SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K/A

(Amendment No. 1)

CURRENT REPORT

Pursuant to Section 13 or 15 (d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): August 21, 2014 (August 15, 2014)

KOPPERS HOLDINGS INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (412) 227-2001

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2.):

Koppers Holdings, Inc. (“Holdings”) filed a Current Report on Form 8-K on August 21, 2014 to report (i) the entrance by Koppers Inc. (“Koppers” or the “Company”), a wholly-owned subsidiary of Holdings, into the Senior Secured Credit Facilities (as defined therein), dated as of August 15, 2014; (ii) the entrance by Koppers into Amendment No. 1 to a Stock Purchase Agreement, dated as of August 15, 2014; (iii) the termination of a debt financing commitment letter, dated as of April 13, 2014 related to the Senior Secured Credit Facilities; and (iv) the completion of the Company’s acquisition of Osmose, Inc. and Osmose Railroad Services, Inc. (the “Osmose Entities”). Pursuant to Item 9.01(a)(4) of Form 8-K, this Amendment No. 1 to Form 8-K is being filed in order to include the required historical financial statements of the Osmose Entities and the related pro forma financial information not later than 71 calendar days after August 21, 2014, the date that the initial Current Report on Form 8-K was required to have been filed to report the completion of the acquisition of the Osmose Entities.

On August 15, 2014, Koppers completed the Acquisition pursuant to a Stock Purchase Agreement (the “Stock Purchase Agreement”) entered into on April 13, 2014 by and among Koppers, Seller and the Osmose Entities, as amended by the Amendment. The Osmose Entities and their subsidiaries are engaged in the business of manufacturing wood preservatives worldwide and providing railroad services in North America. The Acquisition was previously announced by Holdings in its Current Report on Form 8-K filed on April 14, 2014.

The Acquisition closed for an aggregate cash purchase price of approximately $494.1 million (the “Closing Purchase Price”), which included $27.3 million of cash in foreign countries as well as the value of an anticipated 338(h)(10) tax election. The Closing Purchase Price included estimated net working capital adjustments and is subject to certain post-closing adjustments, including, but not limited to, final net working capital adjustments. The Closing Purchase Price was funded by borrowings under the Senior Secured Credit Facilities, as more fully described in the Current Report on Form 8-K filed on August 21, 2014.

The above description does not purport to be complete and is qualified in its entirety by reference to the Stock Purchase Agreement, a copy of which was filed as an exhibit to Holdings’ Quarterly Report on Form 10-Q for the quarter ended June 30, 2014 and is incorporated by reference into this Current Report. The required historical financial statements of the Osmose Entities and the related pro forma financial information are contained herein under Item 9.01 of this Current Report.

(a) Financial Statements of Businesses Acquired.

Osmose Entities

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(b) Pro Forma Financial Information.

(d) Exhibits. The following exhibits are filed herewith:

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Dated: October 30, 2014

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