Experimental Evaluation of the Sustainable Performance of Filtering Geotextiles in Green Roof Systems: Tensile Properties and Surface Morphology After Long-Term Use

Abstract

Green roofs are increasingly being adopted as sustainable, nature-based solutions for managing urban stormwater, mitigating the urban heat island effect, and saving energy in buildings. However, the long-term performance of their individual components—particularly filter geotextiles—remains understudied, despite their critical role in maintaining system functionality. The filter layer, responsible for preventing clogging of the drainage layer with fine substrate particles, directly affects the hydrological performance and service life of green roofs. While most existing studies focus on the initial material properties, there is a clear gap in understanding how geotextile filters behave after prolonged exposure to real-world environmental conditions. This study addresses this gap by assessing the mechanical and structural integrity of geotextile filters after five years of use in both extensive and intensive green roof systems. By analyzing changes in surface morphology, microstructure, and porosity through tensile strength tests, digital imaging, and scanning electron microscopy, this research offers new insights into the long-term performance of geotextiles. Results showed significant retention of tensile strength, particularly in the machine direction (MD), and a 56% reduction in porosity, which may affect filtration efficiency. Although material degradation occurs, some geotextiles retain their structural integrity over time, highlighting their potential for long-term use in green infrastructure applications. This research emphasizes the importance of material selection, long-term monitoring, and standardized evaluation techniques to ensure the ecological and functional resilience of green roofs. Furthermore, the findings contribute to advancing knowledge on the durability and life-cycle performance of filter materials, promoting sustainability and longevity in urban green infrastructure.

1. Introduction

Urban development often occurs at the expense of green spaces [1], leading to increased impervious surfaces, greater stormwater runoff, and exacerbation of the urban heat island effect. One sustainable solution to mitigate these negative impacts is the implementation of green roofs [2]. Green roofs are engineered vegetative systems installed on rooftops, consisting of layered components that support plant growth, regulate water flow, and insulate buildings. They are typically classified as extensive or intensive: the former are lightweight with shallow substrate layers and drought-tolerant vegetation, while the latter incorporate deeper substrates capable of supporting larger plants, including shrubs and small trees [3]. Due to lower structural load requirements, extensive systems offer wider application. However, their limited subgrade thickness often results in reduced water retention capacity—as observed by Stovin et al. [4] who reported 20 mm of water retention in a system with a subgrade depth of 80 mm. Over the past decades, diverse [5] green roof solutions have been developed through research and practice, each consisting of essential layers: vegetation, a growing medium, a filter layer, and drainage. Proper layer selection—adapted to local climate and environmental context—is crucial for maximizing the long-term performance and environmental benefits of green roofs [6]. These systems provide multiple ecosystem services: reduce runoff and urban flooding, mitigate air pollution, moderate microclimates, sequester carbon, reduce building energy consumption, and enhance biodiversity [7,8,9,10,11]. Recognizing these benefits, many cities have incorporated green roofs into their stormwater management strategies, offering financial incentives such as grants, low-interest loans, and tax relief [12,13]. Further, green roofs are increasingly used in both new construction and retrofitting of existing buildings. The expected service life of properly designed and maintained green roofs is estimated at 40 to 55 years [14], largely depending on the quality and durability of the materials used in each structural layer. Since the 1960s, significant advancements have been made in green roof design—replacing traditional sand or gravel filtration layers with modern drainage systems and geosynthetic materials [15,16]. Life cycle assessment (LCA) studies confirm that, across material sourcing, construction, operation, and end-of-life phases, green roofs offer superior environmental performance compared to conventional roofing systems [17,18].

