Metal roof coating: the cheapest way to extend a sound metal roof

A metal roof that leaks has usually not worn out. Metal panels last a long time, and on most leaking metal roofs the panels are fine. What has failed is several thousand rubber washers under the fastener heads, or the laps between panels, or the surface finish that was protecting the metal from ultraviolet light.

That is the roof a coating is for, and on that roof it is one of the better value decisions in building maintenance. It addresses the actual failure mechanism at a fraction of replacement cost, and it resets the maintenance cycle rather than deferring a problem.

On a different roof, one whose panels have corroded through or whose insulation is already soaked, the same coating is money spent hiding a problem that keeps progressing underneath. So the first question is never which coating. It is which roof.

What a coating is doing

A metal roof coating is a liquid applied membrane. It goes on in several passes with reinforcing fabric at the laps and around every detail, and it cures into one continuous surface across the whole roof.

That continuity is the point. An exposed fastener metal roof has thousands of individual penetrations, each sealed by a washer that hardens, shrinks and cracks on its own schedule. Chasing them one at a time is endless. A coating turns that surface into a single membrane with no individual seals to fail.

It also reflects heat, which on a hot climate building reduces cooling load and keeps the metal itself cooler, and cooler metal moves less and ages more slowly.

When coating is the right answer

Four conditions, and all four should be true.

1. The panels are sound. Surface rust is acceptable and treatable. Perforation is not.
2. The attachment is sound. Fasteners hold, or can be replaced with oversized ones that do.
3. The assembly beneath is dry. On an insulated metal roof this needs checking rather than assuming.
4. The existing finish will accept a coating. Established by a test, not by eye.

Where those hold, coating is straightforwardly cheaper than replacement and there is little argument for tearing a working roof off.

When it is the wrong answer

The disqualifying conditions are specific and worth being blunt about.

Perforated panels. A coating bridging a hole is a coating over a hole. It will fail there first.

A failing deck or structure. Nothing applied to the surface helps a deck that will not hold a fastener.

Trapped moisture below. This is the one most often missed, because the insulation on an insulated metal roof sits where nobody can see it. Coating over a wet assembly seals the water in, where it keeps destroying the insulation and corroding the panels and fasteners from the inside. The roof needs opening up, not covering over.

Loose panels. If the attachment has let go, fix the attachment. A coating adds no uplift resistance whatever.

The preparation is the job

Owners buy the coating. What they are actually paying for is the preparation, and the proportion of cost that sits in preparation rather than in material is what separates a system that lasts from one that peels.

Fasteners first. Every exposed fastener checked individually. Loose, backed out, stripped or sitting on a cracked washer, all replaced with a larger diameter fastener and a new washer. Ask how many the quote allows for. On a large roof there can be several thousand, and a quote with no allowance is a quote from somebody who did not walk the roof.

Washing. Pressure washing to remove dirt, chalking and loose rust, then full drying.

Rust treatment. Back to sound metal, or treated with a product specified for the purpose.

An adhesion test. A small test area, left a few days, then checked. Some factory finishes resist coatings, and a coating that will not bond peels in sheets after one summer. This step costs almost nothing and is the only way to know before the whole roof is committed. It is also the step most often skipped.

Primer, then details, then field. Laps, fastener heads, penetrations and terminations are detailed with reinforcing fabric before the field coats go on.

Film thickness, and how to check you got it

Coating systems are specified in mils, wet and dry, and the service life the manufacturer states assumes that thickness. A thinner application is a shorter roof, and it is invisible on handover day.

There is a simple check available. Ask for the specified thickness in writing, then ask for the material usage on completion: how many pails, over what measured area. The arithmetic takes a minute and tells you whether you received the system you bought. Any contractor working properly will have those figures and no objection to sharing them.

Where coating fits in a maintenance plan

The useful way to think about coating is as a scheduled intervention rather than a repair. A metal roof reaches a point, usually when the fasteners start to weep and the finish begins to chalk, where coating it costs a fraction of replacing it and buys a long extension.

Catch it there and the economics are excellent. Leave it another decade and the panels begin to corrode at the fastener holes, the insulation below gets wet, and the window closes. Comparing coating against replacement sets out how that comparison should be made.

What the warranty actually covers

Coating warranties are worth reading rather than filing, because they vary more than membrane warranties do and they tend to carry conditions an owner can fail without noticing.

Three questions cover most of it. Does the warranty cover material only, or material and labor. Does it require a stated film thickness, and is that thickness what was specified in your quote. And does it require documented maintenance, which on a coated roof usually means an annual inspection and the drains kept clear.

The third condition is the one that quietly voids cover. A coating warranty that depends on annual inspection records is worth nothing to an owner who has none, and the inspections are inexpensive, so the sensible step is to put them on a calendar in the week the work completes and name the person responsible.

It is also worth asking what happens at a repair. A coated roof that is later punctured needs patching in the same coating system, and a different product applied over it can fail at the junction and compromise the warranty on the surrounding area.

Standing seam and exposed fastener roofs coat differently

The two common metal profiles present different problems to a coating, and a quote that treats them the same has not been thought through.

An exposed fastener roof is mostly a fastener problem. Thousands of penetrations, each with a washer, each a candidate for replacement. The preparation is therefore dominated by fastener work, and the coating’s main contribution is turning all those individual seals into one surface.

A standing seam roof has its fasteners hidden and its seams raised above the water line, so leaks there come from the seams themselves, the end laps, and the details around penetrations. The coating work concentrates on reinforcing those raised seams with fabric, and the seams move as the panels expand, so the reinforcement has to accommodate that movement rather than simply bridge it.

There is a third consideration on standing seam roofs. The clips allowing the panels to move are part of why the roof works, and a coating applied so stiffly that it restrains that movement will split along the seams in its first hot summer. Ask how the seam detail accommodates panel movement, because the answer separates a specified system from a sprayed one.

Where to go next

If the roof is leaking now and the condition of the panels is unknown, an assessment comes before any coating quote, because the quote depends entirely on what the assessment finds. If the roof is a membrane rather than metal, the preparation and the approval requirements are different, and coating services generally covers those.