Waterproof Your Metal Roof

Water doesn’t just fall straight down onto your roof and roll off into the gutter-especially not in Nassau County, where nor’easters blow rain sideways and wind can push a drop uphill under a poorly sealed lap. Real metal roof waterproofing is about controlling the paths water wants to follow along seams, around fasteners, over flashings, and off edges, not just slapping sealant on whatever looks wet and hoping for the best. This article walks through those controls in the order water actually tries to get in, so you can stop chasing the same leak every storm season.

Most people think metal roofs are automatically waterproof because the panels themselves don’t soak up water like asphalt shingles. They’re right about the panels. Metal itself is waterproof; the weak spots are every place two pieces meet or something pokes through. Vertical seams, end-laps, fastener holes, chimneys, sidewalls, eaves-those are the spots that whisper trouble in year two and shout it by year five if they weren’t detailed correctly the first time.

If you treat leaks as random holes instead of water paths, you’ll keep chasing the same problem.

Water Follows Paths-Waterproofing a Metal Roof Means Controlling Those Paths

Picture a raindrop landing on your standing-seam panel during a calm summer shower. It rolls down the rib, hits the hem at the eave, and exits into the gutter-exactly as designed. Now picture the same drop in a forty-mile-per-hour October wind with rain coming in sideways off the bay. That drop doesn’t travel straight down anymore; it slides up under an end-lap, sneaks along the valley of a vertical seam, or pools against a sidewall flashing until it finds a gap. The panel hasn’t changed, but the path changed, and if the details weren’t built for that path, water gets inside.

On a windy October storm blowing in off the bay, your metal roof doesn’t see ‘normal’ rain. Wind-driven water can travel horizontally for inches before gravity pulls it down, so any seam or joint that relies on gravity alone to keep it dry will fail when the wind kicks up. That’s why proper metal roof waterproofing layers multiple lines of defense-tight seams with sealants, overlaps that shed water even when it’s moving sideways, and flashings that redirect whatever slips past the first barrier. Panel shape and roof slope work together to guide water down and off, but only if every lap, closure, and fastener is set up to handle the worst storm path, not just the easy one.

How a Proper Metal Roof Is Supposed to Shed and Back Up Water

A well-designed metal roof sheds most of its water on the surface-panels overlap in a way that always sends water downhill, seams are raised or sealed so wind can’t push moisture underneath, and the roof has enough slope that even heavy rain keeps moving instead of ponding. That’s the first line of defense, and honestly, if your panels are installed correctly with good lap orientation and enough pitch, ninety percent of the water never gets close to the underlayment. But the other ten percent-the sideways stuff, the capillary draw at a tight seam, the melt that refreezes at the eave-needs a backup plan. That’s where underlayment comes in.

Panels, Slope, and the Backup Goalie Underneath

Under your panels, a good underlayment is the backup goalie most people forget about. Synthetic underlayment or high-quality felt sits directly on the decking and catches any water that slips through a fastener gasket, migrates along a poorly sealed lap, or condenses on the underside of the metal during a freeze-thaw cycle. In coastal Nassau towns like Long Beach or Freeport, I always add an ice-and-water barrier at the eaves and valleys-not because it snows that much, but because wind-driven rain and salt air can push moisture into places dry climates never worry about. That sticky rubberized layer seals around every fastener and self-heals minor punctures, so even if a panel detail isn’t perfect, the underlayment gives you a second chance before water hits the decking.

Different parts of Nassau County see different storm behavior, and that changes how you design waterproofing. Long Beach roofs catch direct east wind off the Atlantic, so standing-seam systems there need extra attention at vertical seams and leading edges where uplift can peel a poorly fastened panel. A couple miles inland in Massapequa or Seaford, you get less salt and slightly less wind, but you still see driving rain that tests every lap and flashing during a nor’easter. Further north in towns like Westbury or Garden City, the storms are calmer but you deal with more snow melt and ice-damming at eaves, so underlayment and proper ventilation under the metal become critical to prevent condensation leaks. The point is, Nassau metal roofs need to be waterproofed for movement and wind, not just vertical rain, and underlayment plus slope give you that baseline protection before any detail work starts.

To make all this concrete, here’s how four different raindrops find their way-or don’t-on a properly waterproofed Nassau metal roof:

1. Straight vertical rain: Lands on panel rib, rides the slope down the panel overlap at the eave hem, exits into gutter-panel shape and slope handle it completely.
2. Wind-driven into vertical seam: Hits the raised seam, tries to creep sideways; proper seam closure or butyl tape blocks that path, water diverts down the panel face instead.
3. Sliding along sidewall: Rain runs down siding, meets roof; kick-out flashing or diverter sends it away from the metal edge and into a separate drainage path-never allowed to travel under the sidewall flashing.
4. Draining off eave: Water sheet flows to the leading edge; eave closure or drip-edge trim forces it to drop cleanly into the gutter rather than curl back under the panel hem where it could wick into the underlayment.

