Roughly half the metal roofs I see going up in my area are roof-overs — panels installed directly on top of an existing asphalt shingle roof. The question that stops almost every homeowner is whether you need underlayment for metal roof over shingles, since the old shingles already look like a water barrier. The short answer is yes, and it is not optional in most jurisdictions: code officials, panel manufacturers, and every roofer who has torn off a failed roof-over will tell you the same thing. Skip the separation layer and you risk a voided warranty, an abraded panel coating, and a leak path you cannot see until the ceiling stains show up.
Why the Old Shingles Are Not Enough
Asphalt shingles seem like a ready-made underlayment, but they are actively hostile to the underside of a metal panel. The ceramic granules on a weathered shingle are essentially 20-grit sandpaper. Every time the roof heats up and cools down — and steel panels move as much as 3/4 inch over a 40-foot run between a 40°F morning and a 160°F afternoon deck temperature — the backside of the panel drags across those granules. Within a few seasons that scrubbing wears through the backer coating, and on Galvalume or galvanized steel the abraded spots become corrosion starters that eat the panel from underneath.
There is also a moisture problem. Old shingles are full of exposed nail heads, cracked tabs, and lifted edges. Condensation forming on the cool underside of the metal drips onto that surface and finds every one of those defects. A fresh underlayment layer gives that water a clean, sealed plane to shed down to the eave instead of soaking into the deck. Finally, the underlayment works as a slip sheet during installation, letting panels slide into position without hanging up on shingle edges.
What the Code Actually Says About Roof-Overs
The International Residential Code addresses this in Section R908 (roof recover in the 2021 IRC, formerly R907). Two rules matter most. First, you may install a new roof over no more than one existing layer of asphalt shingles — if the house already has two layers, a full tear-off is mandatory, no exceptions. Second, the existing roof must be in serviceable condition: no water-soaked or deteriorated decking, no rotted framing, and a surface flat enough to give the new roof solid support. Your inspector can require tear-off if the deck is spongy or the shingles are badly curled.
Here is the part people miss: IRC R905.10.1 requires metal roof panels to be installed over an approved underlayment per the manufacturer’s instructions, and a roof recover does not erase that requirement. The old shingles are not an “approved underlayment” — they are an existing roof covering. So in a code-compliant roof-over, the stack is deck, one shingle layer, new synthetic underlayment, then metal. Some jurisdictions in high-wind coastal zones and parts of Florida tighten this further or ban shingle roof-overs entirely, so a 20-minute call to your local building department before ordering panels is money well spent.
Synthetic Underlayment vs. Felt Over Old Shingles
For this specific application, synthetic underlayment wins by a wide margin. Thirty-pound felt will technically pass in many jurisdictions, but felt tears easily when stretched over the uneven ridges of old shingle courses, it wrinkles when it absorbs moisture, and those wrinkles telegraph movement noise under the panels. Synthetics such as Owens Corning ProArmor, GAF FeltBuster, or a mid-weight woven polypropylene run 20 to 40 times stronger in tear resistance and lie flat over irregular surfaces.
A few selection points that matter on a roof-over specifically:
- Temperature rating: Metal over shingles builds serious heat. Choose a synthetic rated to at least 240°F; many budget synthetics are rated for 220°F or less and can degrade under dark panels in a southern exposure.
- Slip resistance: You will be walking on this layer over lumpy shingles. A textured or anti-slip walking surface is worth the extra $10 per roll.
- Cap fasteners: Use 1-inch plastic cap nails or cap staples long enough to bite through the shingle layer into the deck — typically 1-1/4 inch to 1-1/2 inch. Standard 7/8-inch cap nails often will not reach solid wood.
- High-temp ice barrier: In snow country, run a high-temperature self-adhered membrane (Grace Ultra or equivalent, about $190 per 34-foot roll) at eaves and valleys. Standard peel-and-stick ice shield is rated around 180°F and can slide or bleed adhesive under hot metal.
When Battens Change the Answer
Some installers skip direct-deck attachment and screw 1×4 or 2×4 purlins horizontally over the shingles, then fasten the panels to the battens. This creates an air gap, so the panel never touches the shingles and granule abrasion is off the table. Does that eliminate the underlayment? Usually not. Most panel manufacturers still require an underlayment beneath battens for moisture control, and condensation dripping off the panel underside will land on the old shingles either way. The gap also creates a convection channel that can actually increase condensation potential in mixed climates unless it is vented at eave and ridge.
Battens make sense when the shingle surface is too irregular for direct attachment, when you are installing exposed-fastener panels that need consistent screw purchase, or when local practice favors a vented “cold roof” assembly for snow. Figure on $0.45 to $0.80 per square foot extra for lumber and labor. If you go this route, still roll synthetic underlayment over the shingles first — it is cheap insurance and most warranties demand it.
Manufacturer Warranty Requirements
Read the installation manual before you buy panels, because the paint warranty is where roof-overs go to die. Nearly every major manufacturer — McElroy, Central States, ABC, Fabral, Metal Sales — includes language requiring an approved underlayment between their panels and any existing roofing. Some explicitly state that direct contact with asphalt shingles voids the finish warranty, because asphalt oils can leach into and stain the backer coating in addition to the abrasion issue. A 40-year Kynar 500 paint warranty is one of the main reasons you paid $4.50 to $7.00 per square foot for the panels; losing it to save $30 per square in underlayment is a terrible trade.
Keep your receipts and photograph the underlayment installation before panels go on. If you ever file a claim, the manufacturer will ask for proof of the assembly, and a dated photo of labeled underlayment across the whole roof settles the question in thirty seconds.
Condensation: The Quiet Roof-Over Problem
Metal sweats. On clear nights the panel radiates heat to the sky, drops below the dew point, and moisture condenses on its underside. In a direct-attach roof-over, the synthetic underlayment catches that drip and sheds it. But the bigger fix happens below the roof: make sure attic ventilation meets the 1:150 ratio (or 1:300 with balanced intake and exhaust) and that bath fans are not dumping into the attic. A roof-over adds thermal mass and a vapor-tight lid, so an attic that was marginal before can turn actively wet after.
In humid climates, consider a panel with an integral anti-condensation fleece backing or specify a synthetic underlayment with some absorptive capacity. On batten systems, insect-screened vented closures at eave and ridge let the air channel dry itself out. None of this is exotic, but I have opened up five-year-old roof-overs where skipping it rotted the top inch of decking around every fastener.
What the Underlayment Layer Costs
Good news: this is the cheapest component of the whole project. A standard 10-square roll (1,000 square feet) of quality synthetic runs $85 to $160, so material is roughly $0.09 to $0.16 per square foot. High-temp self-adhered membrane for eaves and valleys adds $1.10 to $1.60 per square foot for the limited area it covers. Labor to roll and cap-nail underlayment over an existing shingle roof is quick — a two-person crew covers a 20-square ranch in two to three hours — so installers typically fold it into the metal bid at $0.20 to $0.40 per square foot all-in.
On a typical 2,000-square-foot roof, the complete underlayment layer lands between $400 and $800. Compare that against the $1,000 to $2,000 you saved by not tearing off the shingles, and the roof-over math still works comfortably. The rules here are simple: one existing layer maximum, sound deck, approved synthetic over the shingles, high-temp membrane where ice dams live, and fasteners long enough to reach wood. Follow them and a metal roof-over will outlast every shingle roof on the block.
