Steel Roof Questions
From facility managers, farm owners, and the contractors who work on both.
Condition & Preparation
Can it go over rust, or does the roof have to be blasted?
Over tight, well-adhered rust, yes — SSPC-SP3 power-tool cleaning is an accepted maintenance spec and the membrane isolates the oxide from oxygen and moisture, which stops the reaction. Over loose scale or flaking rust, no. The practical test is a scraper: anything it removes has to come off, or the membrane will take it along when it fails. New or heavily corroded panel gets SSPC-SP6 commercial blast with a 2–4 mil profile.
My roof is galvanized. Does that change anything?
Yes, in two ways. Galvanized and Galvalume surfaces do not accept a direct polyurea bond reliably, so 7001 Primer is required rather than optional. And aged zinc surfaces carry chalk and white rust — zinc oxidation products — that must be pressure washed off first. Prime a chalky panel and you have bonded to the chalk.
How bad is too bad? When should I replace instead?
Perforation you can put a screwdriver through, widespread section loss, panels that flex alarmingly underfoot, or purlins and joists with structural corrosion. A membrane is waterproofing, not steel. We will tell you when the honest answer is replacement — and on bar joist buildings especially, get the structure evaluated, because joists have very little material to lose before it matters.
Do the fasteners need to be replaced before coating?
Any screw that is backed out, stripped, or spinning gets replaced or upsized first — the membrane needs a fastener that is actually holding the panel down. Screws that are tight but have hardened, cracked washers do not need replacing; the stripe pass and field membrane encapsulate the head and washer completely, and from that point the washer is no longer doing any sealing work.
Condensation & Insulation
Water drips off the underside of my roof. Will coating the top fix it?
No — and this is the most important question on this page. If the water appears on cold mornings, there is no stain path from a fastener, and it is worst in the parts of the building with the most humidity, it is condensation, not a leak. Coating the exterior does nothing about it. The fix is closed-cell spray foam in direct contact with the underside of the deck, which moves the dew point out of the steel.
Can I just add batt insulation or a vapor barrier under the roof?
It rarely works on a steel deck. Batts and hung vapor barriers leave air paths at the edges, around purlins, and at every penetration, and moist air will find them and reach the cold metal behind the insulation — where it condenses invisibly. Closed-cell foam works because it is adhered continuously with no gap and is its own vapor retarder. Continuity is the whole mechanism.
Should the foam go on top of the deck or underneath?
Depends on the problem. Underneath solves condensation and is the ag and food-processing answer. On top adds R-value, allows tapering to re-slope a ponding low-slope roof, and puts the insulation inside the weather envelope — the commercial and industrial answer. Some buildings get both. Tell us what you are trying to solve and we will tell you which side.
Can spray foam be left uncoated on top of a roof?
No. Closed-cell foam has no UV resistance and very little impact resistance. Left bare it degrades from the surface down and any foot traffic damages it. The polyurea membrane over it is what makes the assembly a roof — OR-80SLM at 60 mil minimum over foam, with FJ1 in walkways.
Cost, Coverage & Application
How much material does a corrugated steel roof take?
More than the plan dimension suggests. At 100% solids one gallon yields 1,604 mil-sq-ft, but a corrugated or ribbed panel has 15 to 25 percent more developed surface area than its footprint, and the lap and fastener stripe passes add material on top of the field spec. Send us the panel profile and square footage and we will run the real takeoff.
How does this compare in cost to replacing the steel roof?
Restoration is typically a fraction of tear-off and re-panel, and it avoids disposal, structural exposure, and shutting down the building. For ag operations that matters most seasonally — you can coat a storage building between crops in a way you cannot re-roof it. It also usually books as maintenance rather than capital. Where the panel is genuinely gone, replacement is the right answer and coating is not a cheaper version of it.
Can the building stay in use during the work?
Exterior coating, generally yes — the work happens above the building and each sprayed section is weathertight within a minute. Interior foam work requires the space to be clear during application and for a ventilation period afterward, which on an ag building usually means scheduling between crops or between herd movements. We plan around it.
What equipment is required, and can my crew do it?
A plural-component proportioner with a heated hose — Graco Reactor E-20 or E-30 class or equivalent — for both the membrane and the foam. If you already run a foam rig you are most of the way there. If you have no equipment we can refer an applicator in your region or talk you through adding the capability, including training and job support on the first roofs.
Question About a Specific Building?
Panel type, age, what you are seeing inside, and square footage. That is enough for a real answer.
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