Among the essential materials in green roof construction are geotextiles—flat geosynthetics used for filtration, separation, protection, and drainage. Geotextiles may be woven or non-woven, typically made from polypropylene (PP) or polyester (PET). Woven geotextiles are manufactured by interlacing yarns, while non-woven geotextiles are formed by bonding fibers through mechanical, chemical, or thermal processes, sometimes reinforced by calendaring. In green roof systems, geotextiles serve to protect waterproofing and insulation, prevent root penetration (in anti-root variants), filter water, and separate drainage layers from substrates. Green roof construction follows standards and guidelines such as the German (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau) FLL Guidelines [19], the International Building Code (IBC), and American Society for Testing Materials (ASTM) and American National Standards Institute (ANSI) standards, particularly in North America. However, regional differences in climate and materials often limit the applicability of universal guidelines. For instance, as [20,21] note, the FLL guidelines are based on studies conducted in Germany, which may not be directly transferable to other climates. Analysis by Dvorak [22] revealed that, in comparison to the comprehensive scope of the FLL guidelines, North American green roof standards and guidelines lack coverage in several critical areas. These include system compatibility testing, application guidelines for sloped roofs, specifications for filter fabrics, post-construction testing of fabric performance, standards for typical material loads, assessment of post-construction bulk density, root barrier design and testing, evaluation of drainage materials and their performance over time, as well as guidelines for growth media, erosion control, plant establishment, and general maintenance. While core North American sources—such as the ASTM standards, the National Roofing Contractors Association (NRCA) guidelines, ANSI/SPRI VF-1, and the Whole Building Design Guide (WBDG)—have begun addressing many of these aspects, gaps still remain in comparison to the FLL’s integrated approach.

In Poland, the FLL guidelines were translated and adapted in 2015 by the Flat Roof and Façade Contractors Association (DAFA), resulting in the Guidelines for Green Roofs, which incorporate both FLL principles and national regulatory requirements.

Despite the importance of geotextiles, many design specifications focus primarily on mass per unit area, which does not reflect crucial mechanical or hydraulic properties. For filtration applications, recommended parameters include a minimum mass of 100 g/m² [19], puncture resistance > 0.5 kN/m² for extensive roofs, and appropriate pore sizes (0.2–0.6 mm) to ensure water permeability without substrate loss. Water permeability perpendicular to the fabric should exceed 100 L/m²s, as defined by PN EN ISO 11058:2019-07 [23].

Resistance to mechanical damage, microorganism activity, clogging, and root overgrowth is also vital, yet manufacturers often do not declare these properties. For example, while root-overgrowth is essential for enabling plants to access water from the drainage layer—especially in thin extensive systems—it is typically assessed informally by system suppliers rather than through standardized tests. Correct installation practices are essential for maintaining geotextile function. The filter fabric must be laid without folds or gaps, with adequate overlaps and anchoring, especially near vertical elements. It should be protected from UV exposure, as most geotextiles are not UV-stabilized, and must be promptly covered with substrate or ballast. Poor installation can lead to clogging, displacement, or exposure, compromising roof function and longevity. Given the building sector’s considerable contribution to global resource consumption and emissions, even minor components like geotextiles must be evaluated not only for performance but for long-term sustainability. As potential failure points, degraded or clogged geotextiles can reduce system effectiveness, increase maintenance costs, and undermine ecological benefits. Therefore, understanding their behavior over time under real-world conditions is vital for sustainable urban infrastructure. Durable geotextiles support environmental sustainability by maintaining system performance (e.g., stormwater management, biodiversity), economic sustainability by reducing maintenance and replacements, and social sustainability by enhancing livability. This study contributes to this sustainability triad by evaluating the real-life aging of filter geotextiles over five years of use in Polish green roof systems.

Predicting the aging of geotextiles is complex and typically involves laboratory or field exposure to stressors such as moisture, temperature variation, microbial colonization, and root penetration [24,25,26]. Numerous studies have examined geotextile aging using either laboratory [27,28] or field-based methods [24,25,26,29,30,31], and some have compared both approaches [25,27,28,32,33,34]. Aging is most commonly assessed by tracking changes in mechanical properties—especially tensile strength [24,25,26,27,28,32,33,35,36], as well as through microscopy [27,29,31], spectroscopy [32,36], thermal analysis [27,29,30], and chromatography [33].

Although the environmental and functional benefits of green roofs are widely recognized, the long-term durability of key structural components—especially filter geotextiles—remains under-researched. These fabrics play a critical role in maintaining system separation and hydraulic function but are subject to long-term degradation processes under variable, real-world conditions. Existing laboratory studies may not fully capture these complexities. Addressing this knowledge gap is essential for improving the long-term reliability, performance, and sustainability of green roof systems.

