1. UNIT TRIANGLE AND RAFTER THEORY
Presentation 20:
UNIT TRIANGLE AND RAFTER THEORY

2. Unit Triangle Unit Triangle is found on the floor plans.
It may be in many shapes.

3. Unit Triangle vs. House
Doubling the Unit Triangle gives a shape that is similar to the building, but smaller.

4. Each side of the unit triangle has UNIT added to it.
Triangle Names
The sides of the triangles have names.
Length
Rise
Run
Length
Rise
Run
Each side of the unit triangle has UNIT added to it.

5. Triangle Names Thus the Unit Triangle Unit Length Unit Rise
Unit Run
Building

6. Unit/House Relationship
How many units of run will fit into the building triangle?
This building has six unit triangles.
12′- 0″
It depends on the size of the building.

7. Unit/House Relationship
How many units of rise will fit into the building triangle?
In this building, six unit triangles.

8. Unit/House Relationship
How many units of length will fit into the building triangle?
Again, six unit triangles.

9. Unit/House Relationship
There are equal numbers of unit triangles along the run, rise, and length.
If this building is 12′ wide, then six unit triangles fit under the rafter.
12′- 0″

10. Unit Triangle Theory
The base of the unit triangle is always 12″ for a common rafter.
The unit rise is given on the house plans.
If the unit rise is 6″,
13.42″
Then the unit length is … 6″
Pythagorean Theorem
12″ = 1 foot = 1 unit of run
or Rafter Tables

11. Example If building is 20′ wide and unit rise is 6″,
what is the rafter Total Rise and Line Length? 6″
20′- 0″

12. Rafter Calculation Answers
Total Rise = Unit Rise x RUN
6″ x 10 = 60″
Line Length = Unit Length x RUN
13.42″ x 10 = 134.2″ = 134 3/16”
134 3/16″
134 3/16″
60″ 6″
10′- 0″ = 10 units of run

13. Conclusions Unit run for a common rafter is ____.
12″
Unit run for a common rafter is ____.
The unit rise is found _____________.
on the floor plans
Unit rise is converted to unit length by _______________________.
using the Pythagorean Theorem
Run x unit rise = _____________.
Total Rise
Run x unit length = _____________.
Line Length