To find and fix a leak in a metal roof, inspect the panel body, the seams and fasteners, roof penetrations, and the roof edges in that order, since that is roughly where leaks concentrate. Reinforce any cracks, seams or penetrations with fabric embedded in the wet membrane, then apply two full coats of SEMCO Liquid Membrane (about 200 sq ft per gallon per coat, plus 25% on corrugated panels) over the cleaned, prepped surface. No primer is needed on any of SEMCO's prep paths, and the cured membrane has up to 1,300% elongation, so it moves with the panel instead of cracking at the seams the way a rigid coating can.
This guide covers what actually causes metal roof leaks, how to find one, how to fix it step by step, how much membrane a corrugated roof really needs, how a water-based membrane compares to silicone-based and rubber-based coatings, and the questions people ask most before starting.
In this guide
- What causes leaks in your metal roof
- How to find a leak in a metal roof
- How to fix a metal roof leak, step by step
- How much membrane a corrugated roof actually needs
- Water-based membrane vs silicone-based vs rubber-based coating
- Getting the most out of your roof sealant
- FAQ
What Causes Leaks in Your Metal Roof
Metal is one of the most durable and long-lasting roofing materials, but no roof is immune to leaks, especially once it is older or was not maintained well. A humid climate or a lot of rainfall accelerates rust, and if you bought a house with an existing metal roof, you have no guarantee it was installed to a standard that lets it perform the way metal roofing is supposed to.
The most common sources of leaks on a metal roof are:
- Improper installation
- Issues with fasteners backing out over time
- Panel damage
- Open headwall or flashing
- Missing trim
- Loose gutters or downspouts
- Gaps in seams
- Rust damage
Wondering why the same spots keep leaking even after a repair? See our breakdown of why metal roof coatings actually fail, covering fastener back-out, capillary action at seams, and the other root mechanisms behind repeat leaks.
How to Find a Leak in a Metal Roof
If your metal roof is leaking, find the source before you treat anything. Work through these four areas in order.
The roof body
A visual inspection can catch major leaks in the body of the roof. Check for holes from weather or debris, rust damage, missing or loose (or overly tight) screws, and shifted panels.
Connection points
Seams and joints are common leak starting points, usually from loosening screws or water buildup. Check for missing sealant or tape, loose trim or gaps, loose or overly tight screws, and misalignment where panels overlap.
Roof openings
Most roofs are built around vents, pipes and other penetrations (skylights, chimneys and similar), which create opportunities for leaks. Check for cracked seals on flashing, gaps in trim, and rotting sealant.
Roof edges
The roofline edges, including where the roof meets the gutter, are also leak-prone. Check for gutter damage, flashing damage, rotting sealant, and cap issues.
How to Fix a Metal Roof Leak, Step by Step
Once you have found the source, SEMCO Liquid Membrane lets you do the repair yourself, without a contractor and without a separate primer.
Step 1: Prepare the roof
Tighten or replace any loose or damaged screws and fasteners first, before anything else; you want seams as tight as possible before sealing. Remove loose debris, then clean with Prep D: Nu-Lift Cleaner to lift oxidation and residue, then Stone Soap, rinsing with clean water after each. Let the surface dry fully before applying fabric or membrane.
Step 2: Fortify all seams
Loose or open seams on paneling, flashing or joists will not hold up on their own. Lay strips of Fabric Reinforcement over cracks and seams. If gaps are large, use a metal-safe polyurethane-based caulk on any remaining cracks first, smoothed flush, then fabric over vents, flashing and other protrusions, smoothed tightly down.
How to apply the fabric reinforcement:
- Apply a generous coat of SEMCO Liquid Membrane where the fabric will go.
- While the coat is still wet, press the fabric in flat.
- Spread any excess coating that squeezes out from underneath with a brush or roller.
- After 10 to 15 minutes, apply a thin coat of membrane over the fabric to embed it.
Not sure which fabric roll size fits your roof, or when it is actually required versus optional? See our guide to what SEMCO Fabric Reinforcement is and when you need it.
Embeds into the wet membrane over seams, vents, flashing and any crack wider than a quarter inch.
Step 3: Apply SEMCO Liquid Membrane
Using a brush, roller or heavy-duty airless sprayer, apply SEMCO Liquid Membrane in an even coat over the entire roof. You need a minimum of two full coats to waterproof, letting each coat dry before the next, at roughly 200 sq ft per gallon per coat on a flat panel.
Wondering whether a water-based membrane is really the right chemistry for a leak-prone metal roof, versus a silicone-based or rubber-based coating? Jump to the comparison section below for how the three hold up on odor, cure time and flexibility.
