The roof is part of the outer envelope of the buildings, and thus it must be able to protect and isolate indoor environments. The roof is located at the top of a building, so in addition to the above, it has to be an efficient cover for precipitation (rain, snow or hail).

The flat roof can be defined as a roof with a slope lower than 5%. Consequently a flat roof needs to be waterproofed -to guarantee the tightness of the interior space-, but also isolated to keep comfortable temperatures inside the building.

Frequently, the insulation material is placed over the waterproofing membrane resulting in the inverted flat roof. This constructive system is broadly used.

In this type of flat roof, the insulation material limits the thermal variations of the waterproofing membrane (which is the most important material of a flat roof), so the insulation material protects the membrane from thermal degradation. This is the main issue explaining the reason why inverted flat roofs are widely used.

The inverted flat roof preserves the waterproofing membrane from thermal oscillations, so in theory this constructive system enlarges the life cycle of waterproofing membranes and consequently of the complete flat roof. However this consideration does not take into account the interactions between the materials which may cause deterioration.

Figure 1 shows the different materials comprising a stone-ballasted inverted flat roof.

The elements used nowadays to separate materials in the inverted flat roof are the auxiliary separating layers which are normally geo-textiles. The most frequents geo-textiles are made of polyester or polypropylene, as they are good materials to minimize the interactions and incompatibilities even in refurbishment of flat roofs (1). However, polyester geo-textiles do not offer good resistance to the alkalinity of the cement, so they are especially used in gravel ballasted and green flat roofs. Basically, auxiliary separating layers are designed to allow free movement and to provide physical and chemical protection to the materials.

The insulation material used in inverted flat roofs needs to have specific characteristics such as, no water absorption, compressive strength, and microorganism resistance, being the extruded polystyrene (XPS) board the unique alternative currently used.

There are some types of synthetic waterproofing membranes intended to be used in buildings and there are many manufacturers in the global market as for example: GAF (North America largest roofing manufacturer), ICOPAL headquartered in Denmark (annual turnover of approximately 1bill €), FLAG-SOPREMA group (Europe), SIKA (headquartered in Switzerland and annual sales turnover of 5.75 billion Switzerland Francs in 2016). They all produce synthetic waterproofing membranes, but only of plasticized polyvinyl chloride (PVC-P) and thermoplastic polyolefin (TPO). Nevertheless, there is an important market of synthetic waterproofing membranes of EPDM, especially in America, where one of the main manufacturers is FIRESTONE building products, which additionally produces TPO.

Therefore, the most important synthetic waterproofing membranes for the building industry are: plasticized polyvinyl chloride (PVC-P), thermoplastic polyolefin (TPO) and ethylene propylene diene monomer (EPDM) waterproofing membranes.

XPS plates may interact with the plasticized polyvinyl chloride (PVC-P) waterproofing membrane in the inverted flat roof (2), deteriorating the materials and thus reducing the life cycle of the construction system. For this reason, the auxiliary separating layer -placed between the waterproofing membrane and the XPS- is of great importance as it reduces this effect. This is one of the reasons for placing auxiliary separating layers in flat roofs, to cut down degenerative interactions.

Some building roofing standards state the need to avoid interactions and incompatibilities between the different materials. This issue is mainly solved by placing auxiliary separating layers, for instance, in between XPS and waterproofing materials. Regulations state this, even between materials which are not clearly described as incompatibles.

Beforehand, the efficiency of auxiliary separating layers depends basically on their thickness and composition. This paper analyzes the effectiveness of several types and thickness of auxiliary separating layers in order to avoid incompatibilities in inverted flat roofs. Furthermore, the possible interactions between the extruded polystyrene (XPS) and two types of synthetic waterproofing membranes (TPO and EPDM) are also studied.

Sometimes flat roofs must resist a layer of soil with a significant thickness (green roofs), thicker pavements, extra loads or installations which are not placed on independent structures but directly over the roof. Installations such as: air conditioning, absorption and cooling units, solar thermal generation systems, etc. In these cases the pressure between materials is greater than usual, and XPS mitigates the overloads. This paper also analyzes the effect of the pressure between the materials and its consequences.

The pressures between materials of the most common ballasted roofs can be calculated adding the weights of the materials that the waterproofing membrane has over it, and dividing the result by a unit of area, giving the following pressures:

These values were reached considering: two layers of geo-textile of 300 g/m², XPS of 5 cm thick and a specific density of around 33 kg/m³, river siliceous gravel of a particle size between 20 - 40 mm with a density of about 1700 kg/m³and pavements made of mortar and floor tiles or concrete with a specific density of around 2150 kg/m³.

Most polymers are incompatible with each other (3). Furthermore, the materials usually placed in inverted flat roofs are mainly polymers, as for example the geo-textiles used as auxiliary separating layers are normally made of polymeric materials such as polyester or polypropylene. XPS is a polymeric foam, additionally there are different types of synthetic polymeric waterproofing membranes normally used in the building industry, such as the PVC-P waterproofing membranes.

