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2018-06 Washington State Parks and Rec

This research study was initiated by Washington State Parks to investigate the potential of wood coatings to improve the durability and extend the service life, while maintaining the historical appearance of shingle and shake roofs. Washington State Parks oversees the maintenance and the preservation of approximately four hundred historic and modern roofs, mostly covered with locally available western redcedar (Thuja plicata) shingles and shakes that and rarely painted or coated with protective wood finishes. The study is motivated by the rising costs of maintaining and replacing historic roofs, combined with the declining quality of new western redcedar (WRC) stock and the lack of technical information on the performance of wood coatings for protection of WRC shingle and shake roofs.

Ten penetrating, non-film forming, traditional and contemporary, commercially available finishes were selected for the study based on predetermined set of criteria, including aesthetic appearance, service life, environmental and health risks, cost, and efficacy toward biotic and abiotic deterioration. The study encompasses the natural weathering exposure testing over the course of one year at three sites locations within the Washington State Parks, representative of major Pacific Northwest climate regions (Oceanic, Maritime and Arid climates). Field data on the performance of uncoated control and coated sample groups were collected after six and twelve months of outdoor exposure for color change, graying, surface erosion, microbial surface growth, fungal decay, water repellency, checking and cupping. Laboratory testing was performed to determine water permeability of wood coatings, and chemical changes resulting from natural weathering using ATR-FTIR.

Images of shingle samples arranged in a grid by type, location, and treatment phase.

Comparisons for color and graying between uncoated control shingles (white label) and unweathered control following first and second outdoor exposure phases for northwest and southeast exposures at three testing sites.

The results demonstrate pronounced differences in the performance among test groups with respect to the climate conditions and coating formulations. The most prominent changes during the first six months of exposure were loss of color, graying, and appearance of microbial surface growth on tested shingles, with the onset of surface erosion, checking, and cupping over the course of the second six months of exposure. In general, all coatings performed equally well or better compared to uncoated groups. The strong climate effect was expressed as higher risk of UV photodegradation to uncoated wood at Arid site, and higher risk to degradation of coatings at Oceanic and Maritime sites characterized by harsh humid climates. The respective differences among individual coatings were governed by the formulation compositions. The overall best performance was observed for TWP 1500 oil-borne, and Sta Brite R water-borne formulations.

The study will be continued over the course of the second year of natural weathering exposure to supplement long-term data on the performance of investigated coatings. The future findings will support further evaluation and consideration for their use by Washington State Parks.

National Center for Preservation Technology and Training
645 University Parkway
Natchitoches, LA 71457

Email: ncptt[at]
Phone: (318) 356-7444
Fax: (318) 356-9119