Category Framing

When sheathing gable-end truss, check plans to see if a vent opening is shown.

• Toenail through bottom of truss into plate so gable end is on chalk line IV2" in from outer edge of double plate. (See below.)

Step 3-Set Gable Ends

Toenail 16d nails 24" O. C. to hold

gable end while setting gable-end truss.

IV2" chalk line

Truss

Wall

Steps

1. String line from center of gable ends.

2. Lift single truss into place.

3. Center truss on string line.

4. Nail truss to exterior wall on layout.

5. Nail through temporary spacer support near ridge of truss and on layout marked on temporary spacer support.

6. Set six trusses. Then check gable end for plumb and put permanent brace on gable end. Permanent brace should connect top of gable end to an interior wall or a cross support running between the trusses.

7. Every eight trusses, add an additional brace. Refer to truss specifications for additional braces.

6. Lookouts

Lookouts support the roof overhang and fascia.

The skin for the roof, sheathing, is a structural part of the roof diaphragm.

Chanter Six

Rafters that are not supported at the top or somewhere along the span by a beam create an outward force on the wall they rest on. This force is frequently offset by ceiling joists joining the walls on the opposite side of the roof.

The chart below shows a common size of ceiling joist needed for different spans.

The previous chart shows a common size of ceiling joist needed for different spans.

The bottom chord of roof trusses acts as a ceiling joist and provides support for the ceiling sheathing, and support to keep the walls from pushing out.

Attic areas above ceiling joists must be made accessible if there is a clear height of 30" or more.

This requires framing an attic opening. The opening must not be less than 20" by 30" and have at least 30" clear space above. The attic access should be framed similar to an opening in floor joists as illustrated on “Step 5-Frame Openings in Joists" in Chapter 3.

Step 1-Spread Trusses

Trusses spread ready to roll

Gable-end truss sitting on trusses.

When trusses are delivered in stacks, they should be set on the roof to allow for easy spreading. The gable ends should be on top because they go up first. The direction of the ridge is important so they can be spread and tilted up easily.

When spreading the trusses, place them on your layout marks so that when you roll them, you will have minimum moving of the trusses. They are easier to move lying down.

Breaking the process of cutting rafters into the four basic characteristics described in this chapter helps to organize the task, but it is still a complicated process. Probably the best way to learn is to work through the steps in figuring individual rafters. The following illustration is an example of a roof that has a number of different rafters and a ridge board identified. Nine additional examples explain how to find the lengths for these rafters and ridge board based on the illustration.

Finding Common Rafter Length – Example 1 on Roof Example Illustration

The roof span at this area is 28′-0", making the run equal to У2 the span of 14′-0" minus half the thickness of the ridge board (3/4"). That makes the adjusted run 13′-111/4". Multiplying that times the diagonal percent for a 6/12 pitch roof (which is 1.118) gives a run diagonal length of 15′-7". If you add that length to the overhang diagonal of 2′-11/s", the rafter length is 17′-81/s". The overhang diagonal

is found by subtracting the fascia (IV2") from the 2’­0" overhang, which gives (22У2"), and multiplying by the diagonal percent 1.118.

Finding a Jack Rafter Length – Example 2 on Roof Example Illustration

Most hip rafters are on 90° corners, with the hip runs in the middle of the corner. Because the two sides of a triangle made by a 90° angle and two 45° angles are the same, the run of the jack rafter can be easily found.

The distance of your layout to the center of your rafter is the same distance as your run to the center of your hip. Just subtract one half the thickness of the hip at a 45° angle (11/16") from the run, and multiply that figure by your diagonal percentage. Then add on your overhang diagonal length. This will give you your rafter length. In this example, the rafter is on layout at 8′-0", so we subtract 11/16" (half the thickness of the U/2" hip at 45°), giving 7′-1015/16", which is multiplied by the diagonal percent of 1.118. The result is 8′-101/s". Add this to the overhang diagonal of 2′-11/s" (same as common rafter overhang), and we get a jack rafter length of 10-111/4". Note that because the connection angle is 45° , the measurement should be taken to the center of your cheek cut.

