The run or the base number is 12 inches.
Minimum pitch for roof trusses.
The minimum pitch for a roof is 1 4 12 which translates to 1 4 inch rise to 12 inches of run.
Flat the most economical flat truss for a roof is provided when the depth of the truss in inches is approximately equal to 7 of the span in inches.
Note that roof pitch is sometimes used in parallel with roof slope.
I e 4 inches of rise per 12 inches one foot of run.
This table is considerably more useful that the one that was in the previous 1990 edition of the regulations as not only maximum truss spans are indicated but also the allowable and recommended pitch of the roof and the member sizes and grades of timber that are specified in sans 1783 2.
A ratio is the most common way of expressing roof pitch or roof slope but degrees are also possible.
However roof pitch is technically measured and defined differently.
Pitch slope the pitch or slope is the incline of the roof expressed as a fraction describing the rise over the run.
Importance of a roof pitch.
However you can only use this pitch with built up roofing or specialized synthetic roofing.
The picture below shows the pitch of a 7 12 roof slope meaning that for 12 of horizontal measurement roof run the vertical measurement roof rise is 7.
It will use the current cost of wooden rafters based on the average price found at home improvement stores.
For example a slope of 6 12 means for every 12 inches of horizontal distance the slope of the roof will rise 6 inches vertically.
If a roof slope rises say 4 inches when measured 12 inches along the bottom of a horizontal roof truss the roof slope is said to be 4 12.
The minimum roof pitch for shedding snow is around 30 or a 6 12 or 7 12 slope although this is not a definite as the material of your roof the direction of the snow and wind are some factors that can affect whether or not snow will slide as roofs of as little as 10 have been reported to shed snow.
So if your roof rises 4 inches for every 12 inches of horizontal length then the pitch of your roof is expressed as 4 12.
This measurement is best done on a bare roof because curled up roofing shingles will impair your measurement.
The rise will be the elevation change over the 12 inches.
The chief reason for having a roof pitch is to redirect water and snow away from the roof and avoid any percolation that might result from stagnation of water on the roof.
The way pitch also known as roof slope is measured is rise over run.
Difference in slope between the top and bottom chords is at least 3 12 or the bottom chord pitch is no more than half the top chord pitch.
