Concrete

Crack and Joint Repair Before Flooring

Moving vs dormant cracks, which joints must carry through the flooring, rout-and-fill basics, and why no fill guarantees against telegraphing.

Updated · 2 min read

Before any flooring goes down, cracks and joints need a decision: fill them, or carry them through the floor. The answer depends on whether they move. Manufacturers are consistent on one point: no fill guarantees a crack won't show through later.

Moving vs dormant

That's the first question for every crack and joint.

  • Every moving joint and moving crack has to be carried up through the whole flooring system.[1]
  • Dormant cracks and joints can be pre-filled, but that doesn't stop them from telegraphing; no method is known to prevent it.[1]
  • A consultant or engineer should specify crack and joint treatment in writing, and adhesive makers don't warrant against cracks showing through.[2]

Tile has its own rules

Tile has its own movement joint detail, TCNA EJ171, published in the TCNA Handbook. The basics are public.[3]

  • Every tile installation needs room for expansion and contraction. In small rooms, a perimeter gap hidden by base or shoe molding is enough.[3]
  • Movement joints go in more often where moisture or direct sun is expected.[3]
  • There's no single spacing for every job; the EJ171 detail in the Handbook sets the guidelines.[3]

Rout and fill

For cracks that will be filled, open the crack up so the filler can bond. This is typical guidance.[4]

  • Cut to a depth of 3/4" and a width of at least 1/4".
  • Use rigid epoxy in static cracks.
  • Use polyurea or urethane with backer rod in cracks that move.

Check the filler's own limits. Sikadur Crack Fix, for example, isn't for injecting cracks wider than 1/4", goes on at 40 to 95°F, on concrete at least 21 to 28 days old.[5]

Step by step

This covers the common case of dormant cracks under resilient flooring.

  1. Map every crack and joint. Mark which are moving (control joints, expansion joints, active cracks).
  2. Honor moving joints through the flooring with the detail the flooring maker specifies.
  3. Rout dormant cracks as the filler requires and vacuum them clean.
  4. Fill with the product the flooring or patch maker approves. ARDEX uses ARDIFIX for cracks larger than 1/32".[1]
  5. Skim-coat over the filled area if needed. See Patching and skim-coating.
  6. Document what you filled and what you carried through.

Common mistakes

Crack problems come back when the movement wasn't respected.

  • Filling a moving joint with rigid epoxy. It cracks again, usually along the edge.
  • Promising the customer it won't show. No manufacturer does.
  • Filling without routing. The filler sits on top and pops out.
  • Covering a tile movement joint.

Products we carry

These cover patching over filled cracks and opening them up.

References

  1. ARDEX Feather Finish technical data (joints and cracks)
  2. Mapei: Surface preparation requirements
  3. TCNA: Movement joint placement FAQ
  4. Sherwin-Williams: Repairing floor cracking
  5. Sika: Sikadur Crack Fix data sheet

Linked PDF documents are archived copies saved by PreFloor Tools so they stay available if the publisher moves or removes them. Check the publisher's website for the latest revision.

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