Each of those paths has a specific detail designed to control it. If water can’t get in there anymore, it looks for the next weak spot.

Seams, Laps, and Fasteners: The First Places Water Tries to Sneak In

Fasteners don’t fail all at once-they whisper before they shout. A screw driven too tight compresses its neoprene or EPDM gasket until the rubber starts to flatten and crack, and that crack becomes a tiny channel for wind-driven rain to seep through. A screw driven into the wrong spot-say, the flat of a panel instead of the proper rib location-creates a low point where water pools instead of draining, and pooled water always finds a way in eventually. Even screws installed perfectly will see their gaskets age and lose elasticity after fifteen or twenty years of UV and temperature swings, so part of any serious waterproofing inspection is walking the roof with a drill and socket, checking for back-out (screws that have loosened and lifted slightly) and replacing any fastener that shows rust staining or a flattened gasket.

One brutal November nor’easter in Long Beach, I got called to a metal roof that was only two years old but leaking in three separate spots-master bedroom, hallway, and garage corner. The homeowner was furious because the panels and paint looked brand-new, and they’d paid good money for a “quality” install. I climbed up expecting to find wind damage or a puncture, but the panels were fine; the problem was every end-lap where one panel overlapped the next going down the roof. The installer had left those laps open-no butyl tape, no closure strips-so wind-driven rain was sliding right up under the lap and dripping onto the underlayment, which wasn’t sticky ice-barrier in those spots, just regular felt. We pulled those laps apart, added proper butyl tape along the full width of each one, installed foam closures at the eaves to block wind entry, and re-fastened everything with fresh gaskets. The very next nor’easter came through two weeks later with forty-mile-per-hour gusts, and the homeowner texted me a photo of their bone-dry ceiling-first storm without a bucket in the hallway since they’d moved in.

Seams and Laps: Where Sideways Rain Loves to Travel

Vertical seams on standing-seam roofs are designed to shed water, but only if they’re mechanically seamed or clipped tight enough that capillary action can’t pull moisture into the joint. If a seam is hand-crimped loosely or if the sealant inside the seam has dried out and cracked, wind can push a thin film of water right up that seam and under the next panel. Horizontal end-laps-where one panel ends and the next starts going down the slope-are even trickier because they overlap against the direction of water flow. The bottom panel has to slide under the top panel far enough (usually six inches minimum) and be sealed with butyl tape or a high-grade sealant that stays flexible through freeze-thaw cycles. Miss that detail, and every windy rainstorm becomes a leak waiting to happen.

Chimneys, Skylights, Sidewalls, and Edges: Where Asphalt Habits Break Metal Waterproofing

Around chimneys, skylights, and sidewalls, asphalt habits can wreck a metal roof. A lot of installers come from shingle backgrounds where you just tuck flashing under the next course and call it a day, but metal doesn’t work like that-metal expands and contracts a lot more than asphalt, and it needs flashings that move with it. Proper metal-specific flashing means step-flashing pieces or continuous counterflashing that overlaps the panel ribs in a way that allows the metal to slide slightly without tearing the seal, and it means lapping the flashing over the panel (not under) so water running down the wall or chimney is always directed onto the top surface of the metal where it can drain away. Skylights are even worse because they create a low spot and a lot of perimeter to seal; using a curb-mounted skylight with its own built-in flashing kit and then integrating that kit with the metal panels using Z-closures or custom pans is pretty much the only way to avoid a chronic leak.

At a small shop in Baldwin a couple years back, the owner kept finding water stains along one interior wall, always after a heavy rain. He’d had three different guys out, and each one caulked something-first the skylight, then the sidewall flashing, then random seams-but the stain kept coming back. When I walked the roof, I found a section of low-slope metal that drained toward the sidewall with no kick-out flashing or diverter to send that water away from the building. Basically, a couple hundred square feet of roof was funneling its runoff straight into the gap between the siding and the metal edge, and from there it traveled down the wall cavity and into the shop. We fabricated a short metal diverter and installed a proper kick-out flashing that redirected the water path out and away from the wall, then re-sealed the sidewall flashing in the correct lap order. Next storm, dry wall. Three tubes of caulk had done nothing because they were trying to seal a path that shouldn’t have existed in the first place.

Eaves, Rakes, and the Leading Edge

In coastal towns like Freeport and Island Park, your leading edge lives a tougher life than the rest of the roof. Salt spray, direct wind uplift, and the constant freeze-thaw or wet-dry cycling at the eave all attack the hem, the drip edge, and any closure or trim piece that’s supposed to seal the gap between the panel and the fascia. Open eave edges let wind blow up under the panels, which not only risks uplift during a big storm but also channels rain directly onto the underlayment and fascia board where it can rot wood and cause interior leaks. Quality eave closures-whether foam strips custom-cut to match your panel profile or metal closures bent to the right shape-seal that gap while still allowing ventilation if your design calls for it. Rake edges (the gable ends) need similar attention; a proper rake trim piece overlaps the panel edge and extends down over the fascia to keep wind-driven rain from wrapping around and sneaking back under the metal.

