You can install metal roofing over one existing layer of asphalt shingles in most jurisdictions, and it is the reason overlays are popular: skipping the tear-off cuts labor and disposal, so an overlay typically runs 20% to 30% less than a full replacement. The catch is that “how to install metal roofing over shingles” has two right answers depending on ventilation. You either strap the roof with furring strips (purlins or steel hat channel) for an air gap, or you lay a slip sheet directly on the shingles. Get the code, the layer count, and the condensation control right, and an overlay lasts 40 to 70 years. Get them wrong, and you trap moisture against your deck.
Can you legally install a metal roof over shingles?
Yes, in most areas you can install metal panels over one layer of asphalt shingles, but this is a “roof recover,” not a “roof replacement,” and the International Residential Code (IRC) treats them differently. Section R908.3 sets the default rule that replacement includes removing existing coverings down to the deck. The exception in R908.3.1 is what makes an overlay legal, and it lists the specific conditions you have to meet.
Metal panel and metal shingle systems are among the roof coverings IRC R908.3.1 permits over an existing roof. The code allows a recover unless one of its disqualifiers applies, so read those conditions as a checklist before you buy panels. A local amendment can be stricter than the model code, so confirm with your building department and pull a reroof permit.
The IRC R908.3.1 recover conditions (the disqualifiers)
A metal recover over shingles is generally not allowed when any of these apply. Each condition below reflects the intent of IRC R908.3.1 and its related sections; your jurisdiction’s adopted code year and amendments control the exact wording.
- More than one existing layer. Most codes cap the roof at two total coverings. If the shingles are already a second layer, the deck has to come clean before metal goes on.
- Water-soaked or deteriorated shingles. If the existing roof is saturated or the decking under it has rotted, you cannot recover. The deck has to be sound.
- Curling, cupping, or buckling shingles. An uneven base telegraphs through the panels and creates gaps. The surface has to be reasonably flat.
- Excessive accumulated weight. The framing has to carry the combined dead load of both roof systems, which is rarely an issue for lightweight metal but is checked.
Furring strips (purlins) vs slip-sheet underlayment: which method?
The single biggest decision in an overlay is whether you strap the roof or lay panels near-direct on a slip sheet. Furring strips (also called purlins or, in galvanized form, hat channel) create an air gap between the old shingles and the new metal, which drains incidental moisture and vents the assembly. A slip-sheet method skips the strips and separates the two roofs with a synthetic underlayment or high-temp membrane so shingle granules do not abrade the panel backs. Furring costs more and performs better on condensation; slip-sheet is cheaper and faster.
| Factor | Furring strips / hat channel | Slip-sheet underlayment |
|---|---|---|
| Air gap (drainage + venting) | Yes, above-sheathing ventilation | None; panels sit near the shingles |
| Condensation risk | Lower | Higher, especially in cold climates |
| Spacing / layout | 12 to 24 inches on center, run to panel spec | Full-coverage sheet, no layout math |
| Added material cost | Higher (strips or channel + fasteners) | Lower (underlayment only) |
| Best fit | Cold or humid climates, exposed-fastener and standing seam | Mild climates, flatter existing roofs, tighter budgets |
| Fastening base | Screws hit the strips, then the deck | Screws pass through shingles into the deck |
Manufacturers such as McElroy Metal describe both routes and offer proprietary shingle-recover systems that build in above-sheathing ventilation. If you lay panels near-direct, a synthetic underlayment or a high-temperature ice-and-water membrane belongs between the shingles and the metal so the granules do not wear through the coating from the underside.
How to install metal roofing over shingles: step by step
The overlay sequence is inspect, prep, separate, strap or slip-sheet, then panel and flash. Below is the standing-seam and exposed-fastener process most crews follow once the recover has cleared code. Read every step as its own checkpoint; skipping prep is where overlays fail.
- Inspect the deck and shingles. Walk the roof for soft spots, rot, and sag. Any spongy decking means a tear-off in that area, because you cannot recover a bad deck.
- Prep the surface. Reseat popped nails, cut or flatten curled tabs, and remove debris so the base is even. A flat base is a code condition, not a nicety.
- Handle valleys and penetrations. Strip or address old step flashing, pipe boots, and valley metal so the new system flashes cleanly rather than layering over failing details.
- Lay the separation layer. Roll out synthetic underlayment or high-temp membrane over the shingles to protect the panel backs and add a secondary water barrier.
- Install furring strips or hat channel (if strapping). Fasten 1x furring or galvanized hat channel 12 to 24 inches on center per the panel manufacturer, screwing through the shingles into the deck or rafters.
- Set eave, drip edge, and starter trim. Install the perimeter trims that the panels lock or lap into, squaring off the eave line.
- Fasten the metal panels. Run panels up the slope, using gasketed screws for exposed-fastener panels or clips for standing seam, keeping fasteners on the manufacturer’s pattern.
- Flash penetrations and transitions. Add pipe boots, sidewall and headwall flashing, and valley metal sized for the panel profile.
- Cap the ridge and hips. Finish with ridge caps and, where the system calls for it, a vented closure so the assembly can breathe.
