In the days before EPDM rubber, flat roofing materials consisted of tar and gravel. The liquefied tar served as a water sealant; after it cooled, the surface was covered with gravel. The gravel served as a shield to stave off the sun’s heat and UV rays.
These traditional measures failed a lot, though, because of a combination of lack of maintenance and user ignorance. When people move on the roofs, they shift the gravel, exposing the tar to surface conditions. The dried tar cracks and blisters under the sun’s heat, nulling its effectiveness as a water sealant.
Tarpaper was invented in an attempt to answer that dilemma. Tarpaper was a material that essentially combined tar and gravel onto a fibre material so no shifting can take place. But, there were problems with that material as well. Workmanship and installation training varied greatly, and manufacturers produced different results for every application.
Failing flat roofs are very dangerous, as leaks aren’t detected immediately. Water usually soaks the decking first, causing considerable rot damage and exposing people to mould spores that cause breathing complications when inhaled.
Thank goodness for modern flat roofing supplies that doesn’t need any kind of supervision. After all, whoever thinks about the roof unless it has a leak?