# Construction - Roof Slope

# Construction - Roof Slope

Being able to calculate the slope of a roof is very important in construction. It determines the size and type of truss that is used and can play a significant factor in determining the type of roofing material used. In this lesson, you will learn to work with slope in the rise/run, percent, and degrees forms.

Slope is most commonly referred to in the rise/run form. The rise is the height of the peak above the eaves and the run is the

**horizontal**distance from the peak to the eave. The run is**NOT**the length of the roof from peak to eave. A slope that has a height of 5 inches for every 12 inches of span is referred to as a slope of 5:12. Measurements will rarely be in perfect units of 12 so you can use proportional relationships to find the actual dimensions. For instance, if you needed to have a slope of 5:12 and your roof is 30' from edge to peak you can use [math]5/12=x/30[/math] to determine that the height ([math]x[/math]) will be 12'6".If your building had a peak in the center which was 5' above the eaves and the distance from eave to center was 60' then we could find the slope in a similar way. [math]x/12=5/60[/math] would result in an [math]x[/math] value of 1 meaning a slope of 1:12.

To find the slope in percent you simply divide the rise by the run and multiply by 100. A slope of 4:12 would be calculated as [math]4/12*100=33%[/math].

To calculate slope in degrees you need to use trigonometry. The tangent ratio is [math](opposite)/(adjacent)[/math] which is convenient since it is equivalent to [math](rise)/(run)[/math]. For example, if a roof has a slope of 7:12 that means that [math]tan(theta)=7/12[/math] which means [math]arctan(7/12)=30deg[/math].

**Directions for This Lesson:**

Watch the video below to learn more and then try to

**answer the Practice Questions.****Required Video:**

**Practice:**

Extend your learning with these practice questions and then complete the worksheet.

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