Roof Framing

Trusses or "Stick Framing"

There are two basic categories of roof framing: truss and conventional. Truss framing involves the use of components that have been fabricated elsewhere and delivered to the jobsite ready for installation.

Conventional roof framing, on the other hand, is done almost entirely on site where basic construction tools and materials are used to build and install various components that collectively form the roof structure.

Frieze Blocks

What's New...

Made of 2x lumber, frieze blocks are used to help stabilize the rafters and seal the space between the double top plate and the roof sheathing.

Collar Ties

Gable Studs

Collar ties are horizontal framing members fastened to opposing rafters, used to strengthen a roof. The length of a collar tie is typically one-third the span of the roof.

Gable studs are 2x lumber installed at the open ends of a gable roof to provide a surface for applying sheathing and siding. The gable studs run from the top plate of the end wall to the underside of the rafter above.

Splice Block

What's New...

A splice block is a piece of wood that is fastened to the ridge between rafters to reinforce the splice when two pieces of ridge are joined together.

Rafter

Sub-Fascia

Just as joists support floor and cieling frames, rafters provide much of the support for a roof. Usually made of 2x material, the rafters run from the center ridge of the roof out toward a load-bearing exterior wall. The roof sheathing is attached to the upper edges of the rafters.

A sub-fascia is a horizontal piece of framing material installed at the end of the rafters to support the fascia board and to help keep the rafters aligned.

Roof Framing Plan

Quality framing and construction depends on good planning. Before laying out and installing a roof frame, it is necessary to estimate and order materials; perform a variety of mathematical equations; determine the points of intersection of rafters, supports, and load-bearing walls; and decide on the sequence of construction.

All of this begins with consulting the project prints and the roof framing plan.

Roof Framing Plan

In certain areas, roof framing plans are required for all roofs. If the prints do not include a roof framing plan, a working plan can be sketched by referencing the elevation views and floor plans in the prints.

A roof framing plan is a drawing or sketch that shows the intersecting points of roof framing components. A roof framing plan is typically included in the project prints.

Style of the Roof

Using different designs or combinations of roof styles can create a wide variety of pleasing architectural effects.

The tools, materials, and procedures necessary to construct a roof will be determined in part by the style of roof. When designing buildings, architects can use a variety of roof styles, and these have an important influence on the overall appearance of a structure.

The intended purpose of the building also influences the architect's choice of roof style. The most common roof styles are the shed roof, hip roof, and gable roof.

Style of the Roof

Hip roof

A hip roof slopes in four directions. This style of roof has four hip rafters that run at 45 degree angles from the ridge at the outside corners of the structure. A hip roof also has common rafters and hip jack rafters.

Shed roof

A single sloping roof with common rafters resting on opposing walls of unequal heights is called a shed roof. Shed roofs are commonly used for stand-alone shed-like structures.

Style of the Roof

Intersecting Roof

The combination roof formed when two roofs intersect is called an intersecting roof. Included among the common variations are the intersecting gable roof and the intersecting hip roof.

Gable Roof

Many homes and other residential structures have gable roofs. This type of roof slopes in two directions from the ridge and is constructed with common rafters. A gable roof slopes equally on both sides of the ridge and has common rafters extending from the double top plate to the ridge board.

Gable Roof Framing Components

Common Rafter

A common rafter is a roof member that runs from the outside of the wall to the ridge of the roof. It is placed perpendicular to the double top plate which supports the rafter.

A function of a common rafter is to transfer the loads applied to the roof to the exterior walls. Common rafters also support the sheathing and roofing materials, and help tie the structure together. In most floor designs, rafters are attached to the ceiling joists at the top plates. This creates additional support for the overall roof structure.

The frame of a gable roof is a fairly simple structure consisting of common rafters, collar ties, fly rafters, and a ridge board. Together these components form the roof structure.

Gable Roof Framing Components

Collar Tie

Roof frames are strengthened with collar ties that connect an opposing pair of rafters. Collar ties are horizontal framing members used to strengthen roofs by distributing loads. The length of the collar tie is typically one third the span. Collar ties are usually spaced 4'-0" on center.

Ridge Board

Gable Roof Framing Components

The ridge board is the uppermost framing member of the roof. Rafters are fastened to the ridge board, which helps maintain their spacing. The ridge will be laid out to indicate the location of the rafters.

Fly Rafter

There are a variety of different methods that can be used to set the ridge board. The most common method is to use four rafters pinching the ridge and holding it in position as the ridge board is secured to these rafters.

A fly rafter is a common rafter installed at the end of a gable end. Fly rafters are used to create a projection or overhang, and do not have a bird's mouth cut out of them.

