Butyl Flashing-Grade 321

Butyl Flashing-Grade 321 is a single-component, trowel-grade, seam sealer specifically formulated to provide a water-tight, weather-resistant membrane with excellent permeability and UV protection. Butyl Flashing-Grade 321 offers good elongation and tensile strength to resist lateral movement and allow for natural daily expansion and contraction of the roofing substrate.

  • Reduces daily expansion and contraction (thermal cycling) of roofing substrate
  • Ideal for sealing holes, cracks and joints
  • Highly flexible
  • Superior elongation and tensile strength
  • Excellent elongation and tensile strength

2-gallon pail, 5-gallon pail



Butyl Flashing-Grade 321 is recommended as a protective vapor barrier seam sealer for a variety of situations, including all types of seams, fasteners, gaps between metal panels, gutters, vent pipe flashings, air conditioning units and expansion joints. Ideal for metal roof applications with visible rust and all types of flat roofing substrates.

Download Product Data SheetDownload Safety Data Sheet
  • Metal
  • Modified Bitumen
  • Single-ply
  • Smooth BUR

All surfaces to be coated must be cleaned properly. Power washing is recommended. Any existing coating must be checked for good adhesion. Before application, any loosely adhered coating must be removed and bare surfaces must be prepared, cleaned and checked for compatibility.


This product contains VM & P Naphtha and Xylene. Keep away from heat, open flame or sparks. Use only in well ventilated areas with proper protective equipment. Avoid breathing vapor or mist.

Application Coverage (1 gallon) Width Wet Mils Average Dry Mils
Seams 50 linear ft. 4″ 76 30
Flashing Details 25 linear ft. 6″ 126 50

*Coverage rates listed are estimates based on typical conditions.

  • Thick Brush
  • Bulk gun
  • Towel
  • CRRC

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