Characteristics of Semi-Weathering Gray and Semi-Weathering Green Natural Roofing Slates

Posted By On 14 Mar 2019

Whether you are building a new home or replacing an old slate roof, it’s important to understand the various color options available as well as the slate’s weathering characteristics. A natural slate’s color characteristics are described as “unfading” or “semi-weathering”. What makes a natural roofing slate unique and can’t be duplicated by synthetic slate companies are the aesthetic advantages of the color variations of each individual slate.  No two pieces are the alike!

North Country Semi-Weathering Green natural roofing slate
North Country Semi-Weathering Green

Semi-Weathering Gray and Semi-Weathering Green slates are commonly used in blends or standalone “old world” slate colors.  A semi-weathering slate roof once installed and exposed to the environmental elements, a percentage of these slates will weather to a range of buff, brown and tan within a few months up to a few years. It’s important that the home owner understands there is no precise percentage or guaranteed time frame, as slate is a natural product and the quarry’s color variation will vary producing slightly different layers of color because slate is a sedimentary rock.

North Country Semi-Weathering Gray natural roofing slate
North Country Semi-Weathering Gray

North Country Semi-Weathering (Vermont) Gray is a premium quality roofing slate in a rich battleship gray color that deepens over time. A percentage of these slates will weather. They make a popular roof covering alone or in combination with other slates.  These slates are produced by select Vermont quarries.

Samples of semi-weathering gray green slates showing the color variation

North Country Semi-Weathering (Sea) Green is also a premium quality roofing slate. Some slate suppliers may also call them Semi-Weathering Gray/Green. Color variations can vary from quarry to quarry and even within a particular quarry as they extract slates over the years there are layers of different colors. A semi-weathering green slate provides moderate color and texture variations while maintaining a primary background color of green. These slates are also produced by select Vermont quarries.

North Country Semi-Weathering Green roofing slate
Semi-Weathering Green roofing slate

Created by nature over 500 million years ago, a weathering slate will lighten because of its iron compounds and calcite mineral makeup. A weathering slate does not impact the quality or life expectancy of the slate. To ensure the slate’s quality, quarry’s hire 3rd party slate testing laboratories to ensure their product meets the expectations of an ASTM S1 rated roofing slate which is the best your money can buy! The National Slate Association’s recommended slate testing laboratories include; Amber Consulting Co. (Poultney, VT), St. Lawrence Testing & Inspections Co., Ltd (Cornwall, Ontario, Canada), Wiss, Janney, Elstner Associates, Inc. (Northbrook, IL) and PRI Construction Materials Technologies LLC (Tampa, FL).

North Country Semi-Weathering Gray natural roofing slate
North Country Semi-Weathering Gray

If you are installing a new slate roof with weathering slates, a skilled slater will blend slates from multiple pallets to ensure a proper blend of the various tones and avoid blotches on the roof. You may ask your slate roofer to create a mock-up board which will allow the architect and home owner to evaluate various percentages of color combinations being considered. In a multi-colored slate roof, weathering slates are frequently employed, in a combination with unfading or other semi-weathering slates, to soften the transition from color to color on the roof. These mock up boards should include a percentage of slates already showing color changes to buffs and tans in order to get the true representation of the finished roof.

North Country Semi-Weathering Gray natural roofing slate
North Country Semi-Weathering Gray

If you are replacing an old slate roof, look at the back of the slate which hasn’t been exposed to the elements, this will be a good indicator of the original slate’s color. In some cases maybe only a portion of the roof needs new slates. Again, by hiring a skilled slater, they will order material that matches the existing roof and blend the new slates in with the originals to avoid unsightly blotches on the roof.

High quality North Country roofing slates come in a wide variety of shapes, colors and sizes; so while the material is inflexible in the face of nature’s challenges, it is extremely flexible in its applications.  The only limit is your imagination.

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