Gapping and Overhang
You must gap Trex decking both end-to-end and width-to-width. Gapping is necessary for drainage and the slight thermal expansion and contraction of Trex decking boards. Gapping also allows for shrinkage of the wood joist system. » ALWAYS follow Trex gapping guidelines. » Maximum allowable perpendicular overhang for all Trex decking is 3/4 " (19 mm). » All decks require air circulation to keep them dry and looking good. To improve air flow, leave openings under the decking or increase gapping to 3/8 " (10 mm). » For areas that require CCMC Evaluation, refer to the Trex Canadian Decking Installation Guide for gapping requirements.
3/16" (5 mm)
Width-to-Width The minimum required width-to-width gapping is 3/16 " (5 mm). This is allowed for both hot and cold weather installations. For docks and heavily wooded areas, Trex recommends a 3/8 " (10 mm) gap. No gapping should ever exceed 1/2 " (13 mm).
Width-to-Width Gap
3/16 " (5 mm)
1/8 " – 3/16 " (3 mm – 5 mm) 1/8" – 3/16" (3 mm – 5 mm)
End-to-End/End-to-Width and Abutting Gap
End-to-End/ End-to-Width Abutting Gap
Above 40°F* (4.5°C)*
1/8 " (3 mm)
1/4 " (6 mm)
Below 40°F* (4.5°C)*
3/16 " (5 mm)
1/2 " (13 mm)
*Temperature at installation.
End-to-End/End-to-Width Gap Trex decking end-to-end, based upon the temperature at installation. See chart at left. For fastening tips, see page 39.
When you use the recommended hidden fasteners, the placement of the hidden fastener establishes the designated gap size.
When installing fascia, gapping guidelines must be followed.
1/4" – 1/2" (6 mm – 13 mm)
Abutting Solid Objects When decking is abutting a wall, you must also gap it 1/4"–1/2" (6–13 mm) depending on the temperature at installation. See chart to the left.
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