How wide should an acrylic expansion joint be?
Tremflex 834 gives a width rule and a separate movement rating. Check both before settling the joint detail.
For Tremflex 834, Tremco specifies a joint width four times the anticipated movement, with a minimum width of 1/4 in (6.4 mm). That’s a product-specific rule, not a sizing rule for every acrylic sealant. The same sheet lists movement capability of ±12.5%, so don’t use the width calculation alone to approve an expansion joint.
Before you settle on a width, ask the project designer or Tremco what “anticipated movement” covers. One direction, or the full range from open to closed? The sheet doesn’t say.
Start with the movement the joint must take
Get the anticipated movement from the project’s joint design. Keep the units consistent, multiply that movement by four, then compare the result with Tremco’s 6.4 mm minimum.
Assume 2 mm of anticipated movement and the calculation gives you an 8 mm width. That’s an arithmetic example only, not a check of how much your joint will move. You still need to confirm whether Tremflex 834 suits the detail.
Send both the width calculation and the ±12.5% rating when you ask for confirmation. How do they fit together? The sheet gives you both figures but doesn’t explain how the rating relates to its wording about anticipated movement.
Width doesn’t settle bead depth
Keep the specified 1/4 in minimum as your reference when you read Tremco’s metric figures. In the sealant-dimensions section, the minimum expansion-joint width and depth are 1/4 × 1/4 in, also shown as 6 × 6 mm. The joint-design paragraph uses 6.4 mm for the minimum width. Those rounded metric figures aren’t separate design allowances.
Check your joint width before setting the bead depth. Under 1/2 in wide, you may use the same depth as the width; for widths from 1/2 to 1 in (13–25 mm), use a depth approximately half the width. Keep the depth to a maximum of 1/2 in (13 mm). Wider than 1 in (25 mm)? Tremco says to contact its Technical Services or a local Tremco representative.
Check the backing before filling
Start with dry backing. To control depth and help the sealant contact the joint walls as you tool it, Tremco recommends closed-cell or reticulated polyethylene backer rod. If the joint depth leaves no room for backer rod, use adhesive-backed polyethylene bond breaker tape so the sealant won’t bond on three sides.
Apply Tremflex 834 to sound, clean, dry surfaces. Fill from the bottom first, then immediately tool with a spatula against the joint walls. Tremco prefers dry tooling and recommends a surface temperature of at least 5°C during application; below that, it refers installers to its cold-weather application guide.
Before you order
Don’t treat a gap measurement as the anticipated movement, or copy this width rule to another acrylic product. Tremco calls for joints designed to accepted architectural or engineering practices and recommends an adhesion mock-up or field test on the actual job materials.
Send Cranco the proposed product, substrates, joint width and depth, expected movement and exposure. Include the drawing if you have one, and ask for the movement basis to be confirmed with Tremco where needed. You can review [Tremflex 834 product details](/tremflex-834/) before preparing that request.