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Repetitive loads on flat roofs: an improved method to predict potential future damage A flat (or low slope) roof is often exposed to dynamic mechanical loads e.g. by pedestrian traffic or small vehicles. These loads occur during construction of the building or for regular maintenance of installations on the roof. After a few loads some materials tend to lose their compressive strength, resulting in a deeper imprint of e.g. the foot on the waterproofing. The stress in the waterproofing may lead to cracks, or to penetration of a mechanical fixer through the waterproofing if the imprint is close by. Therefore the insulation material and the waterproofing may be severely damaged, resulting in a leaking roof.
Figure 1: Example of a damaged roof In the current practice no methods are available to assess the effect of a dynamic load on the long term performance of a roof construction. Often the compressive strength and/or point load resistance of the insulation material is used as an indication, but this method only applies a load once, and thus can not show deterioration due to repetitive loads on the same spot of the roof.
A new method for assessing the effect of repetitive loads To predict potential damage due to repetitive loads, a new method was developed simulating pedestrian traffic on a roof. A machine was built, simulating the repetitive walking of the foot (with a shoe) of a man of 75 kg with a roll of waterproofing of 25 kg. The number of load cycles can be adjusted, simulating the intensity of the repetitive loading of the roof.
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Tests performed With the new method, a series of tests were performed on various insulation materials, without waterproofing membrane [1]. These tests showed that some modifications were needed to get closer to reality such as a supporting frame to prevent side movement of the samples and waterproofing (EPDM) to distribute the horizontal forces (as in real life). A second series of tests was performed with the modifications, and the results are discussed below [2]. Comparison of the test results and the damages observed in practice shows that the test results correlate with real life, according to independent consultants.
Figure 2: Test equipment
Materials To perform the tests, a standard polyurethane rigid foam (PU) flat roof board was used with a density of 35 kg/mÂł, faced on both sides with 50Âľ aluminium foil [Ins 1]. The material was compared with a commonly used organic insulation [Ins 2] and also with another inorganic insulation [Ins 3].
Results Reduction in thickness and reduction in compressive strength were measured to characterize the materials and also visual changes were recorded, because these are important indicators for potential future damage.
Thickness The observed changes in thickness (figure 2) were rather limited for all products within the 30 cycles. These changes can not explain the serious damage observed in practice.
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Loss of thickness (% of start)
5% 4% 5 cycles
3%
10 cycles
2%
30 cycles
1% 0% PU
Ins 2
Ins 3
Figure 2: Loss of thickness after test
Compressive strength Significant changes in compressive strength (CS) were observed (figure 3). For PU the reduction in CS was limited to less than 20%. For type 2, the reduction of CS was 8%. Type 3 showed a much larger decrease, the remaining CS was about 50% after 5 cycles, and after 30 cycles the remaining CS was less than 15%. Loss of CS (% of start)
100% 80% 60% 5 cycles
40%
30 cycles
20% 0% -20%
PUR
Ins 2
Ins 3
Figure 3: Loss of compressive strength after test
Visual observations After testing, changes in the boards were observed. For PU and type 2 an imprint of the test feet could be seen. For type 3, the effects were, at first sight not severe, but a closer look revealed a three layer structure. The top layer was nearly not damaged; however the bottom layers lost coherence, after 30 cycles. So, although the top layer is hard, the material is very sensitive for repetitive loads.
Conclusions Based on the work performed it can be concluded that the new test simulates reality. The test was therefore used to compare various insulation materials. PU performance was superior to [Ins 3] and also better than [Ins 2] with regard to the repetitive loading. The loss of coherence of [Ins 3] seems to be the cause for the reduction of CS, and is therefore likely to be the basic reason for failure. The fact that [Ins3] still fails although the point load resistance was rather high, is an indication that it is
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not a suitable method to test a material on the ability of a roof to withstand the effects of repetitive loads. This work already caused a discussion in Holland, e.g. on how to adapt building regulations, to incorporate a test which reflects the performance of a material on a roof. The “marathon man” method is under discussion.
Next steps After finalizing this work, the subject was also studied further by BDA, and results were published. To use the method wider, it has to be developed further, and to be verified on a larger scale, and with a wider exposure. After that, work could be started to develop e.g. a European standard. [1] Hendriks, N.A. and K. van Zee: “Development of walkability test on roof insulation”, NVPU report by BDA Keuringsinstituut B.V.: No 0294-L-99/1, August 2, 2002 [2] Hendriks, N.A. and A.R. Hameete: “Development of walkability test on roof insulation”, NVPU report by BDA Keuringsinstituut B.V.: No 0294-L-99/2, April 15, 2003 [3] NEN-EN 12430: 1998 “Materialen voor de thermische isolatie van gebouwen – Bepaling van het gedrag bij puntbelasting”, NEN, Delft
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The information contained in this publication is, to the best of our knowledge, true and accurate, but any recommendation or suggestions which may be made are without guarantee, since the conditions of use and the composition of source materials are beyond our control. Furthermore, nothing contained herein shall be construed as a recommendation to use any product in conflict with existing patents covering any material or its use.
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