Water-based, 14 g/L VOC, up to 1,300% elongation, no primer needed on any prep path. About 200 sq ft per gallon per coat, 2 coats minimum.
How Much Membrane a Corrugated Roof Actually Needs
The single most common complaint on any metal roof coating project is running short partway through, and it almost always comes down to one thing: a corrugated or ribbed panel has more actual surface area than its flat footprint suggests, because the coating has to follow every ridge and valley, not just the horizontal span.
Start from the flat coverage rate, about 200 sq ft per gallon per coat, then add roughly 25% to your measured roof area to account for the corrugation before you size the job. Two worked examples:
- A 1,000 sq ft flat footprint becomes about 1,250 sq ft of actual corrugated surface. At two coats and 200 sq ft per gallon, that is 12.5 gallons per coat, 25 gallons total, so round up to the nearest 5-gallon and 1-gallon combination you need.
- A 500 sq ft flat footprint becomes about 625 sq ft corrugated. At two coats, that is roughly 6.25 gallons per coat, 12.5 gallons total.
Measure the flat footprint first (length times width of the roof plane, not including the 25% yet), add the corrugation allowance, then divide by 200 to get gallons per coat, and double it for two coats. Always round up rather than down, since seams, flashing and penetrations take extra fabric and membrane beyond the flat math.
Water-Based Membrane vs Silicone-Based vs Rubber-Based Coating
| Factor | SEMCO Liquid Membrane (water-based) | Silicone-based coating | Rubber-based coating |
|---|---|---|---|
| Primer needed on metal | No, on any SEMCO prep path | Varies by product | Often required on bare or previously coated metal |
| Flexibility | Up to 1,300% elongation | Elastomeric, figures vary by product | Flexible, but ages and stiffens over time |
| Odor while curing | Water-based, 14 g/L VOC, low odor | Often more noticeable until fully cured | Can carry a strong solvent odor, varies by product |
| Inspection before the roof is left to cure | Visible film, pinholes easy to spot and spot-coat | Visible film, same principle | Visible film, same principle |
| Works beyond the roof | Yes: basements, showers, pools, decks, below grade | Mostly tile and wet areas | Often roofing-specific product lines |
For the deeper version of this comparison with cure-time detail, see our dedicated water-based vs silicone-based metal roof comparison. For how a water-based formulation stacks up against rubber-based coatings generally, see water-based alternatives to rubber-based coatings.
Getting the Most Out of Your Roof Sealant
- Use Fabric Reinforcement, the same material used on seams, to patch holes and cracks before applying sealant.
- Wire-brush areas of significant rust back to sound metal and reinforce with Fabric Reinforcement before coating. No primer is needed.
- Apply in good conditions: no heavy wind, rain, or excessive heat or sun while it dries. Pick a dry day between 50F and 85F, out of direct sun.
- On corrugated roofs, always add the roughly 25% surface-area allowance described above so you do not run short mid-job.
- Keep it dry for the full cure, 7 days minimum.
Frequently Asked Questions
Do I need a primer before coating a metal roof?
No. SEMCO Liquid Membrane does not require a primer on any of its prep paths for metal, including previously coated or rusted panels once they are properly cleaned and, where rusted, wire-brushed back to sound metal.
How many coats does a metal roof actually need?
Two full coats minimum, each at roughly 200 sq ft per gallon on a flat panel, letting each coat dry before the next.
Why do the same spots keep leaking even after a repair?
Usually fastener back-out, capillary action drawing water sideways under a lapped seam, or a coating that was applied too thin to bridge movement at the seam. See our full breakdown of why metal roof coatings fail.
How much extra material do I need for a corrugated panel?
Add roughly 25% to your flat, measured roof area before you calculate gallons, since the coating follows every ridge and valley rather than just the flat span. See the worked examples above.
What temperature should I apply it in?
Between 50F and 85F, out of direct sun, with no rain or dew expected before the coat skins over.
Ready to stop a metal roof leak for good?
SEMCO Liquid Membrane needs no primer on metal, bridges seams with up to 1,300% elongation, and cures in 7 days.
Shop Liquid Membrane →Free shipping on orders over $99. 5-year standard limited warranty.
Related reading:
- SHIELDING YOUR INVESTMENT : THE KEY BENEFITS OF WATERPROOF SEALANTS FOR METAL ROOF MAINTENANCE
- EXPLORING ROOFING MATERIALS : A GUIDE TO SEMCO LIQUID MEMBRANE SOLUTIONS
- Metal Roof Leaking? The Real Reasons Coatings Fail (and How to Fix It for Good)
- Metal Roof Leaking? How to Find and Seal It Before Winter Rain
- How to Waterproof a Concrete or Flat Roof Before Winter Rain
