There is more to scrim than meets the eye.
The scrim is woven from polypropylene threads or filaments. Scrims are used in everyday life to bag oranges, grapefruits and onions in the food processing industry. One can see them at local grocery stores or in the refrigerator.
An internet search for images of “polypropylene scrim” will instantly yield hundreds of examples of scrim material used in everyday life. The technology for manufacturing scrims is well established and widely available. Manufacturing costs have been driven down in a competitive market place.
The scrim material is defined by several factors, including the spacing of the individual filaments and the diameter of the filaments. One technical application of scrims is for use as a precision mesh screening cloth. Example scrim specifications can vary can be found in the table titled “Specifications for Nylon, Polyester and Polypropylene Meshes”.
It is an interesting geometry problem to estimate the diameter of the polypropylene threads when given the weight of the scrim and the spacing of the threads.
Of course, a woven scrim by itself is not very useful as a roofing underlayment. It is necessary to bond a PP sheet to the PP scrim or coat the PP scrim on both sides with a suitable coating. The woven scrim provides good strength and stability to the underlayment while the thin sheets make the underlayment impermeable to water.