-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 IdYfiiBpe9hMv2diehe5XPbQQa2tXU9LYNWIqjTmk7bFYw/o2I/N9+kiCYOgnYdA
 d6hpInAiEJbHwN6aSh4HjQ==

<SEC-DOCUMENT>0001069878-04-000075.txt : 20040813
<SEC-HEADER>0001069878-04-000075.hdr.sgml : 20040813
<ACCEPTANCE-DATETIME>20040813162308
ACCESSION NUMBER:		0001069878-04-000075
CONFORMED SUBMISSION TYPE:	3
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20040812
FILED AS OF DATE:		20040813

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			PIFER PHILIP J
		CENTRAL INDEX KEY:			0001300042

	FILING VALUES:
		FORM TYPE:		3
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-14649
		FILM NUMBER:		04974656

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		540-542-6300

	MAIL ADDRESS:	
		STREET 1:		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:		5406784070

	MAIL ADDRESS:	
		STREET 1:		160 EXETER DRIVE
		CITY:			WINCHESTER
		STATE:			VA
		ZIP:			22603-8605
</SEC-HEADER>
<DOCUMENT>
<TYPE>3
<SEQUENCE>1
<FILENAME>c0001300042d20040813f3f.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2004-08-12</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001300042</rptOwnerCik>
            <rptOwnerName>PIFER PHILIP J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>160 EXETER DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WINCHESTER</rptOwnerCity>
            <rptOwnerState>VA</rptOwnerState>
            <rptOwnerZipCode>22603-8605</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Sr VP Sales &amp; Marketing</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (Right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>36.4</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2005-07-09</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2014-07-09</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>60000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The options become exercisable in four equal annual installments beginning on the date shown as Date Exercisable.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Lynn E MacDonald</signatureName>
        <signatureDate>2004-08-13</signatureDate>
    </ownerSignature>
</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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