If I want a roof at 30 degree pitch and the span is 2 meters how high is the ridge

Google several roof calculators to see which one you like (and has metric)
Here's one
Pitch to Angle - Angle to Pitch Converter (http://www.blocklayer.com/pitchangle.aspx)

Won't be much with a 2 meter span, (little over 6'.) Should be about 2 ft. height.