Question 768390
A fraction is a ratio.  In this case it's rise/half-span.  Half-span means the amount of horizontal distance the roof covers from the highest point to the edge.<P>
If the span is 20 feet, the half span is 10 feet.  The ratio is 6/20 which can be reduced to 3/10.  <P>
Roofers/carpenters and other building contractors often call this 6 in 20 which means 6 feet of rise for every 20 feet of horizontal span.