PVC-P can contain a wide variety of additives (4), such as plasticizers, which increase the plasticity of the material. Plasticizers are included in the manufacture of PVC-P to increase its flexibility, workability or distensibility (5); besides plasticizers of plastic materials can move to another substance or material, such as food products, liquids (6), or even to another plastic (7), this phenomenon is known as plasticizers migration. Moreover, crude polystyrene in favorable conditions is capable of taking plasticizers contained in other plastics as it is the case of PVC-P (8).

Under normal weather conditions XPS interact with PVC-P waterproofing membranes in the inverted flat roof (9). This interaction can be summarized as the migration of plasticizers from PVC-P to XPS (2). This may be considered an incompatibility (10), as eventually PVC-P loses many of its properties –becoming stiffer and even shrinking- (11).

Other common types of synthetic waterproofing membranes are polyolefins, such as the thermoplastic polyolefin (TPO) and the ethylene propylene diene monomer (EPDM), which is an elastomer, viscoelastic amorphous thermo polymer (12). However, the interactions between these materials and the XPS in the inverted flat roof have not been described.

According to ASTM D-1418 – 17, EPDM is an M-class rubber (13). The M class refers to the saturated backbone structure, which includes rubbers which have a saturated chain of a polymethylene type. EPDM materials have excellent resistance to heat, oxidation, ozone and weather aging due to their stable saturated polymer backbone structure. They have good electrical resistivity (as non-polar elastomers), as well as resistance to polar solvents, such as water, diluted acids, alkalis, phosphate esters and many ketones and alcohols (14). However, EPDM materials are totally unsuitable with petroleum based fluids such as all mineral oil products, lubricants, fuels, etc.

EPDM materials are divided into two types: sulfur-cured and peroxide-cured. The sulfur acceleration systems can obtain materials with heat resistance up to 130°C (15) with a better durability. The peroxide-cured compounds are suitable for higher temperatures, a heat resistance up to160°C can be obtained with this system. The amorphous or low crystalline grades have excellent low temperature flexibility with glass transition points of about -60°C (16). These polymers may include additions of fillers and plasticizers (17).

EPDM waterproofing membranes have been used in the low-slope roofing industry for over 40 years and they do not have internal reinforcement. On the contrary, TPO waterproofing membranes do have internal reinforcement and have been used for more than 40 years in the Civil Engineering industry, especially for waterproofing of landfills, ponds and water canals. However, the excessive stiffness of these materials made them unsuitable for building roofings until 1991 (18).

Polyolefins can be defined as every synthetic resin made by olefins or alkenes polymerization. They are hydrocarbon molecules with at least one double carbon–carbon bond. Polyolefins are polymer molecules made using either free-radical, ionic initiators, inorganic (metal oxide) or organic-metallic catalysts (19). Each polymerization process produces a different Polyolefin type, being the linear high-molecular-weight thermoplastic polymers the ones mainly produced.

To a lesser extent than PVC-P, thermoplastic polyolefins do normally contain additives (such as stabilizers, inorganic and organic fillers, antioxidants, antistatic agents, etc.), but they do not contain plasticizers (20).

The thermoplastic polyolefins can be divided in two types: polyethylene and polypropylene, which in turn they are subdivided into several grades regarding their different applications. Commercial polyethylene resins are normally copolymers of ethylene, with varying fractions of α-olefin comonomer, which changes the physical properties and applications of polyolefins. The most common α-olefins are 1-butene, 1-hexene, and 1-octene (21).

The polypropylene resins are mostly isotactic materials nevertheless a few syndiotactic grades are also available. There are two principal types of Propylene copolymers: random propylene/ethylene copolymers, and impact propylene/ethylene copolymers (22).

The TPO waterproofing membranes used in the building industry are most often produced by a mixture of polyolefins which form a stable and flexible polymeric compound.

There is a low knowledge about what materials are really incompatible in flat roofs. Therefore, it is important to clarify the risks of placing -in close contact- certain materials, the quality of auxiliary separating layers to reduce interactions, and the conditions favoring these interactions. Flat roofs are often designed and built improperly because of this lack of knowledge, giving eventually less durability to the system.

Several researchers have studied the interactions and incompatibilities in flat roofs. A previous paper studied the importance of placing auxiliary separating layers in flat roofs, being of crucial importance for roofs’ refurbishment (1). At some time, flat roofs need to be rebuilt, and frequently the new roof is built over the old one, consequently the new waterproofing material is placed over the existing waterproofing layer. In some of these cases the auxiliary separating layer is crucial to keep the integrity of the new waterproofing membrane. The interactions between XPS and PVC-P waterproofing membranes was previously analyzed testing an inverted flat roof (in use) (2) and testing in the laboratory all the materials with different configurations (9) , including two types of geo-textiles as auxiliary separating layers. Finally, other paper analyzing the interactions between XPS and PVC-P was found, considering the deterioration of the PVC-P. This research analyzed, with a scanning electron microscope (SEM), the surface and the elemental composition of different samples (10).

