April , 2021 Dear Stockholder: You are cordially invited to attend the Annual Meeting of Stockholders (the “Annual Meeting”) of Cornerstone Building Brands, Inc. (the “Company”) to be held at 10:00 a.m. EDT on Thursday, May 20, 2021. Due to concerns regarding the ongoing coronavirus (“COVID-19”) pandemic, and to protect the health and safety of our employees, stockholders and other stakeholders, the Annual Meeting will be a virtual meeting conducted solely online via live webcast at www.proxydocs.com/CNR. There is no physical location for the meeting. To participate in the Annual Meeting, you must register in advance at www.proxydocs.com/CNR prior to the registration deadline of 5:00 p.m. EDT on May 18, 2021. As part of the registration process, you must enter the control number provided in your proxy card. After completion of your registration, further instructions, including a unique link to access the Annual Meeting, will be emailed to you. At this meeting you will be asked to: (1) Proposal 1: Elect the four (4) Class I directors named in the accompanying proxy statement to serve until the 2024 Annual Meeting of Stockholders or until their respective successors have been elected and shall have qualified or until their resignation; (2) Proposal 2: Provide an advisory vote on executive compensation; (3) Proposal 3: Ratify the appointment of Grant Thornton LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2021; and (4) Transact such other business as may properly come before the Annual Meeting or any reconvened meeting following any adjournment or postponement thereof. It is important that your shares be represented at the Annual Meeting. Therefore, whether or not you expect to virtually attend the Annual Meeting, please sign and date the enclosed proxy and return it in the enclosed envelope or submit your proxy using the telephone or Internet procedures that may be provided to you at your earliest convenience. Please note that using any of these methods will not prevent you from attending the meeting virtually and voting online at that time. Very truly yours, /s/ James S. Metcalf JAMES S. METCALF Chairman and Chief Executive Officer CORNERSTONE BUILDING BRANDS, INC. 5020 Weston Parkway Suite 400 Cary, North Carolina 27513 NOTICE OF ANNUAL MEETING OF STOCKHOLDERS TO BE HELD MAY 20, 2021 Virtual Meeting Only – No Physical Meeting Location NOTICE IS HEREBY GIVEN that the Annual Meeting of Stockholders (the “Annual Meeting”) of Cornerstone Building Brands, Inc. (the “Company”) will be held virtually, on Thursday, May 20, 2021, at 10:00 a.m. EDT. The Annual Meeting will be held for the following purposes: (1) Proposal 1: Elect the four (4) Class I directors named in the accompanying proxy statement to serve until the 2024 Annual Meeting of Stockholders or until their respective successors have been elected and shall have qualified or until their resignation; (2) Proposal 2: Provide an advisory vote on executive compensation; (3) Proposal 3: Ratify the appointment of Grant Thornton LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2021; and (4) Transact such other business as may properly come before the Annual Meeting of Stockholders or any reconvened meeting following any adjournment or postponement thereof. Only stockholders of record at the close of business on April 8, 2021 are entitled to notice of, and to vote at, the meeting or any reconvened meeting following any adjournment or postponement thereof. In accordance with Delaware law, a list of stockholders entitled to vote at the meeting will be available in electronic form on the day of the Annual Meeting and for ten days before the meeting. To request access to the list, please contact [email protected] We are first sending this proxy statement and the enclosed proxy form to stockholders on or about April 20, 2021. Due to concerns regarding the ongoing coronavirus (“COVID-19”) pandemic, and to protect the health and safety of our employees, stockholders and other stakeholders, the Annual Meeting will be a virtual meeting conducted solely online via live webcast at www.proxydocs.com/CNR. Please register in advance at www.proxydocs.com/CNR prior to the registration deadline of 5:00 p.m. EDT on May 18, 2021. As part of the registration process, you must enter the control number provided in your proxy card. After completion of your registration, further instructions, including a unique link to access the Annual Meeting, will be emailed to you. We believe that it is desirable that as large a proportion as possible of the stockholders’ interests be represented at our Annual Meeting. Whether or not you plan to attend our Annual Meeting, we request that you properly date and sign the enclosed form of proxy and promptly return it to us using the enclosed addressed and stamped envelope. If you are present at the virtual meeting and wish to do so, you may