Products : Setting : Polymer-Modified Thin-Set Mortars : MegaFlex® Crack Prevention Mortar

MegaFlex® Crack Prevention Mortar


The ultimate in flexibility and bond strength for the most demanding installation requirements. Its unique blend of polymers and inorganic chemicals enhance the bond between the tile and substrate for unsurpassed performance as part of a wide variety of new and existing job conditions. Flexible enough to withstand horizontal substrate movement to 1/8" (3 mm). MegaFlex® offers up to 20% more coverage than other thin-sets and exceeds ANSI A118.4E, A118.15E and A118.11 standards without the need for additives. Use it for interior or exterior floors, countertops and walls.

  • Features & Benefits
  • Technical Documents
  • Warranty Information

Suitable Tile Types

  • Vitreous, semi-vitreous or non-vitreous tile: ceramic, mosaic, quarry, cement body tile
  • Impervious porcelain and glass tile
  • Brick and thin brick
  • Cement-based precast terrazzo
  • Natural stone tile

Suitable Substrates

  • Concrete, mortar beds, masonry, Portland cement plaster
  • Liquid applied waterproofing membranes such as RedGard® and Custom® 9240
  • Crack prevention sheet membranes such as Crack Buster® Pro
  • Exterior Grade Plywood (interior residential and light commercial dry areas)
  • Gypsum wallboard (interior dry areas)
  • Existing ceramic tile (scarified)
  • Fully-bonded sheet vinyl flooring (scarified)
  • Plastic laminates (scarified)
  • Cutback adhesive (see preparation instructions)

Composition & Materials

MegaFlex® is a dry, proprietary blend of copolymers, Portland cement, inorganic aggregates and chemicals.


  • Maximum bond strength and flexibility
  • Superior bonding to plywood and to porcelain tile
  • Isolates cracks to 1/8" (3 mm)
  • Excellent handling characteristics through extended open and adjustment times
  • Provides up to 20% more coverage than typical Portland cement mortars
  • Exceeds ANSI A118.4E, A118.15E and A118.11 standards without the need for additives
  • Approved for industry-recommended interior and exterior applications
  • Water submerged installations (14 day cure required)


  • Do not bond directly to hardwood, Luan plywood, particle board, parquet, cushion or sponge-back vinyl flooring, metal, fiberglass, plastic or OSB panels.
  • When setting moisture sensitive natural stone, cement or agglomerate tile use EBM-Lite™ Epoxy Bonding Mortar 100% Solids or CEG-Lite™ 100% Solids Commercial Epoxy Grout.
  • Do not use to install resin-backed stone; use EBM-Lite™ Epoxy Bonding Mortar 100% Solids, CEG-Lite™ 100% Solids Commercial Epoxy Grout or contact Custom's® Technical Services for recommendations.
  • When setting glass tile larger than 6" x 6" (15 x 15 cm), contact Custom's® Technical Services for recommendations.
  • When setting dimensional stone larger than 12" x 12" (30 x 30 cm), contact Custom's® Technical Services for recommendations regarding subfloor deflection requirements.


  • 50 lb (22.68 kg) bag
  • Gray or white

Technical Data

Applicable Standards

American National Standards Institute (ANSI) — ANSI A108.5, A118.4E, A118.15E and A118.11 of the American National Standards for the Installation of Ceramic Tile ASTM International (ASTM)

  • ASTM C109 Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in or [50-mm] Cube Specimens)
  • ASTM C627 Standard Test Method for Evaluating Ceramic Floor Tile Installation Systems Using the Robinson-Type Floor Tester 
  • Resilient Floor Covering Institute - (RFCI) Recommended Work Practices for Removal of Resilient Floor Coverings 
  • Tile Council of North America (TCNA) - TCNA Handbook for Ceramic Tile Installation, TCNA Method EJ171
  • ISO 13007-2


Technical Data Sheet Technical Data Sheet - French Technical Data Sheet - Spanish Material Safety Data Sheet - English

Product Warranty

Custom® Building Products warrants to the original consumer purchaser that its product shall be free from defects in material and workmanship under normal and proper usage for a period of one year following the date of original purchase. Custom's® sole liability under this warranty shall be limited to the replacement of the product. Some states, countries or territories do not allow the exclusion or limitation of incidental or consequential damages, so the above limitation or exclusion may not apply to you. This warranty will not extend to any product which has been modified in any way or which has not been used in accordance with Custom's® printed instructions. Custom® makes no other warranties either expressed or implied. This warranty gives you specific legal rights, and you may have other rights that vary from state to state or from one country/territory to another.  This warranty is not transferrable.

Warranty Duration

When MegaFlex® Crack Prevention Mortar is used as a part of a qualifying full installation system of CUSTOM products, the installation can qualify for up to a lifetime system warranty. CUSTOM will repair and/or replace, at its discretion, the affected area of the system. For more information, find details and limitations to this warranty at

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