<SEC-DOCUMENT>0001127602-12-030942.txt : 20121115
<SEC-HEADER>0001127602-12-030942.hdr.sgml : 20121115
<ACCEPTANCE-DATETIME>20121115160103
ACCESSION NUMBER:		0001127602-12-030942
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20121113
FILED AS OF DATE:		20121115
DATE AS OF CHANGE:		20121115

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Gupp William R.
		CENTRAL INDEX KEY:			0001428106

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-14649
		FILM NUMBER:		121208714

	MAIL ADDRESS:	
		STREET 1:		C/O TREX COMPANY
		STREET 2:		160 EXETER DRIVE
		CITY:			WINCHESTER
		STATE:			VA
		ZIP:			22603

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			TREX CO INC
		CENTRAL INDEX KEY:			0001069878
		STANDARD INDUSTRIAL CLASSIFICATION:	LUMBER & WOOD PRODUCTS (NO FURNITURE) [2400]
		IRS NUMBER:				541910453
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		160 EXETER DRIVE
		CITY:			WINCHESTER
		STATE:			VA
		ZIP:			22603-8605
		BUSINESS PHONE:		5405426300

	MAIL ADDRESS:	
		STREET 1:		160 EXETER DRIVE
		CITY:			WINCHESTER
		STATE:			VA
		ZIP:			22603-8605
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>form4.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
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<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-11-13</periodOfReport>

    <issuer>
        <issuerCik>0001069878</issuerCik>
        <issuerName>TREX CO INC</issuerName>
        <issuerTradingSymbol>TREX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001428106</rptOwnerCik>
            <rptOwnerName>Gupp William R.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O TREX COMPANY, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>160 EXETER DRIVE</rptOwnerStreet2>
            <rptOwnerCity>WINCHESTER</rptOwnerCity>
            <rptOwnerState>VA</rptOwnerState>
            <rptOwnerZipCode>22603-8605</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>CAO, Gen. Counsel &amp; Secretary</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
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                <value>Common Stock</value>
            </securityTitle>
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                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
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            <transactionAmounts>
                <transactionShares>
                    <value>6456</value>
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                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                <transactionPricePerShare>
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            <transactionDate>
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                </transactionShares>
                <transactionPricePerShare>
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            <transactionDate>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
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                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-11-15</value>
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                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionTimeliness></transactionTimeliness>
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                <transactionShares>
                    <value>1142</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>36.95</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
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                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
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                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-11-15</value>
            </transactionDate>
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                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
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                </transactionShares>
                <transactionPricePerShare>
                    <value>36.96</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
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                </sharesOwnedFollowingTransaction>
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                <directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-11-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>209</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>36.97</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                </transactionAcquiredDisposedCode>
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            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
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                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-11-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
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                <footnoteId id="F3"/>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>300</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>36.98</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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        <nonDerivativeTransaction>
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            <transactionDate>
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                <transactionCode>S</transactionCode>
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                <footnoteId id="F3"/>
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            <transactionTimeliness></transactionTimeliness>
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                <transactionShares>
                    <value>400</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>36.99</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
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                <sharesOwnedFollowingTransaction>
                    <value>41107</value>
                </sharesOwnedFollowingTransaction>
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                <directOrIndirectOwnership>
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                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-11-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
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            <transactionTimeliness></transactionTimeliness>
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                    <value>385</value>
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                <transactionPricePerShare>
                    <value>37.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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                </sharesOwnedFollowingTransaction>
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                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Appreciation Right</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>13.44</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-11-13</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>6456</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2010-02-18</value>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-02-18</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6456</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6456</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">6,456 Stock Appreciation Rights (SARs) are being exercised pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on July 31, 2012.</footnote>
        <footnote id="F2">1,786 shares are being surrendered as allowed by the Company's 2005 Stock Incentive Plan to cover the payment of taxes currently due on the exercise of stock appreciation rights.</footnote>
        <footnote id="F3">The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on July 31, 2012.</footnote>
        <footnote id="F4">The price reported in Column 4 is a weighted average price.  These shares were sold in multiple transactions at prices ranging from $37.67 to $38.11, inclusive.  The reporting person undertakes to provide Trex Company, Inc., any security holder of Trex Company, Inc., or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in Footnote (4) to this Form 4.</footnote>
        <footnote id="F5">The stock appreciation right becomes exercisable in three equal installments beginning on the first anniversary of the grant date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Willaim R. Gupp</signatureName>
        <signatureDate>2012-11-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
