UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

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SCHEDULE 13G/A

Under the Securities Exchange Act of 1934

(Amendment No. 2)

Trex Company, Inc.

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(Name of Issuer)

Common Stock

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(Title of Class of Securities)

89531P-10-5

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(CUSIP Number)

December 31, 2011

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(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which the Schedule is filed:

Item 4. Ownership

The Reporting Persons may be deemed to beneficially own shares of the common stock, par value $0.01 per share (the “Common Stock”) of Trex Company, Inc. (the “Issuer”) which are held in certain family trusts, with the Reporting Persons being the trustees or co-trustees thereof. Where the Reporting Person is the sole trustee of a trust, the Reporting Person may be deemed to have sole voting and dispositive power of the Common Stock held in such trust. Where the Reporting Person is a co-trustee of a trust, the Reporting Person may be deemed to have shared voting and dispositive power of the Common Stock held in such trust.

During the period on or about March 18, 2011 through April 5, 2011, the Reporting Persons sold 273,000 shares of the Common Stock of the Issuer on the open market (the “Sale Transaction”).

Number of shares as to which Carl W. Knobloch, Jr. may be deemed to own after the Sale Transaction:

Number of shares as to which Emily C. Knobloch may be deemed to own after the Sale Transaction:

Number of shares as to which William R. Knobloch may be deemed to own after the Sale Transaction:

*Based upon 15,594,825 shares of Common Stock outstanding as of October 26, 2011, as reported in the Issuer’s quarterly report on Form 10-Q for the period ended September 30, 2011.

Each of the Reporting Persons disclaim beneficial ownership of shares held in a trust for which such Reporting Person is not a trustee.

Item 5. Ownership of Five Percent or Less of Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than 5 percent of the class of securities, check the following £.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: February 1, 2012

EXHIBIT INDEX

EXHIBIT A

AGREEMENT OF JOINT FILING

In accordance with Rule 13d-l(k)(1) under the Securities Exchange Act of 1934, as amended, the undersigned acknowledge and agree that this Schedule 13G/A is filed on behalf of each of the them and that all subsequent amendments to this Schedule 13G/A may be filed on behalf of each of them without the necessity of filing additional joint filing agreements. The undersigned acknowledge that each shall be responsible for the timely filing of such amendments, and for the completeness and accuracy of the information concerning him, her or it contained herein and therein, but shall not be responsible for the completeness and accuracy of the information concerning the others, except to the extent that he, she or it knows that such information is inaccurate.