Liquid waterproofing membranes: polyurethane, bitumen-polymer

What is a liquid-applied membrane?

A liquid-applied membrane is a waterproofing compound that is applied by brush, roller, squeegee or airless sprayer and, once cured, turns into a continuous elastic film without joints. Unlike roll materials, it does not need to be cut to size and welded: it goes around pipe penetrations, rainwater outlets, parapets and complex junctions, and damage is easily repaired locally. Cold-applied membranes require neither an open flame nor expensive equipment — this is their main difference from hot-spray polyurea, which is described in the “Polyurea” section.

Types

One-component polyurethane — cures by atmospheric moisture. Convenient on roofs, balconies and terraces, including over old bituminous coverings with a suitable primer. Aromatic grades change colour and chalk in the sun, so on exposed roofs they are covered with a light-stable topcoat. In the catalogue: Sikalastic M 640.
Pitch-modified polyurethane — for below-ground structures and places where the membrane will be covered: foundations, basements, tunnels, ground-bearing floors, car park decks. An example is Fosroc Nitoproof 600, which according to its data sheet is also used with aggressive groundwater.
Water-based hybrid polyurethane — solvent-free, UV-resistant, for exposed roofs. Light-coloured grades reflect sunlight and reduce roof heating: according to its data sheet, Fosroc Nitoproof 725 has a solar reflectance index (SRI) of 106.
Two-component polyurethane — cures faster and more predictably in a thick layer; some systems, such as Sikalastic-822, are designed for mastic asphalt to be laid over the membrane on bridges.
Bitumen-rubber and bitumen-polymer — thick mastics for below-ground structures and complex details; their requirements and classification are set by EN 15814. In the catalogue: the two-component Fosroc Proofex LM membrane, designed according to its data sheet for a hydrostatic head of up to 70 m.

How to choose a membrane

Exposed or protected

The first question is whether the membrane will be visible. An exposed roof needs a UV-resistant material or a system with a topcoat; the data sheets of roofing membranes also state their fire classification for external fire exposure. Membranes that are covered with backfill, a screed, tiles or asphalt are chosen for their resistance to water, soil and loads, not for light stability.

Substrate

Concrete, cement screed, an old bituminous roof, metal, timber — for each substrate the manufacturer specifies its own primer and sometimes mandatory reinforcement: over bituminous membranes, for example, Sikalastic M 640 is reinforced over the whole area. The moisture content of the concrete is limited: to no more than 4 % in the Sikalastic M 640 and Sikalastic-822 data sheets and to no more than 6 % for Fosroc Nitoproof 725. The strength of the substrate is also specified: according to the Sikalastic M 640 data sheet, the concrete must withstand a compressive stress of 25 N/mm² or more, and the pull-off strength of the surface layer must be 1.5 N/mm² or more.

Water and structural movement

Roofs and terraces are constantly subjected to temperature fluctuations, so membranes with high elongation and the ability to bridge cracks in the cold are chosen for them. Below ground, the deciding factors are the groundwater head and its aggressiveness, as well as protection against damage during backfilling. For permanent immersion — tanks, swimming pools, channels — only materials for which such use is explicitly stated in the data sheet are used; many roofing membranes are not intended for it.

Moisture in the structure

A dense membrane holds back not only water from outside but also moisture from inside. ShNK 2.03.11-24 does not permit protective coatings that prevent moisture from escaping on concrete and reinforced concrete structures that are not fully protected and are exposed to moisture and sub-zero temperatures. Therefore, where such structures are coated on one side only, the vapour permeability of the material and the escape route for moisture are checked at the design stage.

Bridges and car parks under asphalt

Waterproofing under mastic or hot-mix asphalt must withstand the laying temperature of the asphalt and the shear loads from traffic. Only systems for which the manufacturer has confirmed such compatibility are used here — with a specific primer, tack coat and grade of asphalt.