Finding a Ridge End Common Rafter Length—Example 3 on Roof Example Illustration

As long as you use the top cut illustration in “Connection # 1," then this rafter will be cut the same length as the king common rafter adjacent to it.

Finding a Hip Length—Example 4 on Roof Example Illustration

Finding the hip length requires an additional step and uses the hip-val diagonal percent. First find the hip run. It is the diagonal created by a triangle in which the other two sides are the run of the ridge end common and the line from the hip corner to the ridge end common. In this case, the span is 40′, so the run is 20′, and the distance from the corner is also 20′. Using the calculator, enter 20′ for the run, 20′ for the rise, and press the diagonal button.

The result is 28′- 37/16". This distance is the run of your hip.

Subtract half the distance of the ridge at a 45° angle, which for a 11/2" ridge is 11/16", leaving an adjusted hip run of 28′- 23/8". Then find the hip overhang length using a similar procedure. The sides are 2′, which leads to a 2′-915/16" diagonal. Then subtract 11/2" at a 45° angle for the fascia (which is 2V8"), so the hip overhang run is 2′-713/16". Add this figure to the 28′- 23/8" hip run, and you get a hip rafter run of 30′-103/16". Multiplying that number by the hip – val diagonal percent of 1.061 results in a hip rafter length of 32′-83/4". Remember, these lengths are to the middle of the rafter, and each end has two 45° connection angle cuts at a 6/12 hip-val pitch angle.

Finding a Valley Rafter Length— Example 5 on Roof Example Illustration

This valley will be the same length as the hip rafter for the 28′-0" span section, except for the end cuts. On the bottom, the 45° cuts will be concave ( < ) instead of convex ( > ) like the hip. At the top, there will be a full-width 45° cut. The top-end adjustment will require you to subtract one half the thickness of the ridge at 45°, which is 11/16".

Finding the valley rafter length is similar to finding the hip length, and requires the following steps:

• Span = 28-0"

• Run = 14′-0"

• Top adjustment = subtract У2 ridge board at 45°

= 11/i6".

• Hip run = 19′-99/i6" = On the calculator enter 14′-0".

Charts provide bird’s mouth plumb line lengths.

– Then press the run button, enter 14′-0".

– Then press the rise button and then the diagonal button.

Overhang hip run = 2′-713/16"

– On calculator enter 1′-10 1/2" run then 1′-10 1/2" rise, then press diagonal.

Add the hip run and the overhang hip run =

19′-99/іб" + 2′-713/іб" = 22′-53/8".

Subtract for the top adjustment %" on a 45° = 11/16" (See “Connection #2“ illustration on previous page.)

Adjust the hip rafter run = 22′-53/s" – 11/16" = 22′-45/16".

Hip rafter length =

22′-45/16" x 1.061 (hip-val diagonal percent) = 23′-811/16".

The top will be a 45° saw cut for the connection angle at a 6/12 hip-val cut for the pitch angle.

The bottom will be concave ( < ), two 45° saw cuts at a 6/12 hip-val cut.

Finding a Valley-to-Ridge Jack Rafter-Example 6 on Roof Example Illustration

There are a couple of ways to find the length of this rafter. The ridge location is easy to establish as half the span of 40′, making it 20′. The valley point can be determined by figuring the distance the valley runs before the rafter starts. In this case, since the rafters all conveniently line up and run at 24"

O. C., the easiest method is to count the rafter spaces from the other side of the roof. In this example, there are seven rafter spaces; therefore the run will be

14′. Subtract half the distance of the 45° bottom cut for the valley rafter (11/16"), and half the thickness of the ridge board (3/4"), and the run will be 13′- 103/16". The rafter length will be 13′-103/16" x 1.118 (diagonal percent) resulting in a 15′-513/16" rafter length. The connection angle at the top will be a 90° saw cut, and the pitch will be at a 6/12 common cut on the speed square. The bottom will be a 45° saw cut at a 6/12 common cut. The measurement will be to the center of the 45° cheek cut.