revoke the proxy and vote in person. If, however, you hold your shares through a nominee or broker, you must follow the instructions of the nominee or broker to revoke your voting instructions. By order of the Board of Directors, /s/ Alena S. Brenner ALENA S. BRENNER Executive Vice President, General Counsel and Corporate Secretary Cary, North Carolina April 20, 2021 Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting to Be Held May 20, 2021 via live webcast at www.proxydocs.com/CNR The Notice of Annual Meeting of Stockholders, our Proxy Statement and our Annual Report to Stockholders are available at www.proxydocs.com/CNR. PROXY STATEMENT FOR ANNUAL MEETING OF STOCKHOLDERS To Be Held May 20, 2021 TABLE OF CONTENTS SOLICITATION OF PROXIES 2 OUTSTANDING CAPITAL STOCK 2 QUORUM AND VOTING 6 PROPOSAL 1: ELECTION OF DIRECTORS 9 PROPOSAL 2: ADVISORY VOTE ON EXECUTIVE COMPENSATION 14 PROPOSAL 3: RATIFICATION OF APPOINTMENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM 16 MANAGEMENT 17 COMPENSATION DISCUSSION & ANALYSIS 19 EXECUTIVE COMPENSATION 33 OTHER COMPENSATION INFORMATION 40 BOARD OF DIRECTORS 43 CORPORATE GOVERNANCE 51 LEADERSHIP STRUCTURE OF THE BOARD 54 COMMUNICATIONS WITH OUR BOARD 55 DELINQUENT SECTION 16(a) REPORTS 55 TRANSACTIONS WITH RELATED PERSONS 55 AUDIT COMMITTEE AND AUDITORS 58 STOCKHOLDER PROPOSALS AND DIRECTOR NOMINATIONS 59 ANNUAL REPORT 60 STOCKHOLDERS SHARING THE SAME LAST NAME AND ADDRESS 60 MISCELLANEOUS 60 QUESTIONS AND ANSWERS ABOUT THE ANNUAL MEETING 60 CORNERSTONE BUILDING BRANDS, INC. 5020 Weston Parkway Suite 400 Cary, North Carolina 27513 (866) 419-0042 PROXY STATEMENT FOR ANNUAL MEETING OF STOCKHOLDERS TO BE HELD MAY 20, 2021 Virtual Meeting Only – No Physical Meeting Location This proxy statement is furnished to stockholders of Cornerstone Building Brands, Inc. (“Cornerstone Building Brands,” the “Company,” “we,” and “us”) in connection with the solicitation of proxies to be used at our Annual Meeting of Stockholders (the “Annual Meeting”) to be held virtually solely online via live webcast on Thursday, May 20, 2021 at 10:00 a.m. EDT at www.proxydocs.com/CNR. By granting a proxy, you authorize the persons named in the proxy to represent you and vote your shares at the Annual Meeting. Those persons will also be authorized to vote your shares to adjourn the Annual Meeting from time to time and to vote your shares at any adjournments or postponements of the Annual Meeting. Stockholders have a choice of voting over the Internet, at www.proxypush.com/CNR (registered stockholders only), by telephone using the number 1-844-927-1815, or by mail using a traditional proxy card. If you give a proxy on the enclosed form, or by telephone or the Internet, you may revoke it at any time before it is exercised at the Annual Meeting by (1) delivering written notice of revocation to the Corporate Secretary of the Company, (2) signing, dating and delivering to the Corporate Secretary of the Company a later dated proxy at our principal executive offices, which are located at 5020 Weston Parkway, Suite 400, Cary, North Carolina 27513, or (3) virtually attending the Annual Meeting and voting online. Virtual attendance at the Annual Meeting will not, in itself, constitute revocation of a completed and delivered proxy card. If you are a street name stockholder (meaning that your shares are held in a brokerage account by a bank, broker or other nominee) and you vote by proxy, you may change your vote by submitting new voting instructions to your bank, broker or nominee in accordance with that entity’s procedures. We are first sending this proxy statement and the enclosed proxy form to stockholders on or about April 20, 2021. ACTION TO BE TAKEN AT ANNUAL MEETING When you have appropriately specified how your proxy should be voted, the proxy will be voted accordingly. If you properly complete and return a proxy, but do not indicate any contrary voting instructions, your shares will be voted as follows: x FOR Proposal 1, the election as directors of the nominees listed under “Election of Directors;” x FOR Proposal 2, an advisory vote on executive compensation; x FOR Proposal 3, the ratification of Grant Thornton LLP as the Company’s independent registered public accountants for the fiscal year ending December 31, 2021 (“Fiscal 2021”); and x At the discretion of the proxy holders, either FOR or AGAINST any other matter or business that may properly come before the Annual Meeting. As of the date hereof, our Board of Directors (our “Board”) is not aware of any other such matter or business to be transacted at our Annual Meeting. If other matters requiring a vote of the stockholders arise, the persons designated as proxies will vote the shares of common stock of the Company, par value $0.01 per share (the “Common Stock”), represented by the proxies in accordance with their judgment on those matters.