Rusted Metal Building Roof: Restore With Coatings or Retrofit?

If the steel under the rust is still firm and the corrosion sits on the surface, a restoration coating system is usually the better buy: the roof is cleaned, primed, its fasteners and laps are treated, and a new protective film goes over metal that still has life in it. If the panels are pitted through, rust has eaten into the laps, the fasteners no longer hold, or the insulation below has failed, spend the money on a retrofit and put a new metal roof over the old one. What decides it is how much steel is left, not how orange the roof looks from the parking lot.

Owners of rusted warehouse, workshop and light industrial roofs around San Antonio and Austin often hear two opposite opinions, one contractor selling a coating and another selling a new roof. The checks below settle that argument on the roof itself, with evidence you can see and measure.

Start with how much steel is left

Most metal building roofs are light-gauge steel, commonly 26 or 24 gauge, so there is very little metal in reserve. Rust is not one condition. It shows up in three stages, and each stage points to a different answer.

• Surface rust: the paint has chalked, worn or flaked and the exposed steel carries an orange bloom, but a wire brush takes it back to firm, smooth metal.
• Pitting and scale: the rust comes away in flakes or layers, and the steel left underneath is cratered and rough. The panel is now thinner than it was.
• Perforation: the steel has rusted through. From inside the building on a bright day you see pinpoints of daylight, usually along laps, at the eave and under rooftop equipment.

Surface rust is a coating job. Isolated perforation can still be a coating job once the damaged panels are replaced. Pitting across large areas, or holes in many places, means the steel is running out, and a coating would only be holding together a roof that is failing underneath it. We take thickness readings with an ultrasonic gauge at prepared spots and compare them with the original panel gauge, so the decision rests on measurements rather than opinion.

Read the fasteners and laps

An exposed-fastener roof is only as watertight as its screws and joints. Each screw relies on a rubber sealing washer under its head, and years of ultraviolet light and roof heat harden and crack those washers. Daily expansion and contraction also works the screws in their holes, so some back out and others sit in holes that have worn oval.

A coating system can handle tired fasteners. Loose or stripped screws are replaced, often with an oversized screw that bites into fresh metal, and every fastener head is then sealed as part of the system. What no coating can handle is a purlin flange so badly rusted that the screws have nothing left to grip.

Laps tell you even more. Water sits in end laps and side laps longer than anywhere else on the roof, so rust starts there first and works inside the joint where nobody sees it. If a probe goes through the steel at a lap, or rust stains run downslope from most of the end laps, the joints have lost metal. Reinforced seam treatment can bridge a lap that is sound but leaking. It is no answer for a lap that has rusted away.

Look under the roof as well as on top of it

The insulation in most metal buildings is a faced fiberglass blanket draped over the purlins and pinched between each purlin and the roof panel. When the facing tears, humid indoor air reaches the underside of the steel and condensation wets the blanket. When the roof leaks, water soaks it from above. Sagging, stained or musty insulation tells you the roof has been letting water in for a long time.

A coating does nothing for that insulation. If the blanket is wet, collapsed or simply too thin for the way you now use the building, for example a warehouse being converted to cooled storage, the decision tips toward a retrofit. A roof-over leaves room above the old panels for a fresh layer of insulation at full depth.

Look at the purlins while you are inside. Rust running down purlin webs from old leaks needs a closer look whichever route you choose, because the purlins carry the roof in both cases.

When a restoration coating system makes sense

We recommend a coating system when all of these hold true:

• The steel is firm under the rust across most of the roof, and any perforation is confined to areas small enough to replace with new panels.
• Fasteners can be replaced or upsized and sealed, and the purlins still give them a solid bite.
• Laps are leaking or stained but still have their metal.
• The insulation is dry and does the job the building needs.
• Water drains off the roof rather than standing on it.

Done properly, a restoration coating is a system, not a paint job. The roof is pressure washed, loose rust and failing paint are removed, prepared rust gets a rust-inhibitive primer, fasteners are replaced or sealed, laps and penetrations are reinforced with seam sealer or fabric, and the field coat goes on at the thickness the manufacturer specifies. A light-colored topcoat also reflects much of the sun that would otherwise heat the building through a long summer. We are a certified installer for American WeatherStar and Sika, and our metal roof coating service sets out the full scope. For warehouses on Loop 410 or along I-35 toward New Braunfels, our page on commercial roof coatings in San Antonio has the local detail. Plants and flex buildings around Round Rock and Pflugerville are covered under commercial roof coatings in Austin.

When a retrofit is the better investment

A retrofit puts a new roof over the old one on sub-purlins fastened through the existing panels into the purlins. The new roof does not rely on the old steel, so rusted sheeting underneath stops being the problem. We recommend this route when:

• Perforation or deep pitting is spread across the roof rather than confined to a few runs of panels.
• Most end laps have rusted inside the joint.
• Fasteners are failing everywhere and the holes are worn out.
• The insulation is wet or inadequate and you want a real upgrade.
• A previous coating has already peeled or blistered over active rust.
• You want to move from exposed fasteners to a standing seam roof with concealed clips.

The old roof keeps the weather out while the new one goes on above it, so the building stays in use. Our metal roof retrofit service covers the sub-framing, insulation and roof details, and owners of distribution buildings on the I-10 and I-35 corridors can read more about metal roof retrofit in San Antonio.

Why coating a roof that is past saving wastes money

Coating over deep, active corrosion is the most common mistake on rusted metal roofs. Rust keeps growing under the new film, pushes it up in blisters and lifts it away in sheets, and the owner ends up paying twice. The coating adds no strength either. A thinned panel still dents under foot traffic and still tears around its fasteners in a strong wind.

Preparation cost climbs fast on a heavily rusted roof, because every patch of scale has to come off before primer can work. Past a certain point you are paying retrofit-level labor to prepare a roof that will still be thin steel when the crew leaves. When a coating price and a retrofit price for a badly corroded roof are not far apart, the retrofit is almost always the better value.

Many roofs need a split decision

Rust is rarely even across a roof. The eave side that drains into the gutter, the area downwind of an exhaust fan and the panels around old skylights corrode faster than the rest. Where the damage is concentrated, replacing those panels and then coating the whole roof is a sound plan. Where the bad areas have joined up, replacing panel after panel stops making sense, and a retrofit costs less than chasing corrosion across the roof one sheet at a time.

The heat and UV of a Central Texas summer, spring hail and sudden downpours all speed corrosion up once the factory finish has gone. A roof that is borderline today will not stay borderline for long, so it pays to decide while coating is still an option.

What a proper proposal should show you

Whichever route you take, ask for a proposal that shows its work. If you have been searching for metal roof coating contractors near me, use this list to compare the bids you receive:

• A condition map of the roof showing surface rust, pitting and perforation by area.
• Thickness readings, and where they were taken.
• The panels to be replaced, by location.
• The fastener plan: replace, upsize or seal.
• For a coating, the named system, the primer and the specified finished thickness.
• For a retrofit, the sub-purlin layout, insulation, structural review and new panel type.
• How the insulation and purlins were checked from inside.

Send us photos of the roof, the building address and how you plan to use the building over the coming years, and we will tell you plainly whether it needs a coating or a retrofit. To arrange a roof survey in San Antonio or Austin, call (210) 775-0995 or fill out our contact form.