The present study was conducted in Spain, specifically in the Madrid area, and unlike the previous papers, this article shows the results of several laboratory tests performed with XPS and synthetic waterproofing membranes of EPDM and TPO. It also analyzes the efficiency of six auxiliary separating layers (not previously tested) and studies the pressure effect on the interactions.

This study aims to enlarge the knowledge regarding the incompatibilities between the most important synthetic waterproofing materials and the extruded polystyrene used in inverted flat roofs. It also quantifies the pressure effect on the degradation of the PVC-P waterproofing membranes and explores the effectiveness of alternative auxiliary separating layers to reduce interactions and incompatibilities depending on their thickness and composition.

The methodology followed unfolds in three experimental short-term tests (23) performed in the laboratory in order to obtain information about the different conditions of pressure and the behavior of the different types of materials used in the inverted flat roof. The first test analyzed the efficiency of different auxiliary separating layers in the inverted flat roof, the second test studied the pressure effect on the deterioration of a synthetic waterproofing membrane and the third test analyzed the possible interactions between XPS and the waterproofing membranes of EPDM and TPO.

The methodology followed was based on standard ISO 177:2016 (24). This Standard describes a test were plastic materials are exposed to a heat for a period of time in order to determine the tendency of plasticizers to migrate from plastic. The tests were performed using different synthetic waterproofing membranes and XPS as the absorbent material of plasticizers.

The tests followed the procedure of ISO 177:2016 standard, but it was adapted to the conditions of the inverted flat roof, as the reference temperature set by the standard is 70 °C. However at this temperature XPS may present thermal degradation (2). Moreover, the established absorbent material set by ISO 177 is a rubber disc, nevertheless these tests were performed with XPS instead of rubber in order to simulate an inverted flat roof cross section and because XPS is also capable of absorbing plasticizers (8). Also, the pressures of the tests performed were much lower than the one specified by the standard in order to make accurate tests and close to the conditions of the inverted flat roof.

The tests intend to study the efficiency of different auxiliary separating layers in identical conditions; to analyze the effect of the pressure between the samples on the degradation of the materials (in identical conditions and only varying the pressure in the different configurations); and finally to prove the compatibility of XPS and the waterproofing membranes of TPO and EPDM in a test based in the ISO 177:2016 standard and with identical conditions.

The general bases of the tests are described in Figure 2, were the materials involved, the interaction and the effect of the pressure are described. The materials tested were placed in close contact, and under defined conditions of pressure, temperature, and time. The mass loss of the samples after the test was recorded and becomes a measure of plasticizer migration. The more mass lost the greater damage in the waterproofing membrane.

The temperature of all the tests was 50 °C, and every configuration was always tested three times. In the tests performed the XPS was IV type ROOFMATE SL - 30 mm, the cylindrical samples had 60 mm of diameter. The disc shaped waterproofing membranes samples had a diameter of 50 mm. To weigh the samples, a scale with a 0.001g of precision was used. The difference between the previous and the final mass would be the mass loss of this material. The results of every test and configuration are presented showing the average, the standard deviation and the RSD% or relative standard deviation in percentage.

The details of the three different tests performed are shown in the next subsections.

The auxiliary separating layers must be placed always in the inverted flat roof. This is advisable even with compatible materials in order to provide free movement and physical protection to the materials.

The Polyester geo-textile is the best option to be used in inverted flat roofs. However, this material does not offer good resistance to the alkalinity of cement, so for the pavement of roofs the best option is the Polypropylene geo-textile.

The Fiberglass felts provide good protection to reduce interactions. However these materials are not commonly used for this purpose and the material should be properly tested before using it. The microorganism resistance is not totally clear.

The Polyethylene films cannot be used as auxiliary separating layers in the inverted flat roof since they are not effective to prevent the deteriorative interactions.

At low pressures, the mass per square meter of auxiliary separating layers of identical composition is not decisive to cut down interactions. The results obtained are very close to each other independently on the thickness of the auxiliary separating layers analyzed.

There is a clear relation between the pressure applied on the materials of the inverted flat roof and the interaction between them. The more pressure the more chance for a degenerative interaction.

The inverted flat roofs with loads that apply greater pressure than usual have to dispose efficient auxiliary separating layers to separate the waterproofing membrane from XPS.

The waterproofing membrane of EPDM is incompatible with XPS, thus this material must be properly separated to enlarge the life cycle of the roof.

The waterproofing membrane of TPO is compatible with the XPS. However, it is advisable to use auxiliary separating layers to get physical protection and independence of the movements.

The temperature is crucial for the deterioration of incompatible waterproofing membranes in the inverted flat roof. For this reason, in this construction system, thicker XPS plates would reduce the interaction, since it would also reduce the temperature in the contact surface of both materials.