This study aims to analyze the changes in the mechanical and structural properties of non-woven filter geotextiles used as separation layers between substrate and drainage layers after five years of operation. The selected material is one of the most widely used in Polish green roof applications. By comparing initial and post-service mechanical and physical parameters—including tensile strength and surface morphology—this study seeks to determine how degradation affects filtration performance and durability. Although the investigation is limited to a specific product and climatic region, it provides practical insights relevant to Central European conditions. These findings can support more informed material selection and maintenance practices, ultimately contributing to the long-term sustainability of green infrastructure.

2. Materials and Methods

2.1. Geotextil TS 20

This research was conducted in 2013–2018 at the Warsaw University of Life Sciences (SGGW) Water Center. Three containers measuring 0.5 m in length, 0.3 m in width, and 0.3 m in height, each with a volume of 45 L, were used in the experiments on green roofs. An 8 cm layer of gravel was placed in the first container, on which a geotextile (polypropylene geotextile Polyfelt TS 20, strength class GRK 2, weight 125 g/m²) was attached, and on top of that, a 17 cm layer of intensive roof substrate. The choice of smaller containers was due to field conditions, because it was not possible to obtain materials from larger roofs, and the aim of the study was to assess the actual consumption of the filtration layer in field conditions. Despite the smaller scale compared to real green roofs, the dimensions used in the research allowed for the simulation of the performance of the geotextile and substrate over time. Although larger-scale studies could provide additional information, the small-scale experiment allowed for controlled analysis of material properties and their behavior in conditions resembling real-world conditions.

The TS 20 geotextile, commonly used in green roof construction, plays a role between the vegetation and drainage layers, allowing water to drain from the substrate. According to the manufacturer, the geotextile is made of continuous polypropylene fibers, UV stabilized, and mechanically reinforced. It is characterized by high resistance to damage during installation and excellent water permeability and filtration properties. The structure of the geotextile allows for effective water filtration and retention of soil particles due to its high permeability. The lack of weak directions, thanks to the structure of continuous fibers, makes the geotextile uniformly strong.

Geotextile also demonstrates a high capacity to overgrow plant roots so that the roots can take up water from the drainage layer. According to the manufacturer, Polyfelt (TenCate, Linz, Austria), an essential advantage of these materials is that they are highly resistant to prolonged exposure to water, mold, and fungi. Tests were conducted on the geotextile before and after 5 years of use. Figure 1 shows images of the analyzed material.

The images of the tested material (Figure 1) show fibers of similar diameter and stochastic arrangement, with pores of different sizes between them. An optical microscope analysis of the fibers indicates that the pores are found mainly in the outer layer of the geotextile. The fibers have a few darker points visible on their surface (Figure 1, lower right). They result from the extrusion of overheated plastic, which glues and degrades during production, and then its small parts break off and stick to the fibers. Their size is tiny compared to the diameter of the fibers.

2.2. Mass per Unit Area and Thickness

The study analyzed mass per unit area and thickness, because these parameters are key to assessing the properties of filtration materials such as geotextiles, especially in the context of their functioning in green roof systems. Mass per unit area allows us to determine how much material is on a given surface, which affects the strength of the geotextile and its ability to transmit water, as well as its long-term durability. In turn, measuring the thickness of the geotextile is crucial to assessing its structure and the efficiency of filtration and drainage in green roof systems.

The average mass per unit area was obtained in 3 measurements, and the average thickness was obtained in 60 measurements of the tested unused geotextile. The thickness and mass per unit area were measured according to the standard PN-EN 1849-2 [37]. The mass per unit area was determined by weighing a test specimen of a known location with an accuracy of 0.01 g. The specimens were squares with an area of 10,000 ± 100 mm² and were cut out 100 mm from the edges. Three samples were cut from the analyzed material at distances of approximately 500 mm from each other. Before weighing, the specimen was conditioned for ca. 20 h at a temperature of 20 ± 2 °C and relative humidity of 65 ± 5%. The average mass value per unit area was calculated with an accuracy of 0.1 gm⁻². The thickness was determined with an accuracy of 0.01 mm by a mechanical measuring device (micrometer) on samples prepared for strength tests. Six thickness measurements were made on each of the samples.

