You can install metal roofing directly over asphalt shingles in most cases, but the shingles must be in sound condition and you need to follow specific steps to prevent moisture and structural problems.
Metal roofing installed over shingles is common because it avoids the cost and mess of tearing off the old layer. The shingles act as a base, and the metal panels fasten to the roof deck through them. However, this method works only if your existing shingles are not curled, cracked, or missing in large sections. If they are, the metal roof will follow the bumps and gaps underneath, creating weak spots where water can pool or wind can lift the panels.
The process requires proper ventilation planning, correct fastener placement, and flashing details that account for the extra layer. Mistakes at this stage often show up years later as leaks or premature panel failure. Understanding what the job actually involves helps you decide whether to hire a contractor or whether this is a project you can manage yourself.
Key Takeaways
- Metal roofing can go over shingles only if the shingles are flat, intact, and securely fastened; curled or missing shingles mean you must remove the old roof first.
- You must install a moisture barrier (usually synthetic underlayment) between the shingles and metal panels to prevent condensation and rot.
- Fasteners must penetrate through the shingles and into the roof deck itself, not just into the shingles, or the panels will shift and leak.
- Flashing around chimneys, vents, and valleys requires careful sealing because water will flow differently over metal than it did over shingles.
- Most building codes allow this method, but some jurisdictions require a permit and inspection; check your local building department before starting.
Inspect the existing shingles and roof deck
Walk the entire roof in daylight and look for shingles that are curled at the edges, cracked, blistered, or missing. Press down on several spots with your hand to check that shingles are still bonded to the deck and do not move independently. If more than 10 to 15 percent of the roof shows damage, or if shingles are lifting away from the deck, the metal roof will not sit flat and will fail prematurely. In that case, you must remove the old roof before proceeding.
Check the roof deck itself from the attic or crawl space if you can access it. Look for soft spots, water stains, or mold, which indicate rot. Rotted wood cannot support fasteners properly and will not hold the metal roof securely. Small areas of rot can be patched, but widespread decay means the deck needs repair or replacement before any new roofing goes on. This is not a cosmetic issue—a metal roof will trap moisture against a rotted deck and make the problem worse.
Measure the roof dimensions and note the locations of chimneys, vents, skylights, and valleys. These are the places where flashing details matter most and where leaks most often start. Photograph the roof from multiple angles so you have a record of the existing condition before work begins.
Install underlayment and prepare the surface
Lay synthetic underlayment (also called synthetic roofing felt) over the entire shingle surface. This is not optional—it creates an air gap and moisture barrier that prevents condensation from forming between the metal and shingles. Asphalt shingles and metal expand and contract at different rates, and without this layer, moisture will accumulate and cause rot and rust.
find the underlayment with cap nails or staples spaced about 12 inches apart, following the manufacturer's instructions. Overlap seams by at least 6 inches and run the underlayment up and over the ridge. Trim it back slightly from the edges so it does not hang over the fascia, where wind can catch it.
Before laying underlayment, nail down any shingles that are lifting or curling. Use roofing nails (not staples) driven through the shingle into the deck. Do not try to glue shingles down—adhesive will not hold under a metal roof's weight and movement. If shingles cannot be nailed flat, remove and replace them with new shingles that match the existing ones, or remove the entire old roof.
Fasten metal panels through shingles into the deck
Metal roofing panels fasten with screws, not nails. Use fasteners rated for metal roofing—typically stainless steel or galvanized screws with rubber or neoprene washers that seal around the hole. The screw must pass through the shingles and penetrate at least 1.5 inches into the roof deck. If a screw stops in the shingle layer, it will not hold the panel securely and the panel will shift, creating gaps where water enters.
Follow the panel manufacturer's fastening pattern exactly. Most metal roofing requires fasteners at the top and bottom of each panel, and sometimes along the sides. Spacing is usually 12 to 24 inches depending on the panel profile and your local wind load. Fastening too close together can cause the panel to buckle; too far apart and it will move and leak.