Does a metal roof over shingles cause condensation?
An overlay can cause condensation when warm, moist indoor air reaches the cold underside of the panels and there is no path for that moisture to dry. Trapped between two roof layers, it can feed mold, rot, and rust over time. The fix is a drying path: an air gap from furring strips, a high-temp membrane, and adequate attic ventilation working together.
This is the main technical reason strapping beats a near-direct install in cold and humid climates. The air gap created by furring strips or hat channel gives incidental moisture somewhere to go and lets the assembly vent. Pairing the overlay with balanced intake and exhaust ventilation in the attic below matters as much as the layer against the shingles. See our guide to metal roof condensation causes and fixes for the assembly-by-assembly detail.
Metal roof over shingles: pros and cons
The overlay trades a lower upfront cost and faster timeline against ventilation risk and the fact that you never see what is under the shingles. It is the right call on a sound, single-layer roof and the wrong call on a wet or failing one.
| Pros | Cons |
|---|---|
| Costs roughly 20% to 30% less than tear-off (no removal or disposal) | Hides deck problems you cannot see or fix |
| Faster: typical residential overlays run 3 to 7 days | Condensation risk if the assembly cannot dry |
| Metal service life of 40 to 70 years vs 15 to 25 for asphalt | Added dead weight, though metal is light |
| Less landfill waste than a tear-off | Some manufacturer warranties require a tear-off or specific recover details |
| Existing shingles add a small thermal and sound buffer | Not code-legal over more than one layer or a bad deck |
When you should NOT install metal over shingles
Skip the overlay and tear off when the recover conditions fail. A tear-off costs more up front but is the correct move when the base is compromised, because burying a bad deck under new metal just delays and hides the failure.
- Two or more existing layers. You are already at the code cap, so the roof has to be stripped to the deck.
- Rotted or spongy decking. If the deck fails an inspection, recover is off the table. Budget for roof decking replacement instead.
- Widespread curling, cupping, or buckling. An uneven base makes for a wavy metal roof and violates the flat-surface condition.
- Active, unresolved leaks. Covering a leaking roof traps the moisture path underneath the new panels.
- Warranty or permit requirements. If your panel warranty or local amendment requires a tear-off, follow it. Weigh the two paths in our overlay vs tear-off decision guide.
What does it cost to put metal over shingles?
An overlay generally lands 20% to 30% below a comparable tear-off because it removes the demolition, dumpster, and disposal line items. Installed metal roofing commonly runs about $7 to $12 per square foot depending on the metal type and panel system, before the recover-specific extras. Strapping the roof adds material and labor for the furring or hat channel.
The savings shrink if furring, extensive prep, or spot decking repairs are needed, and a heavily strapped standing-seam overlay can approach tear-off pricing. For a full breakdown by panel type and region, see our metal roof cost guide, and for the base process on a clean deck, our metal roof installation walkthrough. Homeowners weighing the whole reroof decision can start at our learn hub.
Frequently asked questions
Can you put a metal roof directly over shingles?
Yes, metal panels can go over one layer of asphalt shingles as a code recover, but “directly” is risky without a separation layer. A synthetic underlayment or high-temp membrane belongs between the shingles and the panels so granules do not abrade the metal from underneath. For better moisture control, most pros add furring strips or hat channel to create an air gap rather than laying panels near-direct.
How many layers of shingles can you have before adding metal?
Most jurisdictions following the IRC cap a roof at two total coverings, so you can generally recover over one existing layer of shingles. If the roof already carries two layers, or a previous overlay, the code requires a tear-off to the deck before new metal. Always confirm the layer limit with your local building department, since amendments can be stricter than the model code.
Do you need furring strips for a metal roof over shingles?
Furring strips are not always required, but they are strongly recommended, especially in cold or humid climates. Strips or galvanized hat channel spaced 12 to 24 inches on center create an air gap that drains moisture and vents the assembly, cutting condensation risk. A slip-sheet method skips the strips and relies on a synthetic underlayment, which is cheaper but offers no ventilation gap.
Does a metal roof over shingles cause condensation?
It can, if warm indoor air reaches the cold panel underside with no way to dry. Moisture trapped between the two roof layers can feed mold, rot, and rust. Controlling it takes an air gap from furring strips, a high-temperature membrane, and balanced attic ventilation. Skipping ventilation is the most common reason an overlay fails prematurely.
Does installing metal over shingles void the warranty?
It depends on the manufacturer. Some metal panel warranties require a tear-off or specify approved recover details, and installing outside those terms can void coverage. Read the panel warranty before choosing an overlay, and confirm the installer follows the manufacturer’s shingle-recover instructions. A local permit and inspection are separate from, and additional to, the manufacturer’s requirements.
How long does a metal roof over shingles last?
The metal itself commonly lasts 40 to 70 years depending on the coating and metal type, far longer than the 15 to 25 years typical of asphalt shingles. An overlay does not shorten the panel’s life when the deck is sound and the assembly can dry. The main risk to longevity is trapped moisture, which is why ventilation and a proper separation layer matter.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.