Metal Roof Coating Systems and Repaint Options
Coatings differ in chemistry, in the standard they're tested to, and in how they behave on a pitched metal roof. These are the four jobs that come up most.
Acrylic Elastomeric Coatings
Acrylic roof coatings are water-based and go on by brush, roller or spray. ASTM D6083 is the specification for liquid-applied acrylic elastomeric roof coatings, and it covers the material's properties, not how to apply it. Application comes from the coating maker's instructions, so ask for the product data sheet and the system it describes for metal: cleaner, primer, seam treatment and the number of coats. If any part of the roof holds puddles after rain, ask whether the product is rated for ponding water before it goes there.
Silicone and Urethane Coatings
Silicone and urethane coatings are the other two common chemistries. The ASTM specifications usually quoted for them, D6694 for silicone and D6947 for moisture-cured urethane, are written for coatings on spray polyurethane foam roofs. Neither is a metal-roof specification. When a silicone or urethane is offered for a metal roof, ask for the manufacturer's own metal-roof system: primer, rust treatment and the tested film thickness. These products show up most on low-slope commercial metal roofs; see Commercial Metal Roofing.
Rust Spot Priming and Seam Sealing
Every coating job starts with the parts that leak and rust first. Loose screws get replaced or reset, laps and fastener heads get sealed (often with reinforcing fabric bedded in the base coat), and rusted areas get cleaned and primed. National Park Service preservation guidance says iron-based roofing has to be undercoated and kept painted to stop corrosion. A coating rolled over loose rust peels off with the rust, so the prep is where most of the labor goes. Cut panel edges, drip lines at the eaves and the ring around each screw head are where rust usually starts, and they deserve the closest look.
Repainting Old Tin and Terne Roofs
On an old terne or tin roof, paint is the protection, so repainting is the maintenance, done on a cycle, by brush, with a primer and finish suited to terne. Terne's coating is itself a lead-tin alloy, and old roof paint may contain lead as well, so ask how scrapings and wash water are contained and cleaned up, especially near gardens and play areas. Inside the historic districts the Board of Architectural Review's rules apply to roof work. They are on Historic Metal Roof Restoration.
Why Charleston homeowners call Charleston Metal Roofing Co.
- Estimate at no cost to you
- Metal, seam and fasteners named on paper
- Your request goes to a single local roofer
- Referral is free; you pay only the roofer you hire
Metal Roof Coating Code and Ratings
The code doesn't make you tear off a metal roof to coat it. The 2021 International Residential Code allows a new protective roof coating over an existing metal roof panel or metal roof shingle without tear-off. The coating is a maintenance layer, and a sound roof under it is still required.
Two claims to treat with care:
ENERGY STAR
The EPA stopped certifying roof products on 1 June 2022. Its last criteria for steep-slope roofs (above 2:12) were an initial solar reflectance of at least 0.25 and at least 0.15 after three years. A coating or panel sold today as "ENERGY STAR certified" is quoting a program that has ended. Ask for the product's measured solar reflectance instead.Factory finish versus field coating
A field coating isn't the same as the baked-on factory paint on new panels. New-panel finishes are covered on Aluminum Roofing and Standing Seam Metal Roofs.
Metal Roof Coating or Replacement
A coating is worth quoting when the panels are still attached and solid, rust is on the surface rather than through it, and the decking or insulation under the roof is dry. It's the wrong fix when:
Panels are perforated
Holes rusted through the steel need new metal, at least in those areas.Panels are loose
A roof that moves in the wind cracks a coating at the seams.The deck is wet
Trapped water keeps rotting what the coating hides. Check the attic before you decide.The roof is the wrong system for the slope
A coating won't stop water that a low pitch lets back under the laps.
If the inspection points to replacement, see Metal Roof Replacement.
Metal Roof Coating Estimate Checklist
Product and standard
The coating's name, its ASTM specification (if any) and the manufacturer's metal-roof system sheet.Surface prep
Pressure washing, rust removal, primer type and where it goes.Seams and fasteners
Screw replacement, sealant and fabric at laps and penetrations.Thickness
Number of coats and the dry film thickness the maker calls for.Exclusions
Which areas, if any, need panel replacement before coating.Weather plan
Coatings need dry weather to cure. Ask how the crew schedules around summer storms and what happens if rain arrives mid-job.Coating maker's terms
Whether the manufacturer offers any coverage on its system, and what inspections or recoats it requires to keep it.