1. Basic Roof Calculations
Span
Unit run
Total run
Unit rise
Total rise
Pitch

2. Safety Notice
The viewer is expressly advised to consider and use all safety precautions described in this presentation or that might also be indicated by undertaking the activities described herein. In addition, common sense should be exercised to help avoid all potential hazards.
Publisher assumes no responsibility for the activities of the viewer or for the subject matter experts who prepared this presentation. Publisher makes no representation or warranties of any kind, including but not limited to, the warranties of fitness for particular purpose or merchantability, nor for any implied warranties related thereto, or otherwise. Publisher will not be liable for damages of any type, including any consequential, special or exemplary damages resulting, in whole or in part, from viewer's use or reliance upon the information, instructions, warnings or other matter contained in this presentation.

3. Basic Roof Calculations
Observe safety rules.
Make sure tools and equipment are available.
Check materials.
Follow the steps described here.

4. Safety
If you are at a worksite, wear all the required safety equipment.

5. Tools and Equipment
Calculator

6. Materials
Paper and pencil

7. Procedure
The steps that follow describe the terms and calculations used for building a roof.

8. Step 1
The span is the distance between the outside edges of opposing exterior walls.

9. Step 1
Measure straight across the building, not at an angle.

10. Step 2 The unit run is used to figure the roof angle.
For a common rafter, the unit run is always 12".

11. Step 3 The total run is one-half the span. Divide the span by two.
Example:
If the span is 24´, the total run is:
= 12´ 24 2
Exception: This rule does not work when the pitch is different on opposite sides of the ridge board.

12. Step 4
The unit rise is the number of inches that a roof rises for every 12" of run (unit run).
The triangle above the roof in this drawing shows the unit rise (8) and unit run (12).

13. Step 4
The angle of the roof will change if the unit rise changes.

14. Step 5
The total rise is the vertical distance from the top of the top plate to the upper end of the measuring line.
Total
rise

15. Step 5
The measuring line is an imaginary line running from the outside wall to the ridge.
Total
rise

16. Step 5
The total rise equals the unit rise multiplied by the total run.
Total
Rise
15'
10'
15 x 8" = 120"
120" = 10´

17. Step 6
The pitch is the angle that a rafter makes with a horizontal plane.
Pitch is the ratio of the unit rise to the unit run.
Pitch is written as a fraction:
unit rise
unit run
In a drawing, the triangle symbol shows the pitch visually.

18. Step 6
Example: In this drawing, the roof has an 8" unit rise and a 12" unit run.
The roof has an “eight/twelve pitch” or an “eight in twelve pitch.”
The pitch is written as 8/12.
Total
rise

19. Step 7
What is the unit rise in the roof shown here?
Answer: 6"

20. Step 7
What is the span?
Answer: 28´

21. Step 8
What is the pitch of the roof shown here?
Answer: 8/12