Drive fasteners straight in, perpendicular to the roof surface. A fastener driven at an angle will not seal properly and the washer will not compress evenly. Use a power drill with a clutch or torque limiter so you do not over-tighten, which crushes the washer and creates a leak. Under-tightening leaves a gap; the washer should be snug but not deformed.
Seal flashing and penetrations
Flashing is the metal trim that directs water around chimneys, vents, skylights, and valleys. When you install metal roofing over shingles, the flashing must be replaced or extended because water flows differently over metal than over shingles. Old flashing that worked fine with shingles may not work with metal panels.
Install new flashing under the underlayment at the top edge and over it at the bottom edge, so water flows down and out without pooling. Seal all flashing seams with roofing cement or metal roof sealant rated for the metal type (aluminum or steel). Do not use tar-based products, which degrade in sunlight and do not adhere well to metal.
Vent pipes, ridge vents, and other penetrations need boots or collars that fit the metal panel profile. Standard shingle-roof boots will not work because the panel is thicker and has a different shape. Order boots designed for your specific metal panel type before installation begins, or you will have to stop work and wait for them to arrive.
Handle valleys and edges carefully
A valley is where two roof slopes meet. Water concentrates there, so the flashing must be watertight. With shingles, water flows across the shingles into the valley. With metal panels, water flows faster and more directly, so the valley flashing must be wider and the sealing must be more precise.
Open valleys (where flashing is visible between panels) are easier to seal than closed valleys (where panels overlap the flashing). If your roof has closed valleys, consider opening them during the metal roof installation so you can inspect and maintain the flashing later. This adds cost but reduces the risk of leaks.
At the eaves and gables, trim the metal panels so they overhang the fascia by 1 to 1.5 inches, the same as the shingles did. Install drip edge flashing under the underlayment at the top and over it at the bottom to direct water into the gutter. Seal the overlap where the drip edge meets the panel with roofing sealant.
Check local building codes and obtain permits
Most jurisdictions allow metal roofing over shingles, but some require a permit and inspection. A few places have restrictions on metal roofing in certain neighborhoods or require specific colors or profiles. Contact your local building department before you start work to find out what applies to your property.
If a permit is required, the inspector will want to see that the old roof is sound, that underlayment is installed, and that fasteners are placed correctly. Taking photos during installation makes the inspection easier and creates a record if a problem develops later. Inspectors also check that flashing is sealed and that penetrations are properly booted.
Some jurisdictions require that the old roof be removed if it is more than one layer thick. If your roof already has two layers of shingles, you may not be allowed to add a third. Check this before you buy materials.
Frequently Asked Questions
Do I have to remove the old shingles before installing metal roofing?
No, if the shingles are in good condition and flat against the deck. However, if shingles are curled, cracked, or missing in patches, removal is necessary because the metal roof will follow the bumps and gaps, creating weak spots and leaks. Removal also reduces the total weight on the roof structure.
What happens if I don't install underlayment?
Moisture will condense between the metal and shingles, causing rot in the shingles and the roof deck, and rust on the underside of the metal panels. This damage is often not visible until it is severe. Underlayment is inexpensive compared to the cost of repairing rot later.
Can I use roofing nails instead of screws for metal panels?
No. Nails do not hold metal panels securely because metal expands and contracts more than wood, and nails will back out over time. Screws with rubber washers are designed to accommodate this movement and maintain a watertight seal.
How long does a metal roof over shingles last?
Metal roofing typically lasts 40 to 70 years depending on the material (aluminum, steel, or copper) and climate. The shingles underneath will eventually deteriorate, but they are protected from weather and will last much longer than they would if exposed. The metal roof is what matters for longevity.
What should I do if I find rot in the roof deck during installation?
Stop work and repair the rotted area before proceeding. Cut out the damaged wood and replace it with new plywood or lumber that matches the existing deck thickness. Fasteners must bite into solid wood, not soft or rotted material, or the roof will fail.