Types of Roofs and Cutting Common Rafters

Dr. David AgnewArkansas State University

Types of Roofs:In Agricultural Applications

Hip

Gothic

Combination

Uneven span

Types of roofs

Types of Rafters

Parts of A Roof…..

Definition of a Rafter

Single timber supporting a roof section.

Kinds of Rafters

Parts of a Rafter

Rafter Slope

Roofs are not flatPros and cons of low or steep roofs Cost of steep is higher for materials Harder and more dangerous to work on if steep Steep roofs have greater ability to withstand load

Pitch Expressed as a Fraction ¼, 1/6, 1/8, etc.

Rise/foot of run Example: 2 in./ ft. Used in more plans today than pitch

Calculating Pitch

Procedures for Rafter Layout

Three TechniquesStepping methodFiguring length using

A2 + B2 = C2

Rafter Table

Procedures for Rafter Layout

Three TechniquesStepping method

# steps= feet of run

Procedures for Rafter Layout

Three Techniques A2 + B2 = C2

Example: If A=4 , B=3 then what would c=?

16 + 9 = 25 then C= sq root of 25 which is 5

A= Run of Roof

Resources Needed to Layout and Cut a Rafter

Timber/Lumber (2x4, 2x6, 2x8, etc)Measuring Device-- Tape MeasureMarking PencilRafter/Carpenter’s SquareSawSawhorses Note: The class project rafter layout will need

some different supplies.

Steps to layout of Rafter Using the Rafter Table.

Determine Rise or PitchDetermine the Span and/or RunUsing the table for common rafters on the Rafter Square Identify the length of rafter per ft of run.

Rafter Layout Continued

Multiply the run times the number from the table. That number is the length of the rafter… But you are not done yet…Identify the end of the board that you want to make the ridge cut. The square has a tongue and a body Tongue is the smaller of the two 18 inches

long and 1 ½ inches wide. Body is larger 24 inches long and 2 inches

wide.

Parts of a Framing or Rafter Square

1.5 inches thick

2 inches thick

Proper Use of the Square

Always work from one side of the square (never flip it over).Remember to either use the numbers from the outer edge or inner edge, but do not use both at one time.

Ridge Cut

Using the appropriate numbers on the square (ex. 4 and 12) place the square near the end of the board where the cut is to be made. See picture below.