5 Layers Under Your Metal Roof & Why Shingles Should Not Be One

As you search for a suitable material for your Charleston-area home’s roof replacement, you think long and hard about metal roofing and your budget. Can a metal roof be installed directly over shingles to save money? What goes under metal roofing? Metal roofing is an attractive, durable alternative to shingles, but it is far more than a mere replacement for them. 

Peeled

If you could peel back the layers of your home’s existing roof, you might be surprised to find all the layers and components beneath the surface. Every roof has its unique qualities, but five layers and parts are typically found under any shingle, metal, or tile roof:

1. Roof deck—The rafters and sheathing frame your home and form the foundation of your roof; these wooden elements are the skeletal support for the other roof layers
2. Ice and water shield—This thick, self-adhesive sponge-like material tightens and forms around nails and staples driven through your roof; this prevents water leaks along valleys and at the edges of your roof, though Charleston homeowners seldom have to worry about wintertime ice dams
3. Underlayment—Synthetic or natural felt in roll form is spread out over your home’s sheathing to provide a water-resistant barrier for the full expanse of your roof; it is stapled down and helps prevent water infiltration
4. Ridge vents and ventilation—Ridge vents run along the highest point of your roof, providing a covered exit for rising air leaving your attic; ridge vents can be supplemented or replaced by gable vents or powered ventilation fans
5. Flashing—Thin, corrosion-resistant metal sheets cover the gaps between unlike materials on your roof, as with the joint around your chimney; flashing is also the general term for any sealing material such as rubber boots around sanitary stacks, flexible plastic curbing around skylights, or even caulking around irregular shapes on your roof

Nothing on your roof is a luxury. Every layer and component plays a part in preventing water from infiltrating your attic. Whether you choose shingles or metal roofing for complete roof replacement, expect your roofer to peel away some of these layers during the tear-off process. Once the sheathing is exposed, your roofer’s highly skilled technicians will determine which sheets must be replaced and which can be re-covered with fresh underlayment. 

The roofing elements keep water from getting into your home, but the final layer atop all those parts plays the most critical role. That layer must:

• Look attractive
• Shed water
• Withstand all types of weather
• Protect against falling tree limbs
• Discourage gnawing rodents and insects

One System

No Charleston-area home should ever have more than one roofing system on it at a time. You may have heard some neighbors claim substantial cost savings in having a new roof installed over an old shingle roof—do not fall for that! 

Several factors work against the idea:

• Local building codes usually forbid two roofs
• The old roof can hide dangerous flaws or defects that go unrepaired when the new roof is installed
• The added weight on rafters and sheathing is something your home doesn’t need
• A new metal roof over old shingles will never fit well; it will decrease both curb appeal and home value

Asking a skilled, professional roofer to lower costs by roofing over the existing shingles, especially when having a beautiful new metal roof installed, is like asking a doctor to put a Band-AidⓇ on a broken arm. 

The old roof may be seriously defective, with a litany of problems:

• Mold-covered sheathing
• Split, insect-damaged, or rotten rafters
• Loose, missing, or broken shingles

When (not if) a water leak makes its ugly, unwelcome appearance inside your home, your roofer will never be able to find the leak source if you have two complete roof systems overhead.

Weight For It

If you are still not dissuaded from asking a roofer to install a fine, durable metal roof over an aging shingle roof, consider the added weight on your home. Rafters are sturdy. Sheathing is strong. But every wooden element has its load limits, and the added weight of a metal roof atop a shingle roof can be too much for your house. 

You might hear rafters crack and shatter when a brisk breeze blows through. You might see a sag in your roof develop over time. You are far better off investing in a new metal roof that sits firmly and securely on your home’s sheathing, with nothing to disturb it. 

Talk with your trusted residential roofer about the best way to install a metal roof. Then watch as your old roof is removed and carted off, a weight off your mind and home. 

Trico Exteriors in Charleston, South Carolina, has the tools, talent, and training to provide your home with a gleaming new metal roof. Contact us today for further information about our roofing services and exterior home solutions, from siding to skylights.