Number of layers already installed.
Depending on the area of the home, you may not be able to add a second layer (a few allow up to 3 layers). Again, make sure your repairs and upgrades don’t become a bottleneck in the future sale of the home.