Finding Valley-to-Hip Jack Rafter Length-Example 7 on Roof Example Illustration

There are different ways to find the run length. Here is a way that has not yet been illustrated. In this example, run length will be figured from the 28′ span length. The run for the 28′ span is 14′.

The top of the rafter is 2′ past the end of the ridge

Jack rafter run lengths equal layout lengths.

board, which will add 2′ to the run going up the hip that it connects to.

The run at the bottom will be shortened by 4′ because it extends up the valley the equivalent of 4′ of run. This leaves 12′ of run. Adjust for top and bottom by subtracting one half of a 45° angle for top and bottom cuts or two times 11/16" (21/8") = 11′-97/8" times 1.118. This makes for a rafter length of 13′-25/8". Both the top and bottom would have a 45° cheek cut for the connection angle and would be marked at a common 6/12 for the pitch angle.

Ridge-to-Ridge Hip Rafter-Example 8 on Roof Example Illustration

In this example, the rafters are so conveniently arranged that we can see the hip rafter goes from the center of one rafter to the center of another rafter with two in between, resulting in a distance of 6′.

Another way to find this length is to calculate the difference in the runs for the ridges that establish the height difference. One has a span of 28′-0" for a run of 14′-0", while the other has a span of 40′-0" for a run of 20′-0".

The difference is 6′-0", the same as we just figured.

Once you have the 6′-0" of run, then you follow the same procedure as with a hip and make the necessary top and bottom adjustments. First establish the hip run. Enter 6′-0" run and 6′-0" rise on the calculator and press diagonal, which gives you the hip run of 8′-5 13/16". The top will be a standard hip connection. Therefore one half the ridge at a 45° angle (11/16") will be subtracted. At the bottom it will be a #2 connection. (Connection #2.) Therefore subtract one half the thickness of the ridge at a 45° angle, or 11/16". The

bottom will also require a square cut 7/16” deep on the end. You can establish the thickness of this square cut by finding the diagonal for the triangle in which the other two sides are the same and created by the balance of the difference between half the distance of the ridge board at 45° and half the distance of the ridge board at 90°.

The 7/16" square cut will not affect your hip run. This means that you can subtract the 11/i6” and 11/i6” to get an adjusted hip run of 8′-311/16". Multiplying 8′-311/16" x 1.061 (hip-val diagonal percent) gives you the ridge to ridge hip rafter length of 8′-93/4 The top cut will be a regular hip cut with convex (>) 45° cuts at a 6/12 hip-val pitch. The bottom will be a 45° cut at a 6/12 hip-val pitch with a 7/16" square cut end.

Finding the Ridge Board Length – Example 9 on Roof Example Illustration

The ridge board runs parallel with the wall at the other end of valley #5 and the hip of the 28′-0" span. That length is 12′-0", so the length of the ridge is 12’­0" with adjustments at the ends. The hip connection is a number 1, so one half of the thickness of the common rafter (3/4") is added. At the other end, it is a connection #2, and the ridge will extend to the next rafter, adding 231/4" to the length.

The ridge board length therefore is: 12′-0" + %" + 231/4" = 14′-0". Both ends will be cut at 90° with square ends.

Summary

Until you have framed many roofs, cutting rafters is always going to be a challenge. Three ways to make it easier are:

• First, use the diagonal percent to find the rafter length.

• Second, figure lengths to the framing points and then make the adjustments.

• Third, become familiar with and use a construction calculator for the math.

If ever you get stumped, you can always organize your thinking by using the four basic characteristics of cutting rafters:

1. Find the length.

2. Adjust for the top and bottom.

3. Figure the angle cuts for the top, bottom, and bird’s mouth.

There are calculators that are made specifically for assisting with construction math. These are very helpful in finding rafter lengths. Construction Master IV® is one available calculator, which we will refer to and use in this chapter to demonstrate the process of finding rafter® lengths. These calculators make it easy to do the complicated math, working in feet and inches. The sequence of buttons takes a little time to master, but once you are familiar with them, you will never go back to pencil and paper.

