# How do you calculate rafter length?

By AerialTakeoff

A **rafter** is a structural component that is used as part of a roof construction. Typically, it runs from the ridge or hip of the roof to the wall plate of the external wall. **Rafters** are generally laid in series, side by side, providing a base to support roof decks, roof coverings and so on.

Just multiply the rise or run by the appropriate **rafter length** factor corresponding to your roof pitch. For example, if your roof pitch is 30ﹾ and your run is 2.4m then reading from the table the **rafter length** factor is 1.15. Multiply this factor by your run which gives your **rafter length**.

#### How do you calculate rafter length on a calculator?

If you are

**calculating rafter length**from the center of the ridge beam (as shown below), you will need to subtract half of the thickness of the ridge beam from the top end of the**rafter**. For example, if the ridge beam is 1.5 inches thick, subtract 3/4 of an inch from the overall**rafter length**.#### How far will a 2×8 Rafter span?

A

**2×8**up to 12 feet; 2×10 to 15 feet and 2×12 to 18 feet. The larger the deck, the larger the**joists**. Furthermore,**how far can**a 8×12 beam**span**?#### How is pitch calculated?

The angle, or

**pitch**, of a roof is calculated by the number of inches it rises vertically for every 12 inches it extends horizontally. For example, a roof that rises 6 inches for every 12 inches of horizontal run has a 6-in-12**pitch**.#### What is the length of a common rafter?

EXAMPLE: Using the 7 Inch Rise table (and our 15′ run), the

**common rafter length**is 17′ 43⁄8″. This is the**rafter length**from the top cut to the seat plumb mark.#### What is a rafter run?

The

**length**of the**rafter**is given from the long point of the ridge cut at the top down along the top edge the board to the same plumb cut on the**rafter’s**notch. The next result is the**run**. This is the horizontal measurement from the outside of the wall to the inside of the ridge.#### What angle do you cut roof rafters?

The ridge end of the

**rafter**, and often the eve end, will need to be**cut**to the**angle**of the**roof**slope. The**roof**on this shed is a 4 in 12 slope which makes it a 18 and one half degree**angle**on the board ends. A**rafter**or speed square will have marks for both of these**angles**.