GacoFlex H.E.R. Polyurethane Roof Sealant

Compatibility warning — do not apply GacoFlex H.E.R. over silicone. H.E.R. is a polyurethane sealant. The manufacturer states: “Do not apply over Silicone coatings or silicone caulks.” It must also not be applied over fresh asphalt coatings, coal tar coatings or plastic roof cement. If you are detailing a Gaco silicone roof system (GacoFlex S42 / SF4200), the silicone-compatible detail product is SF4200 SeamSeal — not H.E.R.

GacoFlex H.E.R. is a flashing-grade, single-component, moisture-cure polyurethane roof sealant with a track record since 1977. It is designed for sealing flashings, penetrations, seams and detail work on flat roofs. Single-step application — no primers, tapes or top coats needed. UV stable without requiring a separate top coat. It cures to a tough, seamless membrane that resists chalking and peeling, and fabric reinforcement is not normally required. Supplied in aluminium grey.

Coverage

9.3 m² per 19 litre pail at a 1.5 mm (60 mil) film — that is 0.49 m² per litre.

The unit here is square metres of area, not linear metres of seam. Gaco publishes no linear-metre rate for H.E.R., so this figure must not be converted into one. Source: PDS — GacoFlex™ H.E.R. Flashing Grade Polyurethane Sealant, revised 09/2025.

A note for anyone reading the manufacturer’s own sheet: the 09/2025 PDS prints “20 square feet / per gallon (9.3m / 3.8 L)”. The metric value in that bracket is a misprint — 20 ft² is 1.86 m², not 9.3 m². The 9.3 m² figure quoted above is per 19 litre pail, and is correct.

Detail geometry

• Around penetrations: a stripe 75–100 mm (3–4 in) wide at 1.5 mm (60 mil).
• Metal seams, airless applied: an extruded bead 19–25 mm (¾–1 in) wide.

Source: ERsystems® H.E.R. Technical Data Sheet, 7/7/2023.

Substrates

Metal, single-ply membranes (PVC, EPDM), asphalt, polyurethane foam, concrete, wood, and block or brick. Never over silicone coatings or silicone caulks — see the warning above.

Technical data

• Solids: 80%
• VOC: 186.9 g/L (EPA Method 24)
• Density: 3.5 kg per 3.8 L (approx. 0.89 kg/L)
• Viscosity: 100,000–170,000 cps
• Elongation: 400% (ASTM D2370)
• Tensile strength: 300–350 psi (2.07–2.41 MPa) (ASTM D412)
• Permeability: 1.2 perms at 0.76 mm (30 dry mils) (ASTM E96)
• Hardness: Shore A 40
• Low-temperature flexibility: pass at −51 °C (−60 °F) (ASTM D412)
• Service temperature range: −53.9 °C to 82.2 °C (−65 °F to 180 °F)
• Substrate application range: 4.4–48.9 °C (40–120 °F)
• Initial cure: 12–24 hours at 24 °C (75 °F) and 45% RH
• Shelf life: 12 months at 10–30 °C
• Pack sizes: 10.1 oz cartridge, 20 oz sausage, and 1, 2.5 and 5 US gallon pails

Technical advice on specification and film build: 01208 832 602.