Final Report: Does the Underlayment Matter?

This paper is the final report of over four years of data acquisition and five years of summer weather in a study of the performance of cedar shingles over various underlayments. The preliminary report, “Does the Underlayment Matter?,” was presented at an ASTM symposium in December 2019. Although this research did not provide definitive answers to all the questions initially proposed, much was learned, including that the original hypothesis was wrong. Major issues involved the calibration of the wood moisture data, the corrosion of moisture pins, and confusion about the location of repaired moisture pins and thermocouples. Techniques were developed to solve all these issues. An advanced corrosion-resistant moisture pin was developed and installed and proved effective in providing optimal data. The conclusions of the study are based on over 8.5 million moisture and thermocouple data points and over 200 charts and tables.