<SEC-DOCUMENT>0001127602-12-010724.txt : 20120315
<SEC-HEADER>0001127602-12-010724.hdr.sgml : 20120315
<ACCEPTANCE-DATETIME>20120315124633
ACCESSION NUMBER:		0001127602-12-010724
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20120313
FILED AS OF DATE:		20120315
DATE AS OF CHANGE:		20120315

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Cline James E
		CENTRAL INDEX KEY:			0001430183

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-14649
		FILM NUMBER:		12693126

	MAIL ADDRESS:	
		STREET 1:		C/O TREX COMPANY INC.
		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>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-03-13</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001430183</rptOwnerCik>
            <rptOwnerName>Cline James E</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>Vice President and CFO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-03-13</value>
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                <transactionCode>M</transactionCode>
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                <footnoteId id="F3"/>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>24000</value>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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                </transactionShares>
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                <transactionShares>
                    <value>300</value>
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                <transactionPricePerShare>
                    <value>29.47</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
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        <nonDerivativeTransaction>
            <securityTitle>
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            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>200</value>
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                <transactionPricePerShare>
                    <value>29.505</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
                    <value>46371</value>
                </sharesOwnedFollowingTransaction>
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                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-03-15</value>
            </transactionDate>
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                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
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                </transactionShares>
                <transactionPricePerShare>
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                </transactionPricePerShare>
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                <sharesOwnedFollowingTransaction>
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                </transactionPricePerShare>
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            <transactionAmounts>
                <transactionShares>
                    <value>300</value>
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                <transactionPricePerShare>
                    <value>28.84</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
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                    <value>28.85</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>44571</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Appreciation Right</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>7.52</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-03-13</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>24000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-03-03</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2018-03-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>24000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7364</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">7,513 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 SARs.</footnote>
        <footnote id="F2">This stock appreciation right becomes exerciseable in approximately three equal installments beginning on the first anniversary of the March 3, 2008 grant date.</footnote>
        <footnote id="F3">24,000 Stock Appreciation Rights are being exercised pursuant to Rule 10b5-1 trading plan adopted by the reporting person on February 28, 2012.</footnote>
        <footnote id="F4">The sales reported in this Form 4 were effected pursuant to Rule 10b5-1 trading plan adopted by the reporting person on February 28, 2012.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ William R. Gupp by power of attorney</signatureName>
        <signatureDate>2012-03-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
