Modified Membrane Testing

Understanding ASTM D2523 vs. ASTM D5147

Both ASTM methods are standard test methods employed by roofing manufacturers to test their modified asphaltic roof membranes. The difference is:

  • D2523 is a method that describes how to perform load-strain tests and is more geared toward testing whole assembled systems
  • D5147 describes the battery of tests the manufacturer can perform on individual modified asphaltic roof membranes and includes a load-strain test in its procedure

Garland uses ASTM D5147 because endless combinations of base sheets, felts, glass bases, cap sheets, etc., as well as adhesion methods used to stick the plies together, exist making testing all system strengths a near impossibility.

Therefore it makes more sense to test individual sheets. Also, an individual membrane can be tested using both D5147 and D2523, but systems are tested using the D2523 method. The load-strain procedures for the two tests are similar, but they do have slight variations, namely:

Specimen shape and jaw separation speed

In D2523, the specimen cut resembles a dog bone with a 1” x 3” test area  while D5147 specifies 1” x 6” strips also with a 1” x 3” test area.

Even though the shape of the specimen is different, the testing area is the same (3 sq. in.)

Furthermore, there is a slight difference in the rate at which the clamps separate, which can have some effect on the ultimate strength, but both test methods yield tensile and elongation in the end.

One can infer from data obtained from D5147 that combinations of high tensile products would, at the very least, yield a total system strength that would have a tensile strength equal to or greater than the membrane with the highest tensile strength. This has been confirmed with testing. For example, the chart below compares the individual tensile strength of StressPly Plus Mineral using the method outlined in ASTM D5147, and tensile strengths using D2523 of a system composed of the same cap sheet and two plies of a Type IV Glass Felt.

Incidentally, HPR Glasfelt’s tensile strength is 44 lbf/in using a different method prescribed in the felt’s specification. That is why its individual tensile strength was not included in the chart. Clearly, the system strength is greater than the strongest ply (in this case, StressPly Plus).

In conclusion, neither test method is incorrect, but because of the myriad of system combinations that exist, it is more beneficial to test individual membranes using D5147. The data from this testing can be used to make inferences on how the combined systems will most likely behave.

In practice, system strength is greater than the strongest ply in the system because of an additive effect that occurs when membranes are combined. Therefore, if a spec is discovered with a system tensile strength value obtained from ASTM D2523, all you need to do is refer to the technical data sheet and look at the tensile values for each layer of the system (usually tested using ASTM D5147).