-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 WRNc4w3di4IAAVAScgXHvco3tj8YyYF2/Ey1gCw49iUM50yXdU1A5TciAQd3qu8J
 x+QxZcTLr35XQ3+gyopiAQ==

<SEC-DOCUMENT>0001127602-11-000520.txt : 20110104
<SEC-HEADER>0001127602-11-000520.hdr.sgml : 20110104
<ACCEPTANCE-DATETIME>20110104145501
ACCESSION NUMBER:		0001127602-11-000520
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20110103
FILED AS OF DATE:		20110104
DATE AS OF CHANGE:		20110104

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Posey Richard E
		CENTRAL INDEX KEY:			0001463823

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-14649
		FILM NUMBER:		11505121

	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>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-01-03</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001463823</rptOwnerCik>
            <rptOwnerName>Posey Richard 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>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-01-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>126</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>25.25</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>254</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Appreciation Right</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>25.25</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2011-01-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>227</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2011-01-03</value>
            </exerciseDate>
            <expirationDate>
                <value>2021-01-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>227</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>227</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">This restricted award will vest on the first anniversary of the grant.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ William R. Gupp by power of attorney</signatureName>
        <signatureDate>2011-01-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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