Learning by Example

The best way to learn cutting of rafters is to completely work through the actual process. The following example uses the diagonal percent method of finding rafter lengths. The example shows how to find the lengths of the parts of a relatively difficult roof. If you are able to work through this example and understand the processes, you should be able to figure out how to cut and stack rafters.

This example starts by organizing the whole process. Some of this information was presented earlier in the chapter, but it’s important to review for every job. We’ll start by outlining the important parts of cutting rafters.

Considerations for Cutting Rafters

When cutting rafters, you need to consider the following four factors:

1. Figuring rafter length

2. Figuring the Adjustment to rafter length at top and bottom

3. Finding the angle cuts at the top, bottom, and at the bird’s mouth

4. Finding the bird’s mouth height

Construction calculator designed to assist in construction math.

Figuring Rafter Length

Figuring Rafter Length Using Diagonal Percent was shown earlier in the chapter with the six ways to figure rafter lengths. (See "F.") When you use the diagonal percent, the most difficult part of figuring rafter length is finding the length of the run. The adjustments that need to be made at the top and bottom of the rafter should be added and subtracted from the run before the rafter length is calculated.

In finding the run, it is best to start with the full run distance from the outside of the bearing wall to the framing point of any connecting framing member. Use the framing point for consistency, and then make adjustments from there.

Figure the Adjustments to Rafter Length at Top and Bottom

Because there are so many different types of connections for rafters, it helps to establish certain standard ways to connect, and measure them in order to find the proper adjustments to length for the top and bottom. Following are some standard connections and their adjustments. They will not apply to every situation, but they will work for the most common roofs.

Adjustments for Common Rafters

1. Subtract half the thickness of the ridge board at the top.

2. At the bottom measure to the outside of the wall framing (not the sheathing).

1. Subtract half the 45° thickness of the ridge board. (See “Connection # 1 [close-up] illustration later in this chapter.)

2. At the bottom, measure to the outside corner of the two connecting walls.

Adjustments for Jack Rafters

1. Measure to the framing point where it meets the hip or valley it is connected to. (See “Adjusting the Top Length for Jack Rafters" illustration later in this chapter.)

2. Subtract half the 45° thickness of the valley rafter.

3. When the rafter rests on an exterior wall, measure to the outside of the wall framing.

Adjust Length for Top of Common Rafter

IV2" Ridge board ^ 12

6

22V2" 24

■ Span 14-0"- Adjustment:

Subtract V2 the thickness of the ridge board. This example = %"

This example:

Plumb cut height at bird’s mouth =

Run diagonal = 6ЧІ/4" x 1.118 =

Overhang diagonal = 22Уг" x 1.118 = 2′-1 J/ls"

Rafter length = Run diagonal + Overhang diagonal = 9′-10/іб"

1. Measure to the framing points where it meets the hip or valley it is connecting to. (See “Adjusting the Top Length for Jack Rafters" illustration later in this chapter.)

2. Subtract half the 45° thickness of the hip or valley rafter at each end.

Adjustments for Miscellaneous Connections Between Hips, Valleys, Ridges, and Rafters

1. Find the combination of cuts that provides the greatest number of standard cuts and still provides a sound structural connection.

2. Measure to the framing point for making adjustments.

3. Connection #2 is an example of a miscellaneous connection where a ridge board, a common rafter, a hip rafter and a valley rafter connect. (See “Connection #2" illustration later in chapter.)

Finding the Angle Cuts at the Top, Bottom, and at the Bird’s Mouth

It is easy to figure the angle cuts if you break them down into two separate angles. The first is the pitch angle, and the second is the connection angle.

The pitch angle is either a common or a hip/valley. If you use a speed square, you don’t even have to calculate it. If you are cutting a rafter that is not a hip or a valley, then use the common scale on a speed square for the pitch of your roof and draw your pitch angle line on the rafter. If you’re cutting a hip or valley rafter, then use the hip-val scale on the speed square. The pitch angle line will be your cut line for your saw cut.

