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Hello All 
I am not even a newbie yet as l am just learning the trade. Need a little explanation from all the experts or not so experts out there as to how this calculation is worked out? I am personally unable to understand the math behind this degree slope, need some guidance please…
Course is Moisture Intrusion Course for a CHI, 2.2.1 Moisture-Resistant Roof Systems
AND
Half way down this 2.2.1 page is the statement, which is as follows:
For roof slopes between 2:12 (two units vertical in 12 units horizontal = a 9.5 degree slope) to 4:12 (four units vertical in 12 units horizontal = a 18.5 degree slope), underlayment should be two layers applied in the following manner;
Can someone please explain the better way of understanding this “formula”, or if this is just an error and it can be fixed would be much appreciated. I just need help and guidance understanding things as I am learning at this point, thank you in advance 
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Think about how flat vs how sloped the roof is. Flatter than 2:12 is a “flat roof” (different roof completely) and steeper than 4:12 is a “normal roof” (single underlayment layer.) That range of 2:12 to 4:12 is the in-between where you can still use shingles, but need double the underlayment because it’s not quite steep enough and therefore doesn’t shed water as well.
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excellent point