A CASE STUDY: Do decking oils and other treatments affect Black Locust wood surfaces?

It is time we addressed the biggest question about Black Locust wood surfaces. A question we get all the time from our clients in architecture, landscape design and the green building industry. Is it necessary to use decking oils and other treatments on Black Locust wood surfaces? Or can decking oils and treatments negatively impact Black Locust wood?

There is great confusion around this topic and we need to set the record straight. Having incorporated Black Locust wood as decking, siding, site furnishing and permeable paver products in residential and urban projects, we have experienced many situations.

In our 30+ years of experience working with the wood, we have a good understanding of Black Locust lumber properties and its characteristics.

As experts, we strongly recommend NOT to use oils and chemicals to treat Black Locust wood surfaces. Through our case study, let’s explore why.  

Black Locust wood left natural versus applying oils and surface treatments

From our very own portfolio we would like to show you a case study of two architectural projects that are parallel in age that incorporates Black Locust wood.

The only difference is one of them decided to later apply oils and other treatments and the other did not, they left Black Locust wood natural. They say a picture speaks a thousand words. We have proof with pictures that show decking oils and other treatments actually seriously impact Black Locust wood surfaces.

I. Case study: Applying oils and other treatments to Black Locust sidings 

Location: Cherry Road Elementary School, Westhill Central District
Surface: Black Locust Lumber sidings
Year of installation: 2015
Material: Black Locust 
Applied surface treatments? yes
Cherry Road Elementary School, Westhill Central District Black Locust Lumber sidings picture taken upon installation BEFORE applying surface treatments.

Black Locust Lumber siding was installed on the exterior of the building for the Cherry Road Elementary School in Westhill Central District over 6 years ago. Black Locust wood has a natural durability of 50+ years. It is naturally rot-resistant and has antifungal properties.

Black Locust is a hardwood that keeps its attractive appearance without the help of chemicals, oils or any other surface treatments. No maintenance is required. 

People traditionally apply surface treatments to woods that are exposed to the outdoor elements. In the case of softwoods preservers or waterproofing treatments are applied and in case of tropical hardwoods oils are applied to maintain their longevity and appearance.

This is unnecessary and not required for Black Locust, If you decide to add surface treatments it will not enhance the longevity and durability of the Black Locust wood, it’s only added for the aesthetic. However, doing so will create a high maintenance item. 

Upon completion the developer of Cherry Road Elementary School decided to apply a stain product to the Black Locust Lumber siding and not follow our and the architectural firms’ guidelines and recommendations.

Cherry Road Elementary School, Westhill Central District Black Locust Lumber sidings AFTER applying surface treatments.

After applying surface treatment to Cherry Road Elementary School’s Black Locust sidings the wood absorbed the oils which got affected by UV light pollutants in the air.

The reason we don’t advise using oils to treat Black Locust is apparent. As the surface treatment wears over time this situation becomes more and more pronounced.

The Black Locust wood maintains its integrity and durability, the aesthetic qualities of the surface, however, now face an expensive cleaning and recovery process. 

II. Case study: Leaving Black Locust decking natural

Location: New York Botanical Gardens Native Plant
Surface: Black Locust Lumber exterior decking
Year of installation: 2016
Material: Black Locust 
Applied surface treatments? none
New York Botanical Gardens Native Plant decking picture taken upon installation

The New York Botanical Gardens’s Native Plant promenade, decking, and exterior site furnishings were built with native, sustainably harvested Black Locust Lumber.

Since sustainability was a key factor in this architectural project, it only made perfect sense to use Black Locust wood. Some surface treatments and oils are toxic to marine aquatic life. Black Locust, in other words, Robinia Pseudoacacia is a natural and sustainable alternative to endangered tropical hardwoods which require chemical treatment for longevity.

Chemically-treated lumber lasts a fraction of the lifetime compared to Black Locust. Black Locust wood is an organic product that does not require treatment to enhance its longevity and durability.

New York Botanical Gardens Native Plant Garden decking picture taken 5 years after installation, no surface treatments were applied.

Left natural, this is what Black Locust Lumber looks like without oils and surface treatments.

As you can see in the picture above, with time and exposure to the natural environment Black Locust wood weathers and develops a silvery-gray patina.

The color is a result of natural elements and surroundings: UV, rain, and snow.

Presently there is no known finish to eliminate UV deterioration for Black Locust wood or any other wood species, UV wood treatments will only delay the natural patina of the wood. 

Conclusions about using oils and surface treatments on Black Locust

As visible from the two case studies of Cherry Road Elementary School and New York Botanical Gardens, we do not recommend using oils or other surface treatments on Black Locust.

The most sustainable solution is to leave it natural. It will develop a silvery grey color over time but it will do so gradually. 

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