Atmospheric pressure at sea level is approx 14.5 psia, but unless otherwise stated all normal standards and specifications use guage pressure, relative to local atmospheric pressure. Local atmospheric pressure changes with altitude.
What is says is that the internal guage pressure is basically 0 psi / 4 inches water column or I think 1kPa has been used before now or the equivalent pressure that is the weight of the roof divided by its square area.
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