Exhibit 99.1 Koppers Holdings Inc. 436 Seventh Avenue Pittsburgh, PA 15219-1800 Tel 412 227 2001 www.koppers.com News Release FOR IMMEDIATE RELEASE For Information: Quynh McGuire, Vice President, Investor Relations 412 227 2049 McGuireQT@koppers.com Koppers Reports First Quarter 2024 Results; Provides Updated 2024 Outlook Completed Acquisition of Brown Wood Preserving Company First Quarter Sales of $497.6 Million vs. $513.4 Million in Prior Year Quarter First Quarter Diluted EPS of $0.59 vs. $1.19 in Prior ...