Terms Describing Rafter Features

Plumb Cut, Level Cut, and Seat Cut

Several different cuts are made in rafters before they are installed. Vertical cuts are called plumb cuts, while horizontal cuts are called level cuts. Although these cut lines do not look plumb or level when marked on the rafter, they will be plumb or level once the rafter is installed.

Carpenters use a number of special terms when discussing the rafter layout process. These terms help them avoid confusion and costly mistakes. The following terms are used to descibe various features of a common rafter.

A seat cut is a type of level cut made in a rafter to provide a surface for attaching it to the double top plate.

Rafter Tail

Terms Describing Rafter Features

In most cases, one end of a rafter will extend beyond the outside edge of an exterior wall. This portion of the rafter is called the rafter tail. The rafter tail projects beyond the face of the wall.

Some of the purposes of a rafter tail include:

Birdsmouth

• To accept the sub-fascia board

• To extend the roof beyond the building

• To help protect the windows

• To provide shade

• To protect the house

A birdsmouth is a notch made in a rafter by making a plumb cut and a level or seat cut. It is used for positioning the rafter on a double top plate.

Terms Describing Rafter Dimensions

"The horizontal distance from the outside of a wall to the center of the roof ridge..."

In carpentry, the term rise typically refers to a vertical direction and the term run typically refers to a horizontal direction.

Carpenters also use specific terms to describe the dimensions of a roof or a rafter. The following terms are used to identify various lengths, widths, and angles measured during rafter layout, and they refer to the sides of a right triangle. The accuracy of these measurements and the calculations based upon them are critical to a successful layout and roof assembly.

In roof framing the term rise and run are used to refer to the direction that dimension is measured. They also correspond to the similar sides of a right triangle.

There are four terms that refer to a horizontal or run dimension. They are:

Terms Describing Rafter Dimensions

• Unit run
• Span
• Total run
• and Projection

There are three different terms in roof framing refering to a vertical or rise dimension. These terms are:

These are all horizontal dimensions, but they are measured from different points.

The rafter is represented by the diagonal side of the right triangle and has four terms related to it. These terms are:

• Unit rise
• Total rise
• Actual rise

These terms identify a different portion of the rafter that must have a certain dimension.

All of these dimensions are vertical, but they are measured from different starting and ending points.

• Unit rafter length
• Line length
• Overhang
• and Total rafter length

Unit Rise, Unit Run, and Unit Rafter Length

Terms Describing Rafter Dimensions

The unit run for the common rafter is always 12", and is the horizontal side of the slope symbol. The unit rise is the number of inches of rise per unit of run and is the vertical side of the slope symbol. The unit rafter length is the length in inches of the diagonal side, or hypotenuse, of the right triangle formed by the unit run and unit rise. This dimension is not included in the slope symbol and must be determined by using the formula:

Slope

2

A + B = C

2

The architect determines the slope of the roof and this is based on the overall design of the building. The slope of the roof is given on the elevation drawing of the building and is symbolized with a small right triangle above the roof line. The number given on the vertical side is the unit rise. (the amount that a roof rises compared to the amount of run)

Where A and B are your unit rise and unit run (they are interchangeable), and C = your hypotenuse or unit rafter length.

The total run of a roof is the horizontal distance from the outside of an exterior wall to the center of the ridge. The total run of a roof equals one-half the span of the roof. Total run is also the horizontal dimension covered by one rafter.

Terms Describing Rafter Dimensions

Total Run

Span

The span is the distance from the outside of one exterior wall to the outside of the opposite exterior wall. When laying out a roof, the span describes the horizontal distance between two walls covered by a pair of common rafters and is measured perpendicular to the direction of the ridge.

Total Rise

Total rise can be calculated by multiplying the unit rise by the unit of run in the total run. The width of the rafter material and the method used to secure the rafter do not need to be considered when perfoming the calculation.

Line Length

Terms Describing Rafter Dimensions

The diagonal distance from the center of the ridge board to the building line or outside edge of the structure is referred to as the line length. The line length is also the hypotenuse of a right triangle formed by the total run and the total rise.

Projection and Overhang

Actual Rise

The horizontal distance from the outside edge of the exterior wall to the end of the rafter tail is called the projection. The overhang is the diagonal distance from the outside edge of the building line to the end of the rafter tail. Can be calculated by multiplying the unit rafter length by the units of run in the projection.

To determine the actual rise the height above plate (HAP) is added to the total rise. Although most rafters have a HAP dimension, if there is no HAP on the rafter then the total rise and actual rise are the same dimension.

