Why You Should Not Put a Metal Roof Over Asphalt Shingles
Metal roofing is one of the best investments a homeowner can make. It lasts 40 to 60 years, handles East Tennessee storms better than asphalt, reduces energy costs, and can lower your insurance premium. We install metal roofs every week and stand behind every one of them.
But there is a shortcut that some contractors offer — and some homeowners request — that undermines everything a metal roof is supposed to deliver. That shortcut is installing the metal panels directly over the existing asphalt shingles instead of tearing them off first.
The pitch sounds reasonable. You save on tear-off labor and disposal costs. The job gets done faster. The old shingles provide an extra layer of insulation. It sounds like a win all around. It is not.
Trapped Moisture Destroys Your Roof Deck
This is the most serious problem, and it is the one that causes the most expensive damage.
Metal conducts temperature differently than asphalt. When outdoor temperatures drop at night, the metal panels cool rapidly. Warm, moist air from inside the home rises into the attic and hits the underside of those cold metal panels. The result is condensation — water droplets forming on the metal surface and dripping down onto whatever is below.
On a properly installed metal roof with a clean deck and vapor barrier, this condensation is managed through ventilation and underlayment. On a metal roof installed over old shingles, the condensation drips onto the shingles, which absorb the moisture and hold it against the roof deck.
Over months and years, this cycle of condensation and absorption creates a permanently damp environment between the metal panels and the decking. The shingles deteriorate. The decking rots. Mold grows. By the time you notice a problem — usually a leak or a mushy spot in the ceiling — the damage is extensive.
We have removed metal roofs that were installed over shingles and found the decking underneath completely rotted out. The homeowner paid for a premium metal roof and ended up with a structural repair bill on top of a second roof installation. They spent more total than if they had done a proper tear-off and metal install from the start.
Old Shingles Create an Uneven Surface
Asphalt shingles are not flat. They overlap, they curl with age, they buckle in spots, and they have raised edges where tabs meet. When you install metal panels over this uneven surface, the panels do not sit flat.
The result is a condition called oil-canning — visible waviness in the metal panels that looks like ripples or dents. It is not structural damage, but it makes an expensive metal roof look cheap. Once oil-canning occurs, it cannot be fixed without removing and reinstalling the panels on a flat surface.
Beyond appearance, an uneven substrate creates gaps between the metal panels and the surface beneath them. These gaps allow wind-driven rain to penetrate, create channels for water to travel horizontally, and reduce the panel's resistance to wind uplift. The metal roof looks solid from the outside, but its weather protection is compromised at every point where the panel does not sit flush.
Proper metal roof installation requires either clean decking or a system of furring strips and battens that creates a level surface above the old roof. When contractors skip the furring strips to save time and money, every problem with the uneven surface gets worse.
Manufacturer Warranties Get Voided
Metal roofing is a significant investment — $14,000 to $28,000 for a typical East Tennessee home. One of the things you are paying for is a manufacturer warranty that can run 30 to 50 years on the panels and coating.
Many metal roofing manufacturers explicitly void their warranty if the panels are installed over existing shingles. They know the problems it causes and they do not want to cover damage that results from improper installation. When you skip the tear-off, you may be giving up the warranty protection that justified the premium price.
Before agreeing to a metal-over-shingles installation, ask the contractor to provide written confirmation from the manufacturer that the warranty will remain valid. If they cannot provide it — or if they dodge the question — that tells you everything you need to know.
Fire Risk Increases
Metal panels themselves are fire-resistant. That is one of their advantages. But old asphalt shingles are combustible, especially after years of UV exposure have dried them out and made them brittle.
When a metal roof is installed over old shingles, you have a combustible layer sandwiched underneath a fire-resistant surface. In a fire scenario — whether from an external source like flying embers from a wildfire or brush fire, or from an internal source like a chimney issue — the shingles beneath the metal can ignite. The metal panels trap the heat and make it harder for firefighters to access and suppress the fire underneath.
This is not a theoretical concern. Fire departments in wildfire-prone areas specifically warn against having combustible materials trapped under metal roofing.
You Cannot Inspect or Repair the Deck
Just like with a shingle overlay, installing metal over shingles means you never see the condition of the decking. Rotted plywood, water-stained sheathing, failed flashing around penetrations, improper nailing from the previous installation — all of it stays hidden.
A metal roof is designed to last 40 to 60 years. If the decking beneath it is already compromised when the metal goes on, you are building a long-term roof on a short-term foundation. The decking will fail before the metal panels do, and fixing it means removing the entire metal roof, tearing off the shingles, replacing the decking, and reinstalling the metal. You pay for the metal roof installation twice.
It Defeats the Purpose of Going Metal
Homeowners choose metal roofing for specific reasons — longevity, storm resistance, energy efficiency, low maintenance, and insurance savings. Every one of those advantages is diminished when the metal is installed over old shingles.
Longevity drops because the deck beneath is compromised. Storm resistance drops because the panels do not sit flat and gaps allow water intrusion. Energy efficiency drops because the old shingles interfere with proper ventilation and reflective performance. Maintenance increases because moisture problems create issues that require attention. Insurance companies may not offer the full metal roof discount if the installation does not meet manufacturer specifications.
You are paying metal roof prices and getting overlay roof performance. That is the worst of both worlds.
What Proper Metal Roof Installation Looks Like
A quality metal roof installation starts with a complete tear-off of the existing roofing material. Every shingle, every piece of underlayment, every piece of flashing comes off. The decking is inspected square foot by square foot. Rotted or damaged sections are replaced with new plywood or OSB.
New ice and water shield goes down in the valleys, at the eaves, and around all penetrations. Synthetic underlayment covers the entire deck. Drip edge is installed at the eaves and rakes. Then the metal panels go on — over a clean, flat, solid, dry surface with proper ventilation above and below.
This is how every metal roof manufacturer designs their system to be installed. It is how the warranty remains valid. It is how the roof delivers 40 to 60 years of performance. Anything less is a compromise that will cost you more in the end.
The Real Cost Comparison
A metal-over-shingles installation might save $2,000 to $4,000 compared to a full tear-off and proper install. On a $20,000 metal roof, that is a 10 to 20 percent savings.
But factor in a voided warranty, a shortened lifespan from moisture damage, potential deck replacement in 10 to 15 years, reduced insurance premium discounts, and lower resale value — and that $3,000 in upfront savings costs you $10,000 or more over the life of the roof.
Our Policy
At Renovation Revelation, we do not install metal roofing over existing shingles. Period. We know the problems it causes, and we will not put our name on work that we know will fail prematurely.
When you hire us for a metal roof, you get a full tear-off, a thorough deck inspection, all necessary repairs, proper underlayment and flashing, and a metal roof installed exactly to manufacturer specifications with a valid warranty.
It costs more upfront than the shortcut. It is worth every dollar.
Call (423) 494-4670 for a free roof inspection and metal roofing estimate. We serve Knoxville, Powell, Farragut, Oak Ridge, Clinton, Jacksboro, and all of East Tennessee.