Roof Waterproofing Coatings · an Amrize brandHigh-build polyurethane detail sealant from the GacoFlex urethane system — seams, laps, corners and penetrations before the U91/U92 field coats.
The jobs this product is specified for, day in day out.
Detailed before the U91 base coat goes on.
Pipes, vents and upstands in the urethane system.
Where the mastic route would hold the job up.
What the data sheet actually gives you on site.
Against a minimum 40 hours for mastic — it is the option that keeps a programme moving.
Specified 76 mm wide detail at 64 mil minimum wet film.
One of three approved seam-detail methods in the Gaco U91/U92 specification.
The facts that matter, straight from the product data.
| Type | Polyurethane roof detail sealant |
| Use | Flashing seams, laps and penetrations — 76 mm crested band at 1.6 mm / 64 mil minimum wet film |
| Overcoat | Ready for the U91 base coat in 12–24 hours at 24 °C |
| Conditions | Apply above 4.5 °C and at least 3 °C above dew point on a rising temperature |
Everything from the manufacturer’s literature — open the part you need.
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.
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.
Source: ERsystems® H.E.R. Technical Data Sheet, 7/7/2023.
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 advice on specification and film build: 01208 832 602.
These products are designed to work together — buying the full system is what qualifies the warranty.
Written by our technical team — free to download, no email required.
PDFU91 + U92 Urethane Application GuideTrafficked decks, metal & asbestos cement roofs, over spray foam — free 7-page guideTell us about the building and we’ll spec the job free — the right system, the right quantities, and trade rates if you’re buying for a contract.
Tell us about your building and we’ll recommend the right system — foam, waterproofing or both — surveyed, specified and quoted at no cost.
