X0206 3 2021-01-01 1 0001316835 Builders FirstSource, Inc. BLDR 0001212607 ALEXANDER MARK A C/O BUILDERS FIRSTSOURCE, INC. 2001 BRYAN ST., SUITE 1600 DALLAS TX 75201 1 0 0 0 On January 1, 2021, the merger (the "Merger") of Boston Merger Sub I Inc., a subsidiary of the Issuer, with and into BMC Stock Holdings, Inc., pursuant to the Agreement and Plan of Merger, dated as of August 26, 2020, was completed. This report reflects the beneficial ownership of the Reporting Person at the time of completion of the Merger and does not include the securities of the Issuer acquired by the Reporting Person upon the completion of the Merger. The Reporting Person will file a Form 4 reflecting the Reporting Person's acquisition of securities of the Issuer in connection with the completion of the Merger. /s/ Minator Azemi, by power of attorney 2021-01-05