Why Coatings Are the Finish Line, Not the Starting Gun

If your plan starts with a five-gallon bucket of coating, we’re starting in the wrong place. I’ve lost count of how many times I’ve been called to a roof where someone paid for a “waterproof restoration coating” and still has leaks-sometimes worse than before-because the crew sprayed or rolled a shiny elastomeric layer right over loose fasteners, open laps, and cracked seams. In Massapequa, one homeowner showed me a roof that looked beautiful from the street: smooth white coating, no rust, almost like new. But inside, they still had water stains in two bedrooms. When we scraped back a section of the coating, we found fasteners that had backed out a quarter-turn and seams with gaps wide enough to slide a business card through. The water was just traveling under that nice coating, using the same old paths. We ended up scraping, tightening every fastener, re-sealing the seams with butyl and mechanical closures, replacing worn gaskets, then-and only then-applying a high-quality acrylic coating as the final layer. That’s the only sequence that works: fix the paths, then coat.

Coatings and sealants absolutely have a place in metal roof waterproofing, but they belong at the end of the checklist, not the beginning. Once your seams are tight, your fasteners are sound, your flashings are lapped correctly, and your edges are closed, then a restoration coating can extend the life of an older roof by sealing micro-cracks in the panel finish, reflecting UV to reduce thermal cycling, and adding one more barrier against moisture intrusion. High-grade polyurethane or silicone sealants are fantastic for bedding counterflashing, sealing reglets, or adding an extra bead along a tricky seam-but only after you’ve addressed the mechanical failure that caused the leak in the first place. Trying to waterproof a metal roof with sealant alone is like trying to fix a flat tire by spraying it with fix-a-flat instead of patching the puncture: it might hold for a little while, but you’re not solving the real problem.

A bucket of coating will never fix a bad flashing or an open lap by itself.

Follow the Water Path: A Simple Waterproofing Checklist for Nassau Roofs

If you’re talking to a roofer about waterproofing your metal roof-or if you’re trying to figure out why you’re still seeing leaks after a repair-walk them (or yourself) through the water path in order. Start at the top: are the panels and slope shedding water correctly, or is something flat or sagging that shouldn’t be? Move to seams and laps: are vertical seams sealed and mechanically tight, and are end-laps overlapped far enough with fresh butyl or tape? Check penetrations: do chimneys, skylights, and sidewalls have metal-specific flashings lapped in the right direction, with kick-outs or diverters to send water away from vulnerable joints? Inspect edges: are eave and rake closures in place and intact, keeping wind from getting under the panels? Only after all of those details are fixed should you talk about coatings or additional sealants as a final protective layer. That’s the sequence that actually keeps water out for the long haul. If you’re in Nassau County and you want someone to walk your roof and trace those paths with you-no high-pressure sales, just a straightforward assessment of what’s letting water in and what it’ll take to stop it-TWI Roofing has been doing exactly that kind of work on homes, garages, and small commercial buildings from Long Beach to Garden City for years. We’re happy to climb up, follow the water, and give you a plan that makes sense.

Real metal roof waterproofing isn’t complicated-it just requires following the water instead of guessing at leaks. Panel by panel, seam by seam, you trace where a drop wants to go in a storm and you make sure there’s a detail blocking that path at every step. Underlayment backs up the panels. Sealed laps and tight seams stop sideways travel. Correct flashings and kick-outs divert water around penetrations and edges. Fasteners with good gaskets keep holes from becoming leaks. Coatings and sealants finish the job once the mechanical details are right. That’s the order, that’s the system, and that’s what keeps Nassau County roofs dry through nor’easters, summer downpours, and everything in between.

Most of the chronic leaks I see aren’t from bad materials-they’re from details that were skipped or misunderstood during installation, then patched over with quick fixes that ignore the water path. A coating sprayed over an open lap. Caulk slapped on a loose fastener. Asphalt-style flashing jammed against metal that needs room to move. Those shortcuts might buy you a season, but they don’t solve the problem, and eventually the water finds a new way in. When you approach waterproofing as a system-every detail working together to guide water safely off the building-you stop chasing leaks and start trusting your roof again.

If you’ve been living with buckets in the hallway or water stains that come back every storm, you don’t need magic goop or a total tearoff. You need someone to walk the roof, follow the water paths, tighten what’s loose, seal what’s open, flash what’s exposed, and then-only then-talk about coatings if they make sense for your situation. That’s how TWI Roofing approaches every metal roof in Nassau County, and it’s how we’ve kept buildings dry from Long Beach to Westbury for years. Reach out if you want a straight answer about what’s letting water in and what it’ll actually take to stop it-no hype, no shortcuts, just the details done right so the next storm stays outside where it belongs.