The connection angle depends on a lot of factors, but 45° and 90° are the most commonly used angles. Basically, you will be setting the angle of your saw at the connection angle and cutting the cut line created by the pitch angle. For 90° corners on hips and valleys, the connection angle for jack rafters will be 45°. For standard, common rafters, the top connection angle is 90°.

Finding the Bird’s Mouth Height

The height of the bird’s mouth will affect the height of the roof and possibly the interior design of the ceiling. The most common detail for a bird’s mouth has the bird’s mouth cut starting at the inside corner of the wall.

On a hip and valley, the inside corner won’t align with the wall. Since the height of the hip and valley bird’s mouth must be the same as the common rafter bird’s mouth, you can simply measure the common rafter height and transfer it to the hip and valley bird’s mouth. The chart on page 103 shows some common bird’s mouth heights.

On hip rafters, you measure the height to the outside edge of the hip, whereas on valley rafters, it’s a little tricky. You need to measure to the center of the valley, which is slightly higher than the outside edge.

Set first jack rafter on 16” or 24" spacing with common rafters.

Measure length from common rafter to first jack rafter and then use standard jack rafter differences, as given in framing square table, to measure lengths of remaining jack rafters along the hip rafter.

12

For 17 pitch

Framing Square Segment

For every 12" of common rafter run, there is 16.97" (17" approx.) of run for hip and valley rafters. Multiply the run in feet of the common rafter by 16.97" (17" is commonly used) to get the run of the hip or valley rafter.

Adjustments for the Top and Bottom Cuts

The cut mark will be made similar to the common rafter cut mark, except that the hip-val scale on the speed square will be used instead of the common scale to mark the line to cut. (See “Rafter Cut Length" and “Angle Cuts" earlier in this chapter)

If a framing square is used, apply the same procedure shown previously, except use 17" instead of 12" along the blade of the framing square.

These procedures assume a hip or valley corner of 90 degrees.

Bottom Adjustment

Hip Rafter Length (HRL) = Hip rafter length before adjustments (HRBA) – У diagonal thickness (DT) of rafter – thickness (T) of rafter (У thickness top and bottom combined).

HRL = HRBA – У DT – T

These figures are based on a 90-degree building

corner.

Valley Rafter Length = Valley rafter length before adjustment – % diagonal thickness of rafter.

(The % thickness factors cancel each other out.)

The length of a rafter can be found by determining the horizontal length (run) that it covers, and the pitch of the roof. The constant relationship

between these factors is defined as the diagonal percent. This percent is constant for any common or jack rafter on any roof that has the same pitch. To find the length of a rafter, multiply the length of the run by the diagonal percent. For example, if you have a roof with a 6/12 pitch and a run of 6′-111/4", you multiply 6′-111/4" by 1.118 (diagonal percent for a 6/12 pitch) and find that your rafter length is

7′-91/16". With a construction calculator, enter 7′ x 1.118, and it will read 7′-91/16". The illustration below provides the diagonal percent for common pitch roofs. To figure the length of hip and valley rafters, use the hip-val diagonal percent shown on the chart.

When you use the diagonal percent, the most difficult part of figuring rafter length is finding the length of the run. The adjustments that need to be made at the top and bottom of the rafter should be added and subtracted from the run before the rafter length is calculated. In finding the run, it is best to start with the full run distance from the outside of the bearing wall to the framing point of any connecting framing member. Use the framing point for consistency, and then make adjustments from there.

The illustrations in this section show details on: Cut a pattern first and try it for fit before cutting

A Rafter cut length all the rafters. A framing square can also be used

B Bird’s-mouth to mark cut lines. (See illustration in “Framing

C Angle cuts Square Stepping Method" earlier in this chapter.)

A. Rafter Cut Length

Rafter length (See previous section.)

Length of overhang (Find in same manner as rafter.)

See cutting bird’s-mouth on next page.

Building line

Mark tail cut same as ridge cut.

Set speed square to H (which is given on plans).