Table 1. Basic parameters of the geotextile declared by the manufacturer [38].

Parameter	Units	Value	Tolerance
Mass per unit area 1 m−2	gm−2	125	-
Tensile strength in MD/TD	kNm−1	10/10	−1.3/−1.3
Elongation in MD/TD	%	90/75	±27/±23
Dynamic puncture resistance (hole diameter)	mm	24	+4
Conventional pore size (O 90)	µm	110	±34.5
Water permeability in the direction perpendicular to the surface of the product	Lm−2s−1	115	−34.5
Water permeability in the product plane (20 kPa, i = 1, hard/hard)	Lm−1s−1	1.0 × 10−3	−7.5 × 10−3

shows the basic parameters of the geotextile declared by the manufacturer.

The manufacturer claims a filter layer with relatively high weight (125 gm⁻²) and high tensile strength (10 kNm⁻¹ in both directions), with satisfactory permeability and porosity parameters. Tolerances, especially ±34.5 μm for pore size (O90), suggest the possibility of variation in nominal mesh diameter from about 75 to 145 μm.

2.3. Strength Tests

The mechanical features of the tested geotextile during tension were investigated according to the standard PN-EN 12311-2 [39] in the Instron 5982 (Norwood, MA, USA) testing machine equipped with mechanical jaws. For strength tests, two sets of samples of each material were prepared—five samples for the longitudinal (machine) direction (MD) and five for the transversal direction (TD). The samples measuring 200 mm by 200 mm had been cut out randomly from the tested part of the material (using a template) at a distance no smaller than 100 mm from the material edge. The specimen length of 200 mm was selected according to PN-EN ISO 10319 [40] to ensure a distance between jaws of 100 mm. Notably, in the method used in this study, the width of the specimen is greater than the length (measured between the jaws). This is distinct from the methodology for calculating the mechanical properties of other materials, such as building films (cf. [35,36]). Like other geosynthetics, geotextile tends to narrow under load in the area of the test section. A larger specimen width ensures that the effect of the constriction is reduced, and it is more likely that the test behavior of the specimen will be similar to the behavior of the material in the construction. Before the tests, the samples had been conditioned for 24 h at a temperature of 20 ± 2 °C and relative humidity of 65 ± 5%. The static tensile test was performed at 20 ± 2 °C at a constant machine clamp extension velocity of 20 %/min. An initial load of 15 N was applied to determine the starting point for measuring elongation, and then the test continued until the specimen broke.

The tensile strength $T_{max}$, expressed in kNm⁻¹, was calculated based on PN-EN 10,319 [40] from the following equation:

$$T_{max}=F_{max}·c,$$

(1)

where

$F_{max}$—maximal tensile force (kN),

$c={\displaystyle \frac{1}{B}}$,

$B$—specimen width (m).

2.4. Microscopic Analysis of Sample Surfaces

In addition to mechanical tests, the microstructure analysis of the geotextiles’ surface was carried out. Its goal was to determine the scale of changes in the structure of tested samples after the destruction and find relations between porosity parameters and strength parameters, whose dependence can allow predicting the specific performance characteristics of geotextile.

In the microscopic tests, samples were taken randomly from different areas of the geotextile material to obtain representative data of the geotextile surface throughout its structural range. The decision to take samples randomly was aimed at obtaining average results that best reflect the actual porosity and presence of micro-damage on the material surface. Although the focus was not directly on areas of contamination, the analysis showed that the results included various types of damage, including micro-defects that may be associated with clogging processes. The samples were selected to be representative, without favoring specific areas, allowing for reliable results regarding the overall material properties.

