.::. Some Random Terms .::. • Air entrainment When air is mixed with water in a hot tub or whirlpool tub recirculation system when water rushes past an orifice and draws in air to be mixed with the water flow. .... • Span The horizontal distance between supports for beams, joists, rafters, etc. .... • Plies A term used when a film or surface is constructed by adding one film over another. A built-up type roof for residences us usually fabricated with three separate plies cemented together with asphalt. ....