previous
  • Basement Remodeling Tips
    Basement Remodeling Tips
  • Energy-Smart Details
    Energy-Smart Details
  • 7 Smart Kitchen Solutions
    7 Smart Kitchen Solutions
  • Hot Water Now
    Hot Water Now
  • Remodeling Articles
    Remodeling Articles
  • Tips & Techniques for Painting
    Tips & Techniques for Painting
  • Video Series: Tile a Bathroom
    Video Series: Tile a Bathroom
  • 7 Trim Carpentry Secrets
    7 Trim Carpentry Secrets
  • 7 Small Bathroom Layouts
    7 Small Bathroom Layouts
  • Custom Flooring Inspiration
    Custom Flooring Inspiration
  • Master Carpenter Videos
    Master Carpenter Videos
  • Video: Install a Fence
    Video: Install a Fence
  • All about Roofing
    All about Roofing
  • Read FHB on Your iPad
    Read FHB on Your iPad
  • Design Inspiration
    Design Inspiration
  • Magazine Departments
    Magazine Departments
  • Clever daily tip in your inbox
    Clever daily tip in your inbox
  • Pro Tool Rental. Learn More.
    Pro Tool Rental. Learn More.
  • 9 Concrete Countertops Ideas
    9 Concrete Countertops Ideas
  • A New Approach to Classic Cabinets
    A New Approach to Classic Cabinets
next

A metal roof over old asphalt shingles?

Q: My house has two layers of asphalt shingles that are in excellent condition, but I’d like to cover them with steel roofing. Is it absolutely necessary for me to remove the shingles?





A: Harrison McCampbell, AIA, roofing and waterproofing specialist, replies: I think that putting a metal roof over shingles is unwise for at least three reasons: rot, rust, and weight. There could be some rotted decking and/or rafters below that may not be visible unless you tear off the old asphalt shingles.

Because your house is in Florida, humidity and condensation come into play. Moisture can collect on the underside of a metal roof (which usually doesn’t have a protective coating) and can cause rust. Even though steel is a galvanized metal, the roof still can rust, especially as close to the coast as your house is. You should follow the manufacturer’s instructions for installing the metal roof, but in general, you need to provide some ventilation under the metal to allow moist air to escape.

Also, two layers of shingles weigh quite a bit. Adding more weight to the rafters probably isn’t a good idea, but that depends on the size and span of the rafters.

For the reasons I’ve noted, I think you should tear off the old shingles, properly prepare the roof decking, and install the metal roof correctly.

Give the finish on your metal roof careful consideration. In a harsh sun environment such as Florida’s, you need to use a metal-roofing finish that will stand up over time. Certain colors, such as red or blue, tend to oxidize more quickly than other colors, and the method of finishing the metal panels is critical. Roof coatings such as Kynar will last a long time and won’t fade, but they’re also expensive.

Installing a metal roof may well be the last roofing decision you’ll ever make, so it’s worth the time, effort, and money to do it right.


From Fine Homebuilding 165, pp. 106 September 1, 2004