H = the amount of rise per foot of run.

This speed square, as an example, is set to H = 4.

12

Shown on plans like this: 4 H Usually shown on elevation sheet above the roof.

Steps (illustrated below):

1. Mark rafter length.

2. Mark building line at rafter length for the correct pitch.

3. Mark parallel plumb line a distance equal to width of wall and toward interior of building.

4. Mark seat cut square (90°) from building line at rafter length and bottom of parallel line.

5. Cut bird’s-mouth.

A framing square can also be used to mark a bird’s-mouth. (See illustration in “Framing Square Stepping Method" earlier in this chapter.)

Ridge board ready for rafters

Height of Temporary Ridge Board Support

Ceiling joist

Temporary

sheathing

Height of temporary ridge support =

Rise + length of plumb from bird’s-mouth seat cut

– width of ceiling joist

– thickness of temporary sheathing

– ridge board width

Formula for figuring temporary ridge board support height

To start the ridge board layout, plumb a line up from a string held tight between roof layout marks on opposite walls. From that point, mark the ridge board layout to match the roof layout on the double plate.

Begin by setting the end rafters, as shown. Set the remaining rafters in the order that works best for you. As in lining a wall, attach a string in a similar manner along the edge of the ridge board. This will guide you in keeping a straight ridge board while you set the remaining rafters.

Here two pair of common rafters are fitted into place to secure the ridge board. Be sure to set end rafters first.

The length of the hip and valley rafters can be found by using any of the six common rafter methods previously described, and then making adjustments for the run and the top and bottom cuts.

This method is probably the easiest to use. To find the lengths of rafters, you make an actual size drawing of the rafter on the floor and then measure the length.

All the information you need to use this method should be on the plans. First you will need the pitch. (See Method C mentioned previously.) Second is the span, which is the distance from the support on one side of the rafter to the support on the other side. Third is the width of the rafter, the length of the roof overhang, and the size of the exterior wall.

Steps (shown in Line Chalking Sequence illustration):

1. Chalk a straight line longer than the length of your rafter, which would represent the bottom of the ceiling joist if there were a ceiling joist.

2. Chalk two lines perpendicular to the first line to represent the exterior wall.

3. From the point where your inside exterior wall line crosses the ceiling joist line, measure out and up according to your pitch xp2. For this example let your pitch equal 6г2. Therefore, for every 12” you measure out, you measure 6” up. The longer the distance out, the greater your accuracy. Make sure that the line up is exactly perpendicular or square. You can use surrounding walls that are square to measure from or measure a 3-4-5 triangle.

5. Measure the distance for the span along the ceiling joists line, then make a perpendicular line up and mark the ridge board.

6. Measure the distance of the roof overhang and draw in the fascia board.

With these lines in place, you can measure all the lengths you will need to cut your rafters.

4. Chalk a line for the thickness of the rafter.

Roof Production

Rafter Pr°fiies E. Computer Software

Method

Using the methods described on previous page to find the lengths and angles for cutting rafters is not easy, but it is at least organized—and with a calculator that works in feet and inches and that figures the diagonals automatically, the process is straightforward. However, the easiest method is to use the computer. There are software programs currently available that will do all the work for you and produce a sketch of each rafter. Solid Builder is one of these programs.

The illustrations on this page were done in Solid Builder. “Roof Production" identifies the type of roof parts, the quantity, lumber, and strength. “Rafter Profiles" illustrates the individual rafters with the balance of information you will need for cutting the rafters.

The hardest part of producing these computer­generated diagrams is learning the software and then entering the information needed for

each structure in order to generate the diagrams. However, for the architect who has already drawn up the plans, or the builder who is working with computer-generated plans, it is an easy task to produce these rafter profiles. If computer-generated rafter profiles were prepared and attached to plans, it could really make framing roofs a breeze.

When cutting rafters, you need to consider the following four factors:

1. The length: determined by two factors— distance spanned and slope.

2. The adjustment to the length at the top and bottom. The top and bottom adjustments can depend on a number of factors and are almost always a little different. The two main factors are the distance from the true ridge or framing point, and the connection with other framing members.

