Originally Posted By: rmeyers
This post was automatically imported from our archived forum.
The gravel or pebbles are actually a ballast to help hold the system in place from the effects of "wind lift". The base sheets were generally spot mopped to the roof plywood or solid mopped over insulation board. Nailing was not standard for the felts unless the slope required back-nailing to hold the sheets in place for installation.
These systems were very heavy, with standard specifications often using 300lbs of slag or 400lbs of gravel or 500lbs of white marble chips per square for the surface. (In addition to the varying layers of felt and tar.)
The stones also helped protect the tar surface from weathering. (White chips reflecting heat etc.)
The systems were great while they lasted but when problems started, they were hard to maintain because leaks were often difficult to track down and generally costly to repair properly.
Today's adhered or mechanically fastened single membrane systems have now replaced many of these systems and is the system of choice for most designers when working with "flat" roofs. These modern systems have also made our job as inspectors easier in tracking down roof leaks.(Usually at flashing details.)
Just a few more rambling comments from an old contractor turned inspector!
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