The samples were subjected to microscopic surface testing using the Keyence digital imaging microscope of the VHX-6000 series, with a universal zoom lens VH-Z20R/Z20T and Keyence wide-area 3D measurement system VR-5000 series. These images allowed for an evaluation of the geotextiles’ surface porosity and observation and measurement of the magnitude of existing inclusions, micro-damages, and surface topography. Images from a wide-area 3D measurement system allowed the evaluation of the surface porosity measurement following ISO 25,178 standard [41]. The porosity of the geotextile surface is calculated as the ratio total area of pores $A_p$ to the full area $A$:

$$p={\displaystyle \frac{A_p}{A}}100\%$$

(2)

3. Results

3.1. Mass per Unit Area and Thickness

Table 2. Basic data for the tested geotextile TS 20.

Measured Mass Per Unit Area (gm−2)	Measured Thickness (mm)
110 ± 2%	0.335 ± 2%

presents the average mass per unit area and the average thickness.

The FLL guidelines [19] recommend using a filter geotextile with a minimum weight of 100 gm⁻². Still, weight is not an authoritative indicator for determining the quality of geotextiles. In the case of a filter geotextile, particular attention should be paid to puncture and tensile strength, conventional pore diameter, water permeability, resistance to microorganisms and atmospheric agents, and the degree of penetration by plant roots. Table 2 shows the tested filter fabric’s average surface mass and thickness. The higher the surface mass of the non-woven material, the better, as it is less susceptible to damage during installation and more effective in long-term protection. It should be noted that the measured film thicknesses do not differ from those declared by the manufacturer. The manufacturer declares it a geotextile filter fabric with high mechanical strength (GRK class 2).

The manufacturer declared 125 g-m⁻², but the measurement showed 110 g-m⁻² (12% less). The 12% lower surface weight may be due to production batch differences or compression of the nonwoven during manufacturing, or other sample test conditions. In practice, the slightly lower mass translates into lower puncture resistance and faster wear, as confirmed by the lower-than-declared strength parameters presented in the next section.

3.2. Strength Tests

In the strength tests, none of the samples slipped out from the jaws during the test, nor broke in the distance lower than 10 mm from the machine clamps. Each sample broke in or near the middle, which proves that the tests were performed correctly. The test results in the form of strain–stress curves are presented in Figure 2. The self-similarity in each set is maintained in both directions of tension. Figure 3a–c shows maximum tensile strength, strain at maximum tensile strength, and strain at break, respectively, for the analyzed geotextiles with two tension directions (MD and TD). The average tensile strength with standard deviation was presented in

Table 3. Tensile strength with a standard deviation of the geotextile.

Conditions of Use of the Geotexile	Tensile Strength ${\mathit{T}}_{\mathit{m}\mathit{a}\mathit{x}}$ (kNm−1)	Standard Deviation 1 SD (kN m−1)
unused (MD tension)	7.60	1.13
unused (TD tension)	6.74	0.45
used (MD tension)	8.11	0.24
used (TD tension)	7.43	0.51

¹ $SD=\sqrt{{\displaystyle \frac{1}{n-1}}}\sum _{i=1}^n{\left(x_i-AM\right)}^2$; where $AM={\displaystyle \frac{1}{n}}{\sum }_{i=1}^n{x}_i$, $i=1,2,3,\dots ,n$; where $n$—number of samples; $x_i$—i-th measurement.

.

In the performed tests, the highest average tensile strength was reached in MD tension for both samples—unused and used. The tensile strength was highest for the used samples in both directions.

At the same time, it should be noted that the used filter geotextile had lower strength in the longitudinal (machine) direction (MD) and the transversal direction (TD), as declared by the manufacturer, by 24% (MD) and 32.6% (TD).

The declared range of tensile strength in both directions was 8.7–10 kN-m⁻¹. According to test results, even for the unused condition, the tensile strength (MD = 7.6 kN-m⁻¹/TD = 6.74 kN-m⁻¹) does not reach the lower limit of the declaration. After five years, the values increased slightly (MD = 8.11 kN-m⁻¹/TD = 7.43 kN-m⁻¹), but were still below the declaration. Values of about 7–8 kN-m⁻¹, instead of the expected 8.7–10 kN-m⁻¹, indicate that the manufacturer declared values at optimal test conditions, i.e., right after manufacture. In addition, one can see the discrepancy between MD and TD, which reflects the anisotropy of the fibers and the direction of threading. The declared elongation at break is 90% (MD)/75% (TD) with tolerances of ±27 and ±23 in the respective directions. Test values are 70–80% in MD and 60–65% in TD for the new material, which for the MD direction also falls below the lower limit of the declaration.

