Products : CustomTech Flooring Prep : Patching : GenPatch™ General Purpose Calcium Aluminate Based Patch

GenPatch™ General Purpose Calcium Aluminate Based Patch

GenPatch™ is a fast-setting, calcium aluminate compound that provides a smooth finish on interior subfloors prior to the installation of a wide variety of floor coverings. Mix with water only to patch and level concrete and plywood up to 2" (50 mm) thick. Install floor coverings in just 30 – 90 minutes.

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Key Features

  • Patches and smooths interior subfloors
  • Apply from featheredge to 2"
  • Install flooring between 30 - 90 minutes


Suitable as an Underlayment for:

  • Carpet
  • Wood, parquet
  • Vinyl composition tile (VCT)
  • Sheet vinyl flooring
  • Laminated flooring
  • Pavers, brick
  • Stone, terrazzo tile


  • Quick-setting formula permits floor covering installation in as little as 30 – 90 minutes
  • Smooths the finishes on interior subfloors
  • Patches and levels concrete and plywood to 2" (50 mm)


Applicable Standards

ASTM International (ASTM)

  • ASTM C627 Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or (50-mm) Cube Specimens)
  • ASTM F1869 Standard Test Method for Linear Shrinkage
  • ASTM F710 Standard Test method for Flexural Strength

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.

American National Standards Institute (ANSI) ANSI A108.01 and A108.02 of the American National Standards for the Installation of Ceramic Tile

Conformance to Building Codes

Installation must comply with the requirements of all applicable local, state and federal code jurisdictions.

Composition of Product

GenPatch is a blend of calcium aluminate cement, inorganic fillers and polymers.

Environmental Consideration

Custom® Building Products is committed to environmental responsibility in both products produced and in manufacturing practices. Use of this product may contribute to LEED® certification.

Suitable Substrates

  • Absorbent concrete
  • Lightweight concrete
  • Gypsum-based underlayments
  • Existing ceramic tile
  • Primed cement terrazzo
  • Exterior grade plywood & OSB
  • Cutback adhesive residue
  • Approved and prepared moisture control membranes

General Surface Prep

USE CHEMICAL-RESISTANT GLOVES, such as nitrile, when handling product.

All surfaces must be structurally sound, clean, dry and free from contaminants that would prevent a good bond. Concrete must be fully cured. Refer to final flooring manufacturer's requirements for maximum moisture vapor transmission limitations. Smooth concrete surfaces, existing glazed tile, terrazzo, or polished stone may need to be roughened or scarified. For increased performance in demanding applications, concrete surfaces can be mechanically profiled and prepared by shotblasting, sandblasting, water-jetting, scarifying, diamond-grinding or other engineered approved methods (reference ICRI CSP 3 standards for acceptable profile height). Sheet vinyl must be well bonded and stripped of old finish.

Bonding to OSB and Gypsum Surfaces

OSB or gypsum-based underlayments must first be treated with RedGard® Waterproofing and Crack Prevention Membrane and must obtain a minimum 2000 psi (13.8 MPa) compressive strength at the recommended cure time. The underlayment must be sufficiently dry and properly cured to the manufacturer's specifications for permanent, non-moisture permeable coverings. Surfaces must be structurally sound and subject to deflection not to exceed the current industry standards All OSB and gypsum-based underlayment surfaces to receive GenPatch must be primed with properly applied sealer or a primer coat of RedGard®, consisting of 1 part RedGard® diluted with 4 parts clean, cool water. The primer can be brushed, rolled or sprayed to achieve an even coat. Apply the primer coat to the floor at a rate of 300 ft/gallon (7.5 M/L). Drying time depends on site conditions, but is normally less than 1 hour. Extremely porous surfaces may require 2 coats.

Health Precautions

This product contains Portland cement. Avoid eye contact or prolonged contact with skin. Wash thoroughly after handling. If eye contact occurs, flush with water for 15 minutes and consult a physician. Use with adequate ventilation; do not breathe dust and wear a NIOSH approved respirator. If ingested, do not induce vomiting; call a physician immediately.

Limitations to the Product

  • Do not bond to hardwood, Luan plywood, particleboard, parquet, cushion or sponge back vinyl flooring, metal, fiberglass or plastic.
  • Do not use when temperature is below 50°F (10°C).
  • Not for use in areas with continuous water exposure (ie: showers, swimming pools, spas, water features and fountains).

Bonding to Concrete Surfaces

Contaminants or curing compounds should be mechanically removed before installation. Concrete must be free of efflorescence and not subject to moisture beyond the floor covering manufacturers' limits or hydrostatic pressure. Lightweight concrete surfaces must have a tensile strength in excess of 200 psi (1.4 n/mm²).

Bonding to Plywood Surfaces

Plywood floors, including those under resilient flooring, must be structurally sound and must meet all industry guidelines. For questions about proper subfloor installation, call Custom® Building Products.

Bonding to Cutback Adhesive

Adhesive layers must be removed. Use extreme caution; adhesives may contain asbestos fibers. Do not sand or grind adhesive residue, as harmful dust may result. Never use adhesive removers or solvents, as they soften the adhesive and may cause it to penetrate into the concrete. Adhesive residue must be wet-scraped to the finished surface of the concrete, leaving only the transparent staining from the glue. To determine desirable results, do a test bond area before starting. Refer to the RFCI Pamphlet, "Recommended Work Practices for Removal of Resilient Floor Coverings", for further information.

Mixing Ratios

Mix 1 gal (3.78 L) clean, cool water to a 25 lb (11.34 kg) bag of GenPatch or for smaller jobs, use 3 parts powder to 1 part water.

Mixing Procedures

Slowly add powder to liquid while mixing with a high speed drill (650 RPM or more) and mixing paddle to a lump-free consistency. Mix amounts that can be applied in 10 minutes.

Application of Product

Apply material into all voids up to 1" (25 mm) thickness using a broad knife or trowel and finish flush with surface. For skim coating, use a smooth edged trowel to flatten the surface area. Only spot patching should be done on wood surfaces. If an area more than 5 ft. (1.5 M) in diameter is required, use an appropriate CustomTech™ self-leveling underlayment.

Curing of Product

Allow to dry for 30 – 90 minutes. Drying time can vary with thickness, temperature and humidity.

Cleaning of equipment

Clean with water before material dries.

Product Maintenance

Properly installed product requires no special maintenance.

Ordering Information


25 lb (11.34 kg) Bags

Warranty Information

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. For details and complete warranty information, visit

Coverage Chart

SQUARE FOOT COVERAGE PER 25 LB BAG (SQUARE METER PER 11.34 KG). Coverage will vary depending on the texture of the surface being smoothed.

1/8" (3 mm)  30 ft² (2.78 M²)  35 ft² (3.25 M²)
1/4" (6 mm)  15 ft² (1.39 M²)  17.5 ft² (1.62 M²)
1/2" (12 mm)  7.5 ft² (0.69 M²)  8.75 ft² (0.81 M²)
1" (25 mm)  3.75 ft² (0.34 M²)  4.38 ft² (0.4 M²)
2" (50 mm)  1.88 ft² (0.17 M²)  2.19 ft² (0.2 M²)

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