What the system consists of

A primer for the specific substrate — consolidates the surface and provides adhesion.
Details — penetrations, rainwater outlets, upstands at parapets, movement joints and internal corners are done first, with a reinforcing fabric or mesh embedded in the membrane.
The main layer — in one or two passes up to the thickness given in the data sheet, often in a criss-cross pattern; on critical areas and weak substrates, with a reinforcing fabric over the whole area.
Topcoat or protection — a light-stable coat on an exposed roof; tiles, a screed or protection boards on trafficked surfaces and under backfill; asphalt on bridges.

Application and inspection

The substrate must be dry and warmer than the dew point — by at least 3 °C according to the Sikalastic M 640, Sikalastic-822 and Fosroc Nitoproof 725 data sheets. The moisture content of the concrete is checked before work starts.
On concrete that “outgasses”, the membrane is applied while the temperature is falling — during the day, when the substrate warms up, air escapes from the pores and bubbles and pinholes appear in the film.
Consumption is checked against the area and the quantity of material used, and the wet film thickness with a comb gauge. Skimping on consumption is the main cause of leaks through “thin” spots.
The intervals between coats are observed as per the data sheet; if the interval is exceeded, the surface is activated with the primer specified by the manufacturer.
Flat roofs, balconies and terraces are checked by flood testing. According to the Fosroc Nitoproof 600 data sheet, for example, this is done no earlier than 2 days after the second coat, with a water depth of at least 50 mm for 24 hours.

Common mistakes

Application to damp concrete or over dew — bubbles, delamination, whitish patches.
An aromatic membrane left in the sun without a topcoat — chalking and loss of thickness within a few seasons.
Too thick a layer of one-component polyurethane in a single pass — bubbles and slow, uneven curing.
Details and penetrations done after the main area and without reinforcement — that is exactly where the leaks occur.
Backfilling the excavation without protection boards and without waiting for full cure.
A roofing membrane used in a water tank, although the data sheet rules out permanent immersion.
Materials from different systems: a primer from one manufacturer, a membrane from another and a topcoat from a third.

Materials in the catalogue

Sikalastic M 640 — Sika — one-component polyurethane liquid membrane for roofs, balconies and terraces.
Sikalastic-822 — Sika — hand-applied polyurethane membrane under mastic asphalt on concrete bridge decks.
Fosroc Nitoproof 600 — Fosroc — pitch-modified one-component polyurethane membrane for foundations, basements and roofs.
Fosroc Nitoproof 725 — Fosroc — UV-resistant water-based hybrid polyurethane membrane for exposed roofs, SRI 106.
Fosroc Proofex LM — Fosroc — two-component bitumen-rubber liquid membrane for below-ground structures, up to 70 m head.

Frequently asked questions

Can a liquid-applied membrane be applied over an old bituminous roof?

Often yes — manufacturers have special primers and reinforcement requirements for this. The old roof is surveyed first: blisters, delamination and wet insulation underneath are dealt with before work starts, otherwise the new membrane will share their fate.

Is a topcoat needed?

On an exposed roof, yes, if the membrane is not UV-resistant or if the colour matters; light-stable hybrid membranes also work without one. Under tiles, a screed or backfill, a topcoat is not needed, but protection against mechanical damage is.

What makes a liquid-applied membrane better than a sheet membrane?

It is seamless, easily goes around complex details and can be repaired locally, and on roofs with many penetrations this is a decisive advantage. Roll materials are quicker to lay on large simple areas and give a factory-controlled thickness. The choice depends on the project; we select roll and sheet membranes to suit the design.

How soon can the membrane be walked on and the protection laid?

According to the data sheet of the specific material: Sikalastic M 640 is ready for use after about 12 hours and fully cured after 7 days; Fosroc Nitoproof 600 is covered with a screed or boards no earlier than 24 hours later at 25 °C.

We will select a membrane for your roof, terrace or foundation and calculate the supply — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.