UNIT TRIANGLE AND RAFTER THEORY

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Presentation 20

• UNIT TRIANGLE AND RAFTER THEORY

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Unit Triangle

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

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Unit Triangle vs. House

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

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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.
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Triangle Names

• Thus the Unit Triangle

Unit Length
Unit Rise
Unit Triangle
Unit Run
Building
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Unit/House Relationship

• How many units of run will fit into the building
  triangle?

This building has six unit triangles.
It depends on the size of the building.
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Unit/House Relationship

• How many units of rise will fit into the building
  triangle?

In this building, six unit triangles.
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Unit/House Relationship

• How many units of length will fit into the
  building triangle?

Again, six unit triangles.
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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.
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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
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Example

• If building is 20' wide and
• unit rise is 6?,
• what is the rafter Total Rise and Line Length?

6?
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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
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Conclusions
12?

• Unit run for a common rafter is ____.

on the floor plans

• The unit rise is found _____________.

• Unit rise is converted to unit length by
  _______________________.

using the Pythagorean Theorem
Total Rise

• Run x unit rise _____________.

Line Length

• Run x unit length _____________.