Ridge Deduction

Terms Describing Rafter Dimensions

The length of the rafter must be adjusted to compensate for the thickness of the ridge board. This is done by deducting one half the thickness of the ridge board. The deduction is measured perpendicular to the plumb line at the ridge.

Total Rafter Length

The diagonal distance from the center of the roof ridge to the end of the rafter tail is the total rafter length. This measurement equals the line length plus the overhang.

Relationship Between Rafter Dimensions

Terms Describing Rafter Dimensions

Height above Plate

The line length of the rafter is directly related to the total run. In other words, as the total run gets longer, the line length gets longer to cover the longer run. The projection and overhang have the same relationship. The same relationship is established between total run and total rise. This direct relationship between run, rafter, and rise is important in determining rafter dimensions.

The vertical distance from the seat cut to the top of the rafter measured along the plumb line of the birdsmouth is called the height above plate or HAP. This distance is almost always called the heel height.

Converting Fractions to Decimals

Math Used to Calculate Rafter Dimensions

When converting a fraction to a decimal, divide the numerator by the denominator. The numerator is the number on top of the fraction while the denominator is the number on the bottom of the fraction.

Example:

Math skills are required for nearly every type of construction work. Basic conversions of fractions and decimals are also very important, especially when laying out rafters.

1/4" = 1 divided by 4 = .25"

2/3 = 2 divided by 3 = .666

3/16" = 3 divided by 16 = .188"

(round to the nearest .001")

Example:

Math Used to Calculate Rafter Dimensions

A carpenter would convert .625" to a fraction by multiplying the decimal by 16. This results in a numerator of 10 and a fractional measurement of 10/16" which reduces to 5/8".

Converting Decimals to Fractions

To convert a decimal to a fraction, multiply the decimal by the intended denominator. The result then becomes the numerator of the fraction. Carpenters often make framing measurements in increments of 1/16", so they would typically use 16 as the denominator.

=

.625 10 5

x16 16 8

10

Math Used to Calculate Rafter Dimensions

Example:

Converting Decimal Inches to Feet

Sometimes carpenters must convert a measurement given in decimal inches to whole feet, inches, and a fraction of an inch. This is often necessary when laying out rafters. The conversion is completed in three steps as follows:

Given measurement: 159.865"

159.865 divided by 12 = 13.322' = 13' with a remainder of .322

Convert decimal remainder of .322 to inches:

.322 x 12 = 3.865" = 3" with a remainder of .865

Convert decimal remainder .865 to a fraction:

.865 x 16 = 13.84

Round 13.84 to nearest whole number = 14 and place over 16 = 14/16 which reduces to 7/8"

Result: 159.865" = 13'-3-7/8"

=

.625 10 5

x16 16 8

10

• Convert inches to feet by dividing by 12

• Convert the decimal of a foot to inches by multiplying by 12

• Convert the decimal part of an inch to a fraction by multiplying by 16 and rounding to the nearest whole number

Calculating Rafter Dimensions

Determining Slope

Usually, the slope of the roof, which is the ratio of the rise to the run, can be found on the elevation drawings or section views of the project prints. When examining the prints, look for the slope symbol. The slope symbol will include the unit rise, and sometimes, if the unit run is omitted it is because it is always 12"

The dimensions used in rafter layout must be precise. Accurate dimensions can be calculated using information obtained from the project prints. The following dimensions are required for laying out an equal-sloped gable roof.

Calculating Rafter Dimensions

Determining the Span and Total Run

Determining Unit Rafter Length

The span is the horizontal distance between two exterior walls covered by a pair of common rafters. The span can be determined from the drawings, but it must always be verified by taking actual measurements. The total run of an equal-slope roof is equal to one-half of the span.

The unit rafter length is the length of the rafter per unit of run. It is the hypotenuse of the right triangle formed by the unit rise and unit run. The unit rafter length is calculated using a version of the Pythagorean Theorem.

2

a + b = c

Where a = rise, b = run, and c = hypotenuse (rafter length)

Example

Calculating Rafter Dimensions

Total run = 12'-7"

(12'-7") = 12 whole units of run with a remainder of 7"

7" divided by 12 = .583 = .59 of a whole unit of run

12 units + .58 units = 12.58 units of run in the total run

Determining Units of Run in the Total Run

The unit of run in the total run equals the number of whole feet in the total run plus any remaining inches. The leftover inches are divided by 12 to convert them to a decimal, and then added to the number of whole units.

Stair gauge buttons can be used on a framing square to save dimensions.

