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By Cody Lee ·

Scrap Cauls for Flat Glue-Ups

Straight scrap boards across a panel keep edges flush while clamps pull the joints closed — wax or tape them so they don't stick.

Made in my shop

Reading time
About 1 min
Pine boards glued and held together with one-handed bar clamps on a painted stool
Inspiration image: Wood glue and clamping the pieces before screwing them togetherImage: "Wood glue and clamping the pieces before screwing them together" by Boohyabuddha (Wikimedia Commons uploader, own work), CC BY-SA 4.0, cropped and resized.

Bar clamps close a joint left-to-right. They don’t automatically keep board faces flush. A pair of straight scraps — cauls — clamped across the panel does that job.

What to use

  • Straight 1x stock, plywood strips, or old decking — anything stiff and true.
  • Longer than the panel is wide so clamps can reach.
  • Optional: plane a slight camber (crown) on the clamping face so pressure hits the middle of the panel, not only the edges.

Setup

  1. Dry-fit the panel and mark your triangle.
  2. Pack packing tape or wipe paste wax on the caul faces so glue squeeze-out won’t bond them to the work.
  3. Alternate bar or pipe clamps over and under the panel.
  4. Add cauls across the top and bottom (or top only on a narrow panel) and clamp them down just enough to flush the joints.
  5. Check with a straightedge before you walk away.

Swipe sideways to see the whole diagram, or tap it to open full size.

End view of a panel glue-up: bar clamps squeeze the boards from the sides, and a straight caul across the top and bottom is clamped at both ends, with a slight crown pressing the middle flat.
Cauls across the top and bottom keep the faces flush while the bar clamps close the joints.

Small assemblies

Cauls help on frames too: a flat board across a door-frame glue-up resists cupping while corner clamps or bar clamps do the squeezing. For more on clamp order and squeeze-out, see glue-ups without gaps.

Common mistake

Cranking the caul clamps until the panel hollows the other way. Aim for flush, not crushed — moderate pressure, then leave it alone for the glue’s clamp time.

Need bar, pipe or parallel clamps? See our clamp picks.