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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.

Run

Run

Rise

Rise

Length

Length

Each side of the unit triangle has UNIT added to it.

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Triangle Names

• Thus the Unit Triangle

Unit Run

Unit R

ise

Unit Length

Unit Triangle

Building

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Unit/House Relationship

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

It depends on the size of the building.

This building has six unit triangles.

12′- 0″

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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.

12′- 0″

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Unit Triangle Theory

• The base of the unit triangle is always 12″ for a common rafter.

6″13.42″

Pythagorean Theorem 12″ = 1 foot = 1 unit of runor Rafter Tables

• The unit rise is given on the house plans.• If the unit rise is 6″, • Then the unit length is …

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Example

• If building is 20′ wide and

unit rise is 6″,

what is the rafter Total Rise and Line Length?

20′- 0″

6″

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Rafter Calculation Answers

• Total Rise = Unit Rise x RUN

10′- 0″ = 10 units of run

6″134 3/

16 ″

60″

134 3 / 16″

6″ x 10 = 60″• Line Length = Unit Length x RUN

13.42″ x 10 = 134.2″ = 134 3/16”

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• Unit rise is converted to unit length by _______________________.

• Unit run for a common rafter is ____.

Conclusions

12″

on the floor plans

using the Pythagorean Theorem

Total Rise

Line Length

• The unit rise is found _____________.

• Run x unit rise = _____________.

• Run x unit length = _____________.