Example

Calculating Rafter Dimensions

Unit rise = 8"

Units of run in the total run = 12.58

Total rise = 8" x 12.58 = 100.64" = 8'-4-5/8"

Determining Total Rise

The total rise is made up of units rises. The number of units of run in the total run will determine how many unit rises are in the total rise. To calculate the total rise multiply the unit rise by the number of units of run in the total run.

Total rise = (unit rise) x (units of run in the total run)

Example

Calculating Rafter Dimensions

Unit rafter length = 14.42"

Units of run in total run = 12.58

Line length = 14.42" x 12.58 = 181.4036" = 15'-1-3/8"

Determining Line Length

The line length of a rafter equals the unit rafter length multiplied by the number of units in the total run. The line length forms the hypotenuse of a right triangle based on the total run and total rise. This hypotenuse is made up of unit rafter length as determined by the units of run in the total run.

Line length = (Unit rafter length) x (Units of run in the total run)

Example

Calculating Rafter Dimensions

Projection = 1'-8"

(1'-8") = 1' = 1 whole unit with a remainder of 8"

8 divided by 12 = .6666' (decimal part of a foot)

= .67' (rounded to hundreths)

1 unit + .67 units = 1.67 units of run in the projection

Determining the Units of Run in the Projection

The projection is the horizontal distance from the wall to the end of the rafter tail. This dimension does not include the thickness of the fascia. The number of units of run in the projection equals the number of whole feet in the measurement plus any remaining inches. The leftover inches are divided by 12 to convert them to a decimal and then added to the whole units. This will equal the units of run in the projection. If the projection is less than a foot then the inches are divided by 12 and the answer is the units of run in the projection.

Example

Calculating Rafter Dimensions

Unit rafter length = 14.42"

Units of run in the projection = 1.67

Overhang = 14.42" x 1.67 = 24.0814 = 2'-0-1/16"

Determining the Overhang

The overhang is the diagonal distance from the wall to the end of the rafter tail. It equals the line length of the rafter projection and can be calculated by multiplying the unit of rafter length by the number of units of run in the projection.

Overhang = (Unit rafter length) x (Units of run in projection)

Example

Calculating Rafter Dimensions

Line Length = 15'-1-3/8"

Overhang = 2'-0-1/16"

Total Rafter Length = 15'-1-3/8" + 2'-0-1/16" = 17'-1-7/16"

Determining Total Rafter Length

The distance from the center of the ridge to the end of the rafter tail is the total rafter length. The total rafter length is equal to the line length plus the overhang.

Total Rafter Length = Line Length + Overhang

Methods of Laying Out Common Rafters

The carpenter will then use the same layout procedure to step off the projection to establish the rafter's overhang. The framing square is then used to layout the cuts required on the rafter.

Step-Off Method

One of the most common ways to determine the length of a rafter is the step-off method.

This method requires the carpenter to hold the unit rise and the unit run on the framing square and move the square along the edge of the length of the rafter material marking off the total run. This will establish the Rafter's line-length.

Methods of Laying Out Common Rafters

If the height of the ridge is predetermined, the pitch is found by dividing the overall rise (in inches) by the number of feet in the overall run. This number over 12 is the pitch.

Line-Length Method

A good way to check the accuracy of your calculations is to use the step-off method.

The line-length method requires a precise mathematical calculation.

2

When using this method, the rafter length is found by converting both overall rise and overall run to the same unit of measurement and using A + B = C to find the hypotenuse (which is the rafter measurement from the center of the ridge down to the intersecting lines of the birdsmouth cut). Remember this measurement is not subtracting anything for half the ridge thickness or allowing any extra for overhang.

Preparing to Lay Out Common Rafters

Regardless of the method used to lay out a common rafter, the carpenter must obtain information provided on the print.

The carpenter must also determine the dimensions of the seat cut for the rafter birdsmouth. This information will enable you to lay out a common rafter pattern which can be used to lay out remaining rafters.

Information Needed to Lay Out a Common Rafter

The next piece of information needed are the slope of the roof and the projection. The projection is a horizontal measurement taken from the face of the exterior wall to the end of the rafter tail.

Several pieces of information are needed to lay out a common rafter. The first of these is the total run, which is half of the span. Although the prints will show the span, the structure is still measured to make sure the dimension is correct.

Buildings are often slightly larger or smaller than the dimensions shown on the prints. If the exterior walls have sheathing, make sure the sheathing is included in the span dimension.

The slope of the roof and the projection can be found by examining a detailed wall section in the prints.

Determining the Seat Cut and HAP

Varying the length of the seat cut will affect the HAP. A longer seat cut will reduce the HAP while a shorter seat cut will increase the HAP. It is important to determine the length of the seat cut so you will be able to accurately determine the HAP.

