Cover Page - shares	3 Months Ended
	Mar. 31, 2021	Apr. 23, 2021
Cover [Abstract]
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Type	10-Q
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001069878
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity Registrant Name	TREX CO INC
Document Period End Date	Mar. 31, 2021
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Trading Symbol	TREX
Entity Shell Company	false
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		115,361,970
Entity File Number	001-14649
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	54-1910453
Entity Address, Address Line One	160 Exeter Drive
Entity Address, City or Town	Winchester
Entity Address, Postal Zip Code	22603-8605
Entity Address, State or Province	VA
City Area Code	540
Local Phone Number	542-6300
Title of 12(b) Security	Common stock
Security Exchange Name	NYSE
Document Quarterly Report	true