Roof Coatings: Beware of Salts

Everyone knows that salts cause corrosion (ask anyone who lives where they salt roads in the Winter), but it is often not appreciated what a huge bearing this has on the performance of roof coatings.

1. Uniform Atmospheric Corrosion
The corrosion rate of the metal in a roof depends directly on the concentration of salts such as sulfates and chlorides. This is because anions in the salts catalyze the oxidation of iron. So how long your roof coating will last depends, among other things, on how much salt (from acid rain and other pollutants) your roof is exposed to.

2. Salty Coatings?
Many roof coatings that meet Title 24 "Cool Roof" requirements are made of acrylic latex emulsions in water. These emulsions can contain salts in the formulation. Putting on one of these "government approved" coatings is like adding pollutants yourself to your own roof.

Recommendations:

1. Perform a roof inspection.
There are many "do it yourself" forms available. Checking for corrosion is always one of the tick boxes. Uniform atmospheric corrosion looks like whole panels or sets of metal fasteners corroding at the same time.

Galvanized metal corrodes white first (zinc oxide), then red (ferric oxide).

Roof coatings should chalk first (oxidized polymer) before failing.

If you see what looks like orange blooms in an otherwise good looking coating, chances are you have corrosion from salts in the formulation.

2. Test products first.
Even good roof coatings will perform differently in different situations. Ask for a test patch and see how the product performs on your roof before coating the entire roof.

Roof Coatings: Restoration or Repair?

Roof coatings typically fall into two groups: Repair and Restoration. Both use similar products, but the objective is different. Restoration will include repairs, but the objective is to avoid roof decay, allowing your roof to live longer.

Coatings for Roof Repair
Many coating systems exist for metal roof repair. Regardless of how many layers, or plys, they contain they are comprised chiefly of elastomeric resins of one kind or another (e.g. acrylic, butyl, polyurethane, silicone). An elastomeric coating has elastic properties, and can stretch and return to its original shape without permanent deformation, accommodating small shifts in roof panels such as those caused by changing temperatures. Each polymer type has advantages and disadvantages:

• Acrylic: Low cost water based coatings are easy to use and clean up. They are permeable which allows moisture to pass through the coating.
• Butyl: Highly resistant to moisture and vapor transmission, these coatings are typically used over asphalt built up roofs.
• Polyurethane: Providing greater ponding resistance, these coatings are also highly resistant to moisture and vapor transmission. Typically used over metal, single ply (PVC, TPO), and EPDM. They can also be used on modified bitumen.
• Silicone: A popular alternative to polyurethane for use over polyurethane foam.

Coatings for Roof Restoration
Used alone, an elastomeric coating system represents a patch on the entire roof, thus avoiding the main problem with small patches - moving leaks around. However, in a roof restoration system consideration must also be given to protecting the deck. Typically this is done with two additional products:

• Sealer: Made from the same elastomeric polymers described above, this product is applied in a thick layer to seal seams, prevent ponding, and protect the metal or wood deck underneath the roofing material.
• Cathodic Protection: A coating containing metals such as zinc or aluminum to act as sacrifical anodes, extending the life of the steel fasteners or metal deck.
  

Roof Maintenance: Restore Don't Repair

Various roof maintenance options can have radically different outcomes. Typical options include:

1. Do nothing. No action or expense is required. Outcome: deck (metal, wood, etc) corrodes and an expensive re-roof is necessary.
2. Repair roof with patches. A roofing contractor is called and goes up on the roof with a bucket of tar, or the equivalent. Outcome: leak is initially stopped, but now water accumulates in a new area so every year more patching is necessary, deck eventually corrodes and an expensive re-roof is again required.
   
3. Restore. The old roofing material is renewed. Re-coating a metal roof is analogous to putting new roof tiles or shingles on a conventional roof. Outcome: leaks are stopped, deck is protected for the life of the coating, tile, or shingle.
   

The interested reader is referred to systems such as Iron Sentry for restoring of metal, built up, or single ply roofs MCA for clay roof tiles, and Elk for shingles.

Metal Roof Repair: Why the leaky roof?

The first, and often the most difficult, step in metal roof repair is to diagnose the problem correctly. Possibilities include:

1. Corrosion

A metal roof is simply a steel deck with a coating on top (galvanized, Kynar, etc) to act as the roofing material. As the coating ages it develops the need for repairs. The chemistry of rust is actually quite complicated and the source can be galvanic, pitting, atmospheric, crevice, or a number of other types.

2. Poor workmanship (Improper design, construction, or maintenance)

Faulty skylights, HVAC units not flashed in correctly, gutters absent or improperly installed, leaky window casings are all more common sources of leaks than corrosion early in the roof’s life.

3. Severe Weather

High winds and heavy snow can overload a roof, causing damage that must be repaired immediately. An inspection should take place as soon as possible after a severe weather event.

4. Wear and Tear

Traffic on a roof, without a correctly designed and installed walkway, will prematurely wear most roofing materials. Coupled with corrosion this can also lead to structural failure.

5. Structural Failure, Building Movement

If the building owner waits too long to make repairs, the iron in the steel panels will start to corrode, leading to panel buckling. In time, the purlins and trusses that support the roof may also corrode. Buildings often settle with time and while this may not be a catastrophic event, coupled with normal material expansion and contraction this can lead to structural failure of the roof.