Exhibit 5.1

May 20, 2013

Commercial Metals Company

6565 N. MacArthur Blvd.

Irving, Texas 75039

Ladies and Gentlemen:

We refer to the Registration Statement on Form S-3, File No. 333-188366 (the “Registration Statement”), filed by Commercial Metals Company, a Delaware corporation (the “Company”), with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration of an indeterminate principal amount of the Company’s debt securities, which Registration Statement became effective upon filing pursuant to Rule 462(e) under the Securities Act. Pursuant to the Registration Statement, the Company is issuing $330,000,000 aggregate principal amount of the Company’s 4.875% Senior Notes due 2023 (the “Notes”). The Notes are being issued under an Indenture, dated as of May 6, 2013 (the “Base Indenture”), between the Company and U.S. Bank National Association, as trustee (the “Trustee”), as supplemented by the First Supplemental Indenture dated May 20, 2013 (the “Supplemental Indenture”) between the Company and the Trustee. The Base Indenture, as supplemented by the Supplemental Indenture, is hereinafter referred to as the “Indenture.” The Notes are to be sold by the Company pursuant to an Underwriting Agreement dated May 8, 2013 (the “Underwriting Agreement”) among the Company and Citigroup Global Markets Inc. and Deutsche Bank Securities Inc., as representatives of the several underwriters named in Schedule II to the Underwriting Agreement.

This opinion letter is being delivered in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act.

We have examined the Registration Statement, the Indenture, the Underwriting Agreement, the Notes in global form and the resolutions adopted by the board of directors of the Company, the Finance Committee thereof, and the pricing sub-committee of the Finance Committee established by the Finance Committee relating to the Registration Statement, the Indenture, the Underwriting Agreement and the issuance of the Notes by the Company. We have also examined originals, or copies of originals certified to our satisfaction, of such agreements, documents, certificates and statements of the Company and other corporate documents and

Sidley Austin LLP is a limited liability partnership practicing in affiliation with other Sidley Austin partnerships.

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instruments, and have examined such questions of law, as we have considered relevant and necessary as a basis for this opinion letter. We have assumed the authenticity of all documents submitted to us as originals, the genuineness of all signatures, the legal capacity of all persons and the conformity with the original documents of any copies thereof submitted to us for examination. As to facts relevant to the opinion expressed herein, we have relied without independent investigation or verification upon, and assumed the accuracy and completeness of, certificates, letters and oral and written statements and representations of public officials and officers and other representatives of the Company.

Based on and subject to the foregoing and the other limitations, qualifications and assumptions set forth herein, we are of the opinion that the Notes will constitute valid and binding obligations of the Company when the Notes are duly executed by duly authorized officers of the Company and duly authenticated by the Trustee, all in accordance with the provisions of the Indenture, and delivered to the purchasers thereof against payment of the agreed consideration therefor in accordance with the Underwriting Agreement.

With respect to each instrument or agreement referred to in or otherwise relevant to the opinion set forth herein (each, an “Instrument”), we have assumed, to the extent relevant to the opinion set forth herein, that (i) each party to such Instrument (if not a natural person) was duly organized or formed, as the case may be, and was at all relevant times and is validly existing and in good standing under the laws of its jurisdiction of organization or formation, as the case may be, and had at all relevant times and has full right, power and authority to execute, deliver and perform its obligations under such Instrument; (ii) such Instrument has been duly authorized, executed and delivered by each party thereto; and (iii) such Instrument was at all relevant times and is a valid, binding and enforceable agreement or obligation, as the case may be, of, each party thereto; provided that (x) we make no such assumptions in clause (i) and (ii) insofar as they relate to the Company and (y) we make no such assumption in clause (iii) insofar as it relates to the Company and is expressly covered by our opinion set forth herein.

Our opinion is subject to bankruptcy, insolvency, reorganization, moratorium, fraudulent conveyance, fraudulent transfer and other similar laws relating to or affecting creditors’ rights generally and to general equitable principles (regardless of whether considered in a proceeding in equity or at law), including concepts of commercial reasonableness, good faith and fair dealing and the possible unavailability of specific performance or injunctive relief.

This opinion letter is limited to the General Corporation Law of the State of Delaware and the laws of the State of New York (excluding the securities laws of the State of New York). We express no opinion as to the laws, rules or regulations of any other jurisdiction, including, without limitation, the federal laws of the United States of America or any state securities or blue sky laws.

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We hereby consent to the filing of this opinion letter as an Exhibit to the Registration Statement and to all references to our Firm included in or made a part of the Registration Statement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.