3. The angle of cuts at the top, bottom, and the bird’s mouth. The angle cuts relate to the pitch of the roof and the position of the framing member the rafters are attaching to.

4. The height at the bird’s mouth. The height of the bird’s mouth can be set by details on the plans, for bearing, or to keep the roof level at the plate height.

There are many different ways to cut and set rafters. It doesn’t matter which method is used as long as the completed roof is structurally sound and looks the way it was intended. Different approaches work best on different types of roofs. Sometimes a combination of methods works best.

This chapter will not discuss the specifics of all the different rafter cutting methods, but will instead describe what is possibly the easiest way for figuring the information needed to cut rafters. Using this method, you will be able to “cut and stack" a roof. That is to say, you will be able to cut all the rafters on the ground and stack them ready for installation before the first one is installed.

Step 1-Find the Lengths of Common Rafters

There are six methods for finding common rafter lengths. Study them all and use the one that works best for you.

They are:

A. The Pythagorean Theorem

B. Framing Square Rafter Method

C. Framing Square Stepping Method

D. Chalking Lines Duplication Method

E. Computer Software Method

F. Diagonal Percent Method

Methods for Finding Common Rafters

A. Pythagorean Theorem

B. Framing Square Rafter Method

E. Computer Software Method

C. Framing-Square Stepping Method

„.і.

-”’VyS’®..

Pythagoras was an ancient Greek philosopher and mathematician. His famous theorem states that the square of the hypotenuse of a right triangle is equal to the sum of the squares of the two other sides. Thus: A2 + B2 = C2 In roof framing:

A = the Rise B = the Run

C (the hypotenuse) = the Rafter Length.

Run = У2 building width – У2 ridge board width H = is given on plans = the amount of rise per foot of run

Rafter Cut Length = Rafter Length + Rafter Tail Length

First, find the run by using this formula:

Run = У building width – У ridge board width

Second, find the rise by using this formula:

Rise = H/12 x Run

Third, find the rafter length by using this formula:
Rafter Length =/(Rise x Rise) + (Run x Run)

To apply this formula, multiply Rise x Rise, and then Run x Run. Add the two products, then press the square root key on your calculator. The result is the Rafter Length.

Finding Rafter Tail (T) Length

First, find the TRun by using the following formula: TRun = Overhang – Fascia

Second, find the TRise using the following formula:

TRise = H/12 x TRun

Third, find the Rafter Tail Length by using the following formula:

Rafter Tail Length =

/(TRise x TRise) + (TRun x TRun)

Note: Be sure to mark crowns on rafters prior to measuring and cutting. Crowns are always up.

Steps

4. Find run of rafter and multiply by multiplication factor.

5. Subtract V2 thickness of ridge board to determine the rafter length.

Step 1 = 4 Step 2 = 4 Step 3 =

Step 4 =

Step 5 =

Rafter Tail Length for overhang is found using the same method as for the rafter length, except that Step 5 (subtraction for ridge board) is eliminated. The rafter tail length is then added to the rafter length to produce the rafter tail cut length.

The pitch of the roof is given on the plans in this

way: 12

6H

The number 12 is constant and indicates 12 inches of run, or horizontal distance. The other number represents the rise and varies, depending on how steep the roof is. In the example below, for every 12" of run, or horizontal distance, there is 6" of rise, or vertical distance. The greater this number, the steeper the roof.

To find the rafter length, first lay the framing square on the rafter at 12" on the blade and the

amount of rise on the tongue, 6". Once the framing square is set with stair nuts, just step off the amount of run along the rafter.

Example

Let the run equal 10′-4"

Let the overhang equal 1′-6"

1. Set framing square with stair nuts: the run 12" on the blade; the rise 6" on the tongue.

2. Step off 11′.

3. Perpendicular to the end lines, mark 4" (top), and 6" (bottom). Place the square at those marks to draw the plumb lines for the ridge and tail cuts.