The panels get all the attention on a metal roof quote. The layer nobody asks about is the one that decides whether the assembly holds up: the underlayment stapled or rolled onto the deck before a single panel goes on. On a San Diego home, that choice changes with how close you are to the ocean and how hot your attic runs in September.
What underlayment does under metal, specifically
Metal panels shed water. They don’t seal it out. Wind-driven rain gets under panel laps and up standing seams, condensation forms on the cold underside of metal on a clear winter night, and every fastener and clip is a small hole in the assembly. The underlayment is what catches all of it and routes it back down to the eave.
That job is different from what underlayment does beneath tile or shingle. Under concrete tile the underlayment is the whole waterproofing plane and the tile is a shade screen. Under asphalt shingle it’s a backup layer against a surface that’s already fairly tight. Under metal it’s a secondary drainage plane that also has to survive being trapped against a material that heats and cools faster than anything else on your roof. Metal moves. It expands in the afternoon, contracts overnight, and the underlayment underneath sits through that cycle every day for the life of the roof.
That’s the reason the generic answer doesn’t transfer. Our roof underlayment types guide walks through felt, synthetic, and peel-and-stick as product families across every roof type. This page is about which of them a metal roofer should be specifying on your house, and why two homes eight miles apart in this county can reasonably get different answers.
Felt, synthetic, and self-adhered high-temp
Asphalt-saturated felt is the legacy product. It’s cheap, it’s been used under metal for decades, and its weakness under metal is heat and time: it dries out, gets brittle, and can bond to the underside of hot panels. If a future roofer ever needs to pull panels to chase a leak, brittle felt tears rather than lifting.
Synthetic underlayment is woven polymer. It’s lighter, it doesn’t wrinkle when it gets damp during a multi-day installation, and it holds fasteners better. Most synthetics are mechanically fastened, which means the assembly still depends on cap nails or staples that penetrate the deck.
Self-adhered membranes are the third category. They stick to the deck and self-seal around fasteners driven through them, which is the whole point on a roof held down by hundreds of clips. The version relevant to metal is the high-temperature grade, because a standard self-adhered ice-and-water membrane can soften under a hot metal panel and the asphalt in it can migrate. Manufacturers publish a maximum service temperature and a maximum exposure time for every one of these products. Ask your roofer for the data sheet on the exact product in the bid rather than accepting “high-temp” as a description. That’s a verifiable claim on paper, and it’s the single easiest thing for a homeowner to check.
The deck decides more than the product does
A self-adhered membrane needs a clean, continuous, solid substrate. It cannot bond to a gapped deck. Plenty of San Diego homes built before the 1970s have skip sheathing, spaced 1x boards laid for wood shake or tile, with nothing continuous to adhere to. Those roofs need new sheathing before any peel-and-stick product is a real option, and that changes the job from a reroof to a partial rebuild.
The City of San Diego treats that as a permit question, not just a cost question. Its Information Bulletin 123 requires a building permit when new sheathing goes over existing skip sheathing, because that needs structural analysis, and when replacement of structural framing, sheathing, or support elements is required. It exempts damaged sheathing replacement up to 25 percent of the total roof surface area on single-family homes, duplexes, and townhouses. It also requires that where a roof already carries two applications of asphalt shingles or built-up roofing, all layers come off down to the deck before new covering goes on. Those are city rules. Unincorporated county areas may set their own, so confirm with the jurisdiction that actually issues your permit.
Soft or delaminated decking shows up as a spongy feel underfoot and as fasteners that won’t hold torque. If that’s what a roofer finds, no underlayment upgrade fixes it. See our roof decking replacement guide for what that scope looks like once it’s opened up.
Where coastal and inland San Diego pull apart
Within a mile or two of the water, from Imperial Beach through Coronado, Ocean Beach, Pacific Beach, Del Mar, Encinitas, Carlsbad, and Oceanside, salt aerosol is the governing condition. It’s not primarily an underlayment problem, it’s a metal and fastener problem, and it’s why the panel alloy, the coating, and the clip and screw metallurgy matter more here than the roll under them. Our post on whether salt damages a metal roof covers that side. What the underlayment contributes on the coast is a continuous seal at every fastener, because a corroding fastener eventually becomes a hole and you want a self-sealing layer under it.
Go inland to El Cajon, Santee, Lakeside, Ramona, Escondido, or Alpine and the driver flips to heat and fire. Attic temperatures under a dark metal panel in an East County summer are the harshest environment a membrane on your house will see, which is exactly where a documented temperature rating earns its cost. Fire zone status matters too. California Building Code Chapter 7A requires a Class A fire-retardant roof covering in designated wildland-urban interface areas when a new structure is built, when more than half the roof area is replaced in a year, or when covering is applied in alteration, repair, or replacement. Metal shingles and sheets are among the materials that generally qualify as Class A. Whether your parcel is in a WUI area is a jurisdiction question, so verify it before material selection, not after.
One thing that does not apply here: Title 24’s cool-roof requirement for steep-slope residential roofs covers climate zones 10 through 15. San Diego is climate zone 7, so a sloped metal roof in the city isn’t caught by that steep-slope rule. Low-slope roofs are a different set of zones and San Diego is in that group, so a low-slope metal or single-ply section has reflectance and emittance requirements to meet.
Penetrations, edges, and what the proposal should say
Leaks on metal roofs cluster at transitions. Pipe penetrations, roof-to-wall junctions, valleys, skylight curbs, and the eave and rake edges are where the underlayment either gets carried properly or gets cut short. A good installer runs the membrane up walls and curbs, laps it correctly into valleys, and sets it in the right order relative to drip edge so water leaving the panel lands on top of the flashing and not behind it. Our flashing leak repair guide covers what failure looks like at those same points years later.
Two details are worth asking about directly. First, whether the underlayment runs full-deck or only at valleys, eaves, and penetrations, because partial coverage is a real specification and it costs less for a reason. Second, how long the membrane will sit exposed before panels go on. Every product has a published exposure limit, and a job that stalls waiting on a panel delivery can blow past it, which is a warranty problem the homeowner never hears about.
Get the whole assembly named in writing. The proposal should list the underlayment by manufacturer and product name, the coverage (full deck versus valleys and eaves only), the deck condition found and what’s being replaced, the panel system and gauge, the fastener and clip material, and who pulls the permit. A bid that says “synthetic underlayment” and nothing else isn’t comparable to another bid, and it gives you nothing to hold anyone to later.
Verify the license of whoever signs it. California’s C-39 roofing classification covers metal roofing systems, and license status, bond, and complaint history are public at the CSLB license lookup. Under Business and Professions Code section 7048, the licensing exemption applies only to work under $1,000 that’s minor and needs no permit, which no metal reroof is.
When to call us
Specifying a metal roof assembly is a job for a licensed roofer who has been on your deck and in your attic, because the deck condition and your exposure decide the answer more than any product comparison does. It’s also permit work in most cases, which means it needs someone who can pull it. Whether you’re planning a full metal roofing installation or a roof replacement where metal is one of the options, the assembly should be written down before anyone orders material. Call us at (760) 750-5557 and we’ll connect you with a licensed local roofer.