X0306 4 2018-08-01 0001069878 TREX CO INC TREX 0001201351 KAPLAN RONALD W C/O TREX COMPANY, INC. 160 EXETER DRIVE WINCHESTER VA 22603-8605 1 Common Stock 2018-08-01 4 S 0 2596 78.19 D 14820 D Common Stock 2018-08-01 4 S 0 1750 78.88 D 13070 D The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $77.74 to $78.74, 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 (1) to this Form 4. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $78.75 to $79.19, 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 (2) to this Form 4. /s/ William R. Gupp by power of attorney 2018-08-03