Sizing Roof Rafters Sizing Roof Rafters

Roof rafters are also called joists. These are what supports the roofing underlayment (usually plywood), the moisture barrier (tar or pitch-impregnated felt or paper) and the actual roofing material being used. Below you will find a short description of how to determine how long your individual joists should be, from centerline ridge to edge of the roof eave.

Determine Pitch or Slope

The first step you need to take is to determine how much slope you want or need on your roof. If you live in an area that receives large amounts of rain and snow, you’re going to want more slope on the roof in order to help limit the amount of buildup on the roof during winter.

Determine Lateral Length

After determining the required pitch, you need to measure to find out the overall width or length of your house. The is measured from wall to wall, with approximately 18 inches added on either side for the eave overhang if so desired.

 Determine Roof Height

Once you know your slope and length, you can determine your overall roof height from the top plate (top of the wall) to the top of the roof. Use a balanced equation for this calculation.

Determine Rafter Length

Once you know the height of the roof and the length of the truss base, you can use Pythagorean Theorem to determine rafter length. Your roof truss is two right triangles back to back. The rafter itself is the hypotenuse or C. The base is A, and the center piece is B.

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