Once the rafter is installed, it will rest on the double top plate. The birdsmouth is the notch in the rafter that fits over the double top plate.

Code typically requires a minimum of 1-1/2" bearing on the top plate, however most jobs using 2 x 4 walls will go with a 4" seat cut (3-1/2" for framing and 1/2" for sheathing). This way the bottom of the rafters dies into the top of the inside of the wall for ease of finishing. Birdmouth cuts should not exceed 1/3 of the rafters thickness.

2 x 8 = 7-1/4" 7.25 / 3 = 2.416 = 2-7/16"

The seat cut dimension will usually be equal to the thickness of the exterior wall including the sheathing.

The stock must be longer than the rafter length to allow for the plumb cut at the rafter tail. The amount of extra length depends on the slope of the roof and the width of the rafter material.

Selecting the Rafter Stock

The first rafter that is laid out and cut is used as a pattern for the remaining rafters needed to frame the roof. For this reason, the stock chosen for the pattern must be of sufficient length and as free of defects as possible.

Typically, a roof with a low slope may require about 6" of extra length while a roof with a steeper slope may require as much as 12" of extra length.

Using the Step-Off Method

Positioning Rafter Stock for Layout

During rafter layout, the rafter is placed across two saw horses with the crown of the stock facing toward you. The crowned edge will be the top of the rafter. Rafter layout is performed from the left to the right which is from the ridge to the rafter tail.

The step-off method of laying out a rafter is a relatively simple process. The only tool needed is a framing square which is used for measuring and marking the rafter material.

Marking the Plumb Line Representing the Center of the Ridge

Orienting the Framing Square Properly

To begin rafter layout, the framing square is held with the tongue of the square in the left hand and the blade of the square in the right. (with this orientation the corner of the framing square will be pointing away from you.

The framing square is oriented properly and held at the left edge of the rafter stock. The unit of rise dimension is held on the tongue of the square (left hand), and the unit run is held on the blade of the square (right hand). The plumb line marked on the tongue side represents the center of the ridge.

Marking the Unit of Run

When using the step-off method for common rafter layout, the unit of run is marked at the point where the 12" dimension intersects the top edge of the rafter. The first unit of run is marked while the framing square is still held with the tongue on the center of the ridge.

Comparing the Step-Off Length to the Line Length

Stepping Off the Common Rafter

The step-off length of the rafter is measured and compared to the calculated line length. The line length measurement starts at the plumb line representing the center of the ridge on the rafter and ends at the plumb line representing the building line.

After scribing the ridge plumb line and marking the first unit of run, the whole units of run are marked along the rafter. This is done by holding the same unit of rise and the same unit of run used to scribe the ridge plumb line, and then sliding the framing square to the right, marking and numbering each whole unit of run. Align the unit rise dimension held on the tongue of the square on the unit run mark established for the previous unit.

The remaining inches of run left in the total run are then marked, the square is then slid along the edge of the rafter until a plumb line can be drawn through the mark (this represents the building line).

Stepping Off the Projection

Laying Out the Birdsmouth

If the rafter has a plumb and seat cut, also called a birdsmouth, the framing square is rotated so the blade slides along the plumb line representing the building line. Slide the framing square along this plumb line until the seat cut dimension desired aligns with the bottom edge of the rafter.

If the rafter has a projection, the whole units of run and the remaining inches of run in the projection must be marked. From the plumb line representing the building line, step off the projection using the same procedure used for stepping off the total run. A plumb line is scribed through this mark to establish the rafter tail plumb line.

The rafter tail plumb line just scribed represents the end of the rafter. Lay out the plumb cut at the tail by measuring down from the top edge of the rafter along the rafter tail plumb line and mark the dimension required. Scribe a level line through this point to complete the step-off procedure if required.

Adjusting Layout for the Ridge

If there is a ridge framing member, the plumb line representing the center of the ridge must be adjusted by an amount equal to half the thickness of the ridge. This deduction is made perpendicular to the plumb line.

Using the Line-Length Method

A tapemeasure is hooked on the ridge end of the rafter stock, and the line length and the total rafter length are measured and marked. After marking plumb lines on the rafter at the ridge, building line, and tail the seat cut is laid out.

The line-length method of rafter layout requires calculation of the line length and total rafter length before the rafter is prepared for layout.

The HAP is also measured since this dimension will be laid out on the remaining rafters. Finally, a plumb cut at the tail is measured and the level cut is marked. The last step is to adjust the length of the rafter for half the thickness of the ridge.