Exhibit 5.1

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March 13, 2017

Patrick Industries, Inc.

107 West Franklin, P.O. Box 638,

Elkhart, Indiana 46515

Ladies and Gentlemen:

We have acted as counsel to Patrick Industriese, Inc., a Delaware corporation (the “Company”), in connection with (i) the registration statement on Form S-3 (Registration No. 333-204777) (as amended, the “Registration Statement”) filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Act”), relating to the proposed public offering on a delayed basis pursuant to Rule 415 under the Act of up to $200,000,000 in aggregate amount of the securities listed therein; and (ii) the prospectus supplement, dated March 8, 2017 (the “Prospectus Supplement”) relating to the issue and sale pursuant to the Registration Statement of 1,350,000 shares of Common Stock of the Company, and up to 202,500 shares of Common Stock of the Company issuable upon exercise of an over-allotment option granted by the Company (the “Shares”).

The Shares are to be issued and sold by the Company pursuant to an Underwriting Agreement dated March 8, 2017 by and among Merrill Lynch, Pierce, Fenner & Smith Incorporated, Wells Fargo Securities, LLC, and Robert W. Baird & Co. Incorporated, and each of the other Underwriters named in Schedule A thereto (the “Underwriters”), and the Company, which will be filed with the Commission as Exhibit 91.1 to the Company’s Current Report on Form 8-K, dated March 8, 2017.

In arriving at the opinions expressed below, we have examined and relied, to the extent we deemed proper, on a signed copy of the Registration Statement, a copy of the Prospectus Supplement (each as filed with the Commission), the Underwriting Agreement, resolutions and minutes of meetings of the Board of Directors of the Company, including committees thereof, each as provided to us by the Company, the Articles of Incorporation and Bylaws of the Company, each as restated and/or amended to date, communications with local Indiana counsel to the Company, and such other documents as we have deemed necessary for purposes of rendering the opinion hereinafter set forth.

In our examination, we have assumed the authenticity of all documents submitted to us as originals, the conformity to the original documents of all documents submitted to us as copies, the genuineness of all signatures on documents reviewed by us and the legal capacity of all natural persons.

Based upon, subject to and limited by the foregoing, we are of the opinion that, as of the date hereof, the Shares have been duly authorized for issuance and, when the Shares are issued and paid for in accordance with the terms and conditions of the Underwriting Agreement, the Shares will be validly issued, fully paid and non-assessable.

We express no opinion as to the applicability of, compliance with or effect of the law of any jurisdiction other than the laws of the State of Illinois and the State of New York and, to the extent relevant to the opinions expressed herein, based on communications with Indiana counsel, the Indiana Corporation Law of the State of Indiana.

This opinion speaks only as of the date hereof and only as to the matters expressly set forth herein. We expressly disclaim any responsibility to advise you of any development or circumstance of any kind, including any change of law or fact that may occur after the date of this opinion that might affect the opinions expressed herein.

We hereby consent to the reference to our firm under the caption “Legal Matters” in the Prospectus Supplement and to the use of this opinion as an exhibit to the Company’s Current Report on Form 8-K. In giving this consent, we do not hereby admit that we come within the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission.

Very truly yours,

/s/ McDermott Will & Emery LLP