The increase in the strength of the filter fabric during operation, compared to the intact sample, may have been caused by the accumulation of debris in the geotextile structure.

3.3. Digital Images of the Geotextile Surface

Figure 4, Figure 5 and Figure 6 show a microstructural image of the nonwoven fabric under investigation, taken with the VHX-6000 series microscope, Keyence, Japan. The polypropylene material contains loosely bound fibers in its structure, which form during production.

Surface irregularities characterize the fiber structure, including numerous pits, gaps, and undissolved polymer particles. These compounds significantly reduce the degree of crystallinity of the yarn and weaken its overall properties.

From the images taken with the Keyence VR-5000 optical microscope, slight dark inclusions are visible (Figure 4)—these are probably the result of extrusion of the superheated plastic, which glues together and degrades during production. Then, small parts of it break off and penetrate the geotextile structure. These are colorless or dull granular bulges with a 20 ÷ 100 µm diameter.

The images of the test material show fibers of similar diameter and random arrangement, with pores of different sizes between them. Analysis of the fibers using an optical microscope indicates that the pores are mainly found in the outer layer of the fibers. All the fibers in the nonwoven fabric are smooth on the surface. Measurements made using a Keyence VR-5000 optical microscope show that the diameter of all the fibers forming the nonwoven fabric varies over a wide range of 20–50 μm. For the samples adopted in the study, the percentage permeability of the material was measured, and the average result was 15%. When the yarn is untreated (i.e., untwisted), it shows a higher proportion of the amorphous phase. The semicrystalline nature of the blend is linked to the presence of a small amount of amorphous phase in the yarn of the polymer plastics. Polypropylene has been referred to as a semicrystalline polymer. An increase in the degree of crystallinity leads to an increase in the proportion of the crystalline phase. This process is associated with better mechanical properties, higher abrasion resistance, and improved performance.

In the polymers in question, the grades also show the existence of a mesophase, which is responsible for the parallel arrangement of macromolecules occurring during the rapid stretching of the yarn during the spinning process. The yarns studied are characterized by a crystalline-amorphous submicroscopic structure, confirming their ability to form crystalline regions.

Tests were carried out on the filter geotextile after 5 years of use as part of a green roof. The material contains loosely bonded fibers in its structure, filled with organic and soil material, Figure 5.

Slight, dark inclusions can be seen, such as the material of organic origin, tightly packed between the fabric fibers. These are probably the result of the extrusion of a substrate of organic and mineral nature, which sticks together and degrades during use. Then, small parts of it break off and penetrate the structure of the non-woven fabric. Remnants of plant-derived material and sand grains can be seen in Figure 6.

The image shows fibers of similar diameter and random arrangement, between which pores of different sizes are tightly filled with residues of biological material and soil material. An optical microscope analysis of the fibers indicates that the pores present in the fabric are maximally filled between the layers of fibers. From the measurements carried out with the optical microscope and presented in

Table 4. Porosity of new nonwoven fabric, the total area of samples A = 9,777,704 µm².

Sample	Average Pore Size (µm)	Standard Deviation (µm)	Total Pore Area Ap (µm2)	Porosity (%)
1	49	28	1,719,797	18
2	49	24	1,577,753	16
3	47	20	1,427,886	15
4	45	19	1,357,848	14
5	44	20	1,175,549	12

and

Table 5. The porosity of nonwoven fabric after 5 years of use as a filtration layer in a green roof structure, the total area of samples A = 9,777,704 µm².

