Flat roof

SBS −20 or APP −10?

Last updated: 26 October 2026

The flexibility of bituminous membrane is not a cold-weather specification but a measure of service life. Sometimes −10, sometimes −20 °C, and sometimes APP rat

The system at a glance

Solutions & Applications · Bituminous Products

Two rolls of SBS at 4 kg/m² appear identical on the quotation but differ in service life on the same flat roof. The difference is not in the weight — it is in the flexural temperature, and this figure is not related to the local climate. It relates to how many deformation cycles the material will withstand before it cracks.

What does the flexural temperature measure?

The test is simple: the sample is cooled and bent around a cylinder. The stated value is the lowest temperature at which the membrane bends without cracking. Below this temperature, the bituminous mixture becomes brittle.

The most common misconception is that −20 °C applies to areas where temperatures reach −20 °C. In Greece, almost no areas reach this temperature, and yet −20 is often required. The reason is different: the flexibility value is an indicator of the quantity and quality of the elastomeric modifier in the mixture.

More SBS also means:

Greater elastic recovery — the membrane stretches and returns to its original shape, rather than becoming permanently deformed.
Better resistance to temperature cycles — and Greek flat roofs undergo hundreds of such cycles each year, ranging from 5 °C at night to 70–80 °C on the surface at midday.
Slower curing — the mixture retains its properties for several years before hardening.

ARGYNET Note

Flexibility is not a cold-weather specification — it is a service life specification. In Attica, a bituminous membrane will never be exposed to −10 °C. However, it will be subjected to daily temperature fluctuations of tens of degrees on its surface, ultraviolet radiation and thousands of cycles of expansion and contraction of the slab.

A material that already starts close to its brittleness limit will reach it within a few years of ageing. A material with a tolerance of −20 °C has a margin: it too ages, but remains functional for much longer. The difference in cost per square metre is small; the difference when the flat roof needs replacing is not.

At ARGYNET, we select flexibility based on exposure and installation method, not on the prefecture.

The three levels

Product	Flexibility	Where it fits
ESHAFIN SBS -10	−10 °C	Applications without exposure: basements, foundations, layers beneath other materials, inverted roofs where the membrane is protected by the insulation
ESHAELASTAN SBS -15	−15 °C	The balanced range: classic flat roofs with protection, areas subject to moderate thermal stress
ESHADIEN SBS -20	−20 °C	Exposed surfaces, large slabs subject to heavy traffic, mountainous and cold regions, projects requiring maximum service life
ESHADIEN ULTRAWHITE SBS -20	−20 °C · cool roof	Exposed flat roofs with a white reflective surface — reduces the thermal stress that damages the membrane

The latter is the most interesting from a technical point of view, as it tackles the root cause: a white, reflective surface reaches a significantly lower temperature than a dark one under the same sunlight. A smaller range of variation means less stress, regardless of the mixture’s flexibility.

The decision rule

Will the membrane be exposed? If so, start at −20 °C. Exposure to UV and temperature cycles is the main mechanism of ageing.
Will it be covered? Gravel, slabs, inverted roof insulation or backfill drastically reduce the load — in that case, −10 or −15 is sufficient.
How much does the substrate move? Large slabs, metal substrates and structures subject to significant thermal expansion and contraction require greater elastic recovery.
What is the scope of the project? For a residential property being sold, the criteria are different from those for a building for private use or a public works project with a requirement for durability.
Is there a second layer? In double waterproofing, the lower layer is not exposed — the cladding is concentrated on the upper layer.

When SBS is not the answer

SBS provides flexibility; APP provides heat resistance. The choice of modifier is a separate decision from the choice of flexibility:

	SBS — elastomer	APP — polymer
Performance	It is extended and repeated	Remains stable at high temperatures
Strength	Elastic recovery, tolerance to substrate movement and cold	Softening point up to 148 °C — does not soften in a Greek summer
Typical scope	Flat roofs, cold regions, exposed surfaces	Flat roofs subject to high thermal exposure, overhangs, bridges in accordance with EN 14695

What else distinguishes two rolls of the same weight?

Reinforcement: high-dimensional-stability polyester provides tensile strength and puncture resistance; glass fibre provides dimensional stability with lower mechanical strength. Two rolls of the same weight but with different reinforcement are not interchangeable.
Coating: mineral granules for exposed top layers; smooth surface for layers that will be covered or overlaid with a second membrane.
Marking: EN 13707 for flat roofs, EN 13969 for basements — with a Declaration of Performance for each product code.

The last line also serves as a practical checking tool for the tender: the marking standard indicates the intended use. A membrane marked for use in underground spaces on a flat roof is not covered by the manufacturer’s declaration, however good it may be.