A polyurethane waterproofing membrane is a liquid that cures on the roof into a continuous elastic sheet. There are no laps, no torch-on seams and no mechanical fixings. That makes it well suited to roofs with many penetrations, upstands and changes of level, which are hard to detail with sheet membranes.
Not all polyurethane membranes behave the same way. One-component (1K) moisture-cured products are rolled or brushed. Two-component (2K) systems can be hot-sprayed and cure in seconds.
This article compares them using Krypton Chemical's Impermax technical data sheets (TDS).
How a liquid polyurethane membrane works
A 1K membrane such as Impermax contains an isocyanate-terminated prepolymer. It cures by reacting with moisture in the air into "an elastomeric, cold-applied polyurethane membrane", fully bonded to the substrate.
A 2K membrane such as Impermax 2K mixes a polyol/polyamine component with an aromatic isocyanate prepolymer in a hot-spray machine. It contains no solvents or mineral fillers. Cure is much faster.
For the polymer background, see the Wikipedia article on polyurethane.
Polyurethane waterproofing membrane: TDS comparison
| Property (TDS) | Impermax (1K) | Impermax ST (1K, thixotropic) | Impermax 2K (hot-spray) |
|---|---|---|---|
| Application | Roller / brush, cold | Roller / brush, cold, verticals | Hot projection machine |
| Hardness (ISO 868) | 65–70A | 65–70A | 83A |
| Max. elongation (EN ISO 527-3) | 450–600% | 421% | 350% |
| Tensile strength | 2.5–4.3 MPa | 3.4 MPa | 13 MPa |
| Tear strength (ISO 34-1 B) | 14 N/mm | — | 43 N/mm |
| Adhesion: concrete / ceramic / PU foam | 2.0 / 2.6 / 1.4 MPa | 2.0 / 2.6 / 1.4 MPa | — |
| ETA | 06/0263 (10 and 25 years) | 06/0263 | 10/0296 (W3, 25 years) |
| Working time / reactivity | Pot life 4–6 h | — | Gel 16 s at 25 °C |
| Typical consumption | 2 coats at 1 kg/m² each | — | 1.8–2.0 kg/m², min. 1.9 mm |
Sources: Impermax TDS, Impermax ST TDS, Impermax 2K TDS. A dash means the value is not stated in that TDS.
Reading the table
- 1K Impermax has the highest elongation and the easiest application. It is the practical choice for balconies, terraces, wet rooms and roofs with complex details.
- Impermax ST is slightly thixotropic, so it can be applied to vertical surfaces "with minimal sagging". Its TDS also lists canals and water tanks.
- Impermax 2K is much stronger (13 MPa, 43 N/mm tear) and cures in seconds. It suits large roofs, green roofs and fast-track projects.
What else the Impermax TDS confirms
Beyond the mechanical values, the 1K Impermax data sheet states:
- External fire: Broof(t1) for any roof slope and Broof(t4) (EN 13501-5).
- Thermal stability up to 140 °C. With Geomax fabric, it resists the 160 °C heat of poured asphalt.
- Water vapour resistance µ above 1000 (EN 1931).
- Root resistance certificates with and without reinforcement.
- BBA certificate 11/4836.
- VOC 184 g/l, below the 500 g/l limit of 2004/42/EC.
The European Technical Assessment matters most. It is the document that ties working life, climate zone and imposed load categories to a defined build-up. EOTA publishes the assessment documents used for liquid-applied roof waterproofing kits.
Green roofs and buried membranes
A green roof places the membrane under permanent moisture, root pressure and soil load. That is a harder service condition than an exposed roof.
Impermax 2K is "specially designed for the waterproofing of vegetable roofs (extensive and intensive) and planters". Its ETA includes root-penetration resistance to EN 13948, and the TDS says it "resists continuous contact with stagnant water (neutral) on decks". It is also partly bio-based, with renewable raw materials of vegetable origin.
See Krypton's notes green roofs written as a system and the Oviedo green roof with Impermax 2K, plus the gardened roofs application page.
Ponding water
Flat roofs hold water at low points. A membrane specification should treat ponding as a load case, not an accident.
The 1K Impermax TDS rates resistance to water at 24 h / 90 °C as 5 (best). Impermax 2K explicitly claims stagnant-water resistance. Read Krypton's note ponding is a design condition for the detailing side.
UV exposure: aromatic membranes need a topcoat
All three products above are aromatic. The Impermax TDS states that a colour change is expected and "does not affect its properties". The Impermax 2K TDS recommends an aliphatic finish (Colodur, Impertrans) to preserve appearance.
The pedestrian-roof system on the roofs, balconies and terraces page shows this: Porosity Sealer, then Impermax, then a Colodur topcoat.
Specification checklist
- [ ] ETA number and working life (10 or 25 years) stated for the full kit
- [ ] Primer matched to the substrate (concrete, ceramic, metal, old bitumen)
- [ ] 1K: two differently coloured coats at 1 kg/m² each, so coverage is visible
- [ ] 2K: 1.8–2.0 kg/m², minimum 1.9 mm, sprayed with wind below 25 km/h
- [ ] Reinforcement at edges, corners and drains
- [ ] Root resistance confirmed for green roofs
- [ ] Aliphatic topcoat where exposed to UV
Key takeaways
- A polyurethane waterproofing membrane gives a seamless, fully bonded roof layer with no laps.
- 1K Impermax: 450–600% elongation, cold-applied, ETA 06/0263.
- 2K Impermax 2K: 13 MPa, cures in seconds, root-resistant, ETA 10/0296 with 25-year W3.
- Aromatic membranes need an aliphatic topcoat where appearance matters.
FAQ
How long does a polyurethane roof membrane last?
Look at the ETA, not the brochure. Impermax is assessed for 10 and 25 years; Impermax 2K for 25 years (W3), for the build-ups defined in those assessments.
Can a polyurethane waterproofing membrane be used on a green roof?
Yes, if it has root-penetration resistance. Impermax 2K is designed for extensive and intensive green roofs and its ETA covers EN 13948 root resistance.
Is a polyurethane membrane better than bitumen?
They suit different situations. A liquid polyurethane membrane has no seams and handles complex details well. The Impermax TDS also lists use over existing bituminous products with a suitable primer.
How many coats of liquid polyurethane membrane are needed?
For 1K Impermax, the TDS describes two coats at 1 kg/m² each, in different colours so missed areas are easy to see.
Further reading
- Roof systems and data sheets at Krypton Chemical Solutions.
- Product range at kryptonchemical.com, including Impermax ST.
About the author
Krypton Chemical, S.L. Marketing Department, Nidal Malkawi
Krypton Chemical has manufactured construction chemicals in Spain since 1999. Technical resources: kryptonchemicalsolutions.com · Products: kryptonchemical.com
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