The Local 18 is part of The Operative Plasterers' and Cement Masons' International Association (OPCMIA). The OPCMIA is a labor union that represents plasterers and cement masons in the construction industry in North America.

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As the oldest Building and Construction Trades Union in the United States, the organization traces its roots back to 1864, during the American Civil War, when an organization known variously as the National Plasterer’s Union or the National Plasterers Organization began to unify the various local craft unions that represented workers in the trade.

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The new union endorsed the Eight hour day movement while pledging to avoid “unnecessary strikes” and instituted apprentice training and regulations. The union amended its constitution in 1887 to include Canadian workers. Reflecting the fact that its members did more than finish cement, the union changed its name in 1951 to the Operative Plasterers’ and Cement Masons’ International Association.

Today, the OPCMIA continues to live by the principles upon which it was founded and which will continue to be its strength in the future. As it has been for over a century, the OPCMIA continues to be proud, strong and united.