Sample	Average Pore Size (µm)	Standard Deviation (µm)	Total Pore Area Ap (µm2)	Porosity (%)
1	44	19	619,530	6
2	45	20	614,709	6
3	48	21	720,184	7
4	47	21	794,403	8
5	45	19	616,182	6

, the percentage of porosity of the unused material is 15% on average, and decreased by 56% after 5 years of use in the green roof structure as a filter layer.

A comparison of 3D images of fragments of a new material sample and after 5 years of use as a filter layer in a green roof structure is shown in Figure 6.

The maximum measured pore size of nonused material, given as the diameter of the equivalent circle, is 328 μm, and the minimum size is 28 μm. The average pore size was c.a. 47 μm.

The diameter, 47 μm, is more than twice the declared nominal value. The pore distribution is therefore much narrower, and the porosity (open voids) is lower than expected. This means that the actual material retains much finer particles. On the one hand, this can improve sediment retention, but on the other hand, it can significantly reduce permeability and more quickly cause blockage of the filter layer.

Narejo [42] confirmed in his studies that one of the main factors determining the retention of suspensions is the pore size of the geotextile. Moo-Young and Tucker [43] showed that particle deposition occurs mainly in smaller pores of 10 ÷ 50 μm, and the migration of small particles penetrating the geotextile depends on the size of the smallest pore size of the geotextile provided by the manufacturer.

Research conducted by Torgol et al. [44] showed that clogging of geotextiles occurs more intensively in the case of cyclic drying. The authors associated this phenomenon with the adhesion of small particles in the geotextile’s pores during the synthetic material’s drying, which results in a decrease in the pore cross-section. Maintaining a continuous flow of liquid through the geotextile prevents the deposition of solid particles. In the study, the filter geotextile is a layer covered with a substrate not exposed to continuous drying or UV radiation. Borzdyńska-Marahori and Ossowski [45] showed that geotextile clogging occurs in two characteristic stages. In the first, the pore space of the synthetic material is filled with particles whose dimensions are similar to the pore diameter of the geotextile. This stage was called by the authors the period of pore space filling. In the second stage, structures are created due to the wedging of smaller particles, which leads to a sudden decrease in the water permeability of the geotextile. The duration of the individual stages depends on the type of geotextile used and the granulometric composition of the suspensions.

For a green roof to function properly, water must enter the drainage and flow away to the receiver. Geotextiles with too large “meshes” will allow mineral parts from the substrate or substructure to enter the drainage. The recommended size of the “meshes,” i.e., the technically conventional pore diameter, should be from 0.6 to 0.2 mm (10). The tests showed that the average pore size in unused nonwoven fabric was 47 μm, and for that used after 5 years, it was 46 μm. According to measurements carried out using an optical microscope, the percentage of porosity of unused material is, on average, 15% and decreases by 56% after 5 years of use in the green roof structure as a filtration layer. Lower porosity of the material can lead to clogging of the geotextile.

The decrease in porosity of the geotextile by more than half (−56%) after 5 years of use as a filter layer is not insignificant—on the contrary, such a loss of free pore volume indicates significant clogging of the clearances with biological sediments and soil particles. In practice, this translates into reduced permeability, because lower porosity equals lower drainage capacity, which results in the risk of stagnation, local seepage, or excessive loading of the vegetation layer. Moreover, a decrease in porosity changes the nature of filtration. When the fine pores are filled, the filter begins to act more like a surface layer (“cake filtration”), accumulating contaminants on top, which can lead to a sharp drop in flow. Shortening the lifespan of the entire green roof system is another effect of a decrease in porosity. Too much water ponding can stress the supporting structure and negatively affect the vegetation.

