Terne Metal Roof Paint Deterioration

Terne-coated metal roofs are an element of historic architecture, somewhat common in Capitol Hill and other historic neighborhoods of Washington, DC. However, the terne coating – an alloy of lead and tin – slowly deteriorates over time when exposed to the elements. Thermal cycling, ultraviolet radiation, moisture, and pollution all contribute to degradation of the roof paint at a microscopic level. As the coating ages, it becomes prone to cracking, oxidation, and delamination, compromising its waterproofing abilities. In this article, we will examine the mechanisms by which terne roof paint and then the successive recoats over the decades fail when subjected to years of weathering. We will identify common causes of coating distress like embrittlement and chalking. And we will overview appropriate repair methodologies to deter further deterioration, whether by spot repairs, overcoating, or full substrate recoating. Proper coating selection, surface preparation, and paint application techniques are key to restoring the watertight integrity and historic finish of terne roofs. With a basic understanding of how terne roof paints age and fail, building owners can implement repairs that preserve these building roofs for years to come. 

terne metal roof paint selection

The outline of last week’s discussion follows below for reference:

  1. Caring for Historic Painted Metal Roofs
  2. Paint Condition Assessment
  3. Repair Approach Selection 

In this week’s discussion we will touch on the following topics.

  1. Causes of Paint Distress
  2. Paint Selection 
  3. Surface Preparation

Causes of Paint Distress

With an accurate status established through a conditions survey on the rooftop, investigation into failure causes follows. Typical culprits include aged oil-based paints embrittling and losing elasticity over decades of expanding and contracting through thermal cycles. Some of the best performing roof paints generally have high ultraviolet resistance, but overall, there are no products available that are applied as coating which have unlimited ultraviolet resistance, in fact ultraviolet rays are one of the most detrimental forces and causes of deterioration on flat roof coatings.   

causes of paint distress

Water entry, ultraviolet exposure, and mechanical abrasion also degrade coatings. Initial layers may become powdery chalk beneath stronger topcoats, eventually undermining adhesion. Inadequate surface preparation or incompatibility between new and existing paint chemistries fosters future delamination as well.

what damages metal roof paint

Repair Approach Selection

Understanding the mechanisms of failure, the repair methodology is tailored to address the specific deficiencies. For minor micro-cracking or small damaged sections, techniques like elastomeric crack bridging paints or patch roofing provide local protection and reinforcement. More extensive coating failures warrant full overcoating with compatible paint systems or complete substrate cleaning and recoating. Roofs nearing the expected age or end of duration of the coating life may receive coating upgrades on remaining service life.

Paint Selection

The properties of new paint materials are important. Elastomeric paints withstand expansion and contraction stresses better than hard oil-based paints. However, the underlying chemical composition must not conflict with existing layers or roof metals. The bond and adhesion is super important.  Drying oils and alkyds often suit historic surfaces. Primer adhesion testing on various small patches can determine coating compatibility. Aesthetics should align with historic colors, in some cases, where applicable, where roofs are visible from the street or sidewalk.  This consideration is rarely applied to the typical DC main low-slope roofs though, because those main roofs are generally too high to see and hidden behind parapets. 

Surface Preparation

Thorough cleaning and removal of all loose paint prevents new coating delamination. Hand tool scraping can prepare the surface while avoiding damage by using plastic tools only and specifically flexible plastic putty knives.  These tools are not highly effective but metal tools and even hard, ridgid plastic tools can damage the thin sheet metal substrate. Power washing removes grime and debris but risks driving water under intact paint and into overlapped but unwelded seams. A mild detergent, solvent , or TSP washing may help.  TSP is our company’s personal choice. Priming immediately after cleaning as soon as it is fully dry is somewhat effective in reducing the risk of re-oxidation. Rusted metal is stabilized or chemically converted then spot primed. With optimal conditions, the finish coating adheres and protects.

base of chimney metal roof
If you look closely at the base of the chimney and the area of horizontal or low slope metal roofing around the base of the chimney you can see that a repair or Patch has been soldered into the low slope plane.

The next picture below shows an example of a metal roof up-turn to form a counterflashing around the base of a rooftop chimney. The counterflashing is not set into a reglet or raggle (or alternatively with a termination bar or other approved methodology) as required by the building code and is not coated with a redundant application of gun applied sealant as is required by modern standards. (Not all modern standards are required for all historic elements, but any proactive steward of the facility should recommend application of a gun applied sealant at the top of a base flashing against a brick chimney substrate, in almost all cases.)

water testing of metal roof
Just from visual observation alone, without actual water testing or any other means of testing, you can see that there are areas of delamination and deep shadows of openings that water could enter around the chimney mortar joints and at the base flashing.

For effective low slope and mansard roof care in Washington, DC, chose a contractor like Dupont Roofing who has a passion for historical methodologies, waterproofing, and building science. Roofs safeguard both historic and modern buildings, which are significant investments. We urge clients and readers to prioritize quality construction, building upkeep, and consider building a connection with us. Explore our blog for more insights. If you have questions about your Washington, DC building’s roof, contact us or reach out through our webform – we’re here to help.

metal roof paint

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