don't know what you mean by acceptable but we have trusses on four foot centers with flat purlins. Average snowfall can be as high as 12 feet. . average snowfall is around 8 foot. This in snow belt of the great lakes. nw pa.  Have not had any trouble with four foot centers and flat purlins. Don't think you have a thing to worry about where your located
http://www.usa.com/edinboro-pa-weather.htm#HistoricalSnow