To mitigate the rapid loss of porosity in filtering geotextiles and preserve their long-term functionality, several strategies can be employed. First, selecting geotextiles with anti-fouling properties can greatly reduce pore clogging: for example, fibers treated with hydrophobic surface finishes or biocidal coatings hinder the adhesion of soil particles and the proliferation of biofilms. Equally important is the use of fabrics engineered with a graded pore structure, in which larger pores near the inflow side capture coarser debris while finer pores deeper in the matrix maintain fine-particle filtration without immediate blockage. A practical complement to these advanced materials is the installation of a prefilter layer, such as a coarse aggregate bed or a secondary, more easily cleaned or replaceable geotextile, which intercepts the bulk of sediment before it reaches the primary filter. Routine maintenance routines, including periodic backwashing (reverse-flow rinsing) where system design allows, or gentle mechanical removal of surface deposits, will also extend service life by flushing or brushing away accumulated solids. In modular green-roof assemblies, designing the filter layer in interchangeable panels permits straightforward replacement of saturated or fouled segments without dismantling the entire roof. Finally, implementing a monitoring program—regularly measuring hydraulic conductivity and pressure drop in addition to porosity—enables early detection of performance decline and timely intervention, ensuring that the geotextile continues to deliver effective drainage and filtration throughout its intended lifespan.

4. Conclusions and Summary

The comprehensive investigation of TS 20 geotextile filter fabric, based on both unused specimens and samples retrieved after five years of service in a green-roof installation, illuminates a strong coupling between mechanical performance and microstructural porosity. Tensile testing shows a modest increase in break stress—from 7.60 to 8.11 kN·m⁻¹ in the machine direction and from 6.74 to 7.43 kN·m⁻¹ transversely—yet these values remain below the manufacturer’s declared minima and mask a more critical phenomenon: a 56 percent reduction in free-pore volume (from 15 percent to 6.6 percent) with virtually unchanged average pore diameter. This homogeneous clogging by biological residues and mineral fines stiffens fiber bundles locally, producing superficially higher tensile readings, but simultaneously transforms the fabric into an impermeable barrier that can no longer fulfil its depth-filtration role.

Such findings demonstrate that residual tensile strength alone is an inadequate predictor of long-term serviceability. As porosity declines, the filtration regime shifts to surface cake formation, precipitating a sudden rise in hydraulic resistance and risking substrate waterlogging, increased structural loads, and premature vegetation stress. To forestall these failures, each delivery of geotextile must undergo laboratory verification of mass, strength, and porosity so that design margins exceed minimum requirements. Filtration layers should incorporate prefilters or multilayer arrangements to intercept coarse sediments and delay clogging, while a structured maintenance protocol—including periodic mechanical cleaning or backwashing before free-pore volume falls below approximately 10 percent—is essential to preserve drainage performance.

Moreover, installation practices and material specifications must satisfy long-term durability standards, with a minimum 120-year service life at 15 °C, while EN 14030 (Methods A and B) requires at least 25 years of durability in soils with pH < 4 or pH > 9 and temperatures above 25 °C.

By emphasizing that open pore structure preservation—not merely tensile integrity—underpins reliable filtration and drainage, this study contributes to the development of robust design strategies and performance-based criteria for sustainable construction. It underscores the necessity of integrated material science, long-term monitoring, and preventive maintenance within the life cycle management of green roof systems.

In this way, the research advances sustainable development goals by promoting resource efficiency, reducing system failure risks, and ensuring that green infrastructure delivers environmental services reliably over its intended lifespan. Incorporating such life-cycle thinking and proactive monitoring aligns with global best practices for resilient, climate-adaptive infrastructure.

The study, although it provided valuable information on the long-term performance of geotextiles in green roof conditions, has some limitations. First of all, the use of small containers of dimensions 0.5 m × 0.3 m × 0.3 m may limit the transferability of the results to a larger scale. In real green roof conditions, geotextiles are exposed to different conditions, such as larger substrate volume, variable climatic conditions, and a variety of loadings. Furthermore, the study included only one type of geotextile (TS 20), which may limit the conclusions to this specific material, and different types of geotextiles may exhibit different properties under similar conditions. Another limitation may be the duration of the experiment (2013–2018), which did not include a longer period of use of the materials. In reality, geotextiles in green roof systems can be used for several decades, making long-term studies necessary to fully assess their durability.

In the future, larger-scale studies are planned, using real green roofs or larger containers, which could better reflect natural conditions. It is also worth considering the study of other types of geotextiles and their comparison in terms of durability and filtration efficiency. Additionally, long-term monitoring of changing weather conditions and the impact of climate change on the performance of geotextiles can provide valuable information on their resistance to various external factors.