10/16/2025
The image is an educational diagram that explains the terminology and calculation of a roof's pitch and the components of a rafter. It shows two scenarios for roof framing: one without a central ridge board and one with a ridge board.
Common Terminology and Dimensions:
The diagram labels the key components of a rafter and its relationship to the roof's geometry:
* Rafter Length: The total length of the rafter from the ridge to the exterior wall.
* Run: The horizontal distance from the outer face of the wall to the center point of the roof (where the rafter meets the ridge).
* Rise: The vertical distance from the top of the wall to the peak of the roof.
* Tail or Eave: The part of the rafter that extends beyond the exterior wall, forming the roof overhang.
* Tail Run: The horizontal distance of the overhang.
* Plumb Cut: The vertical cut at the top of the rafter that allows it to fit snugly against the adjacent rafter or the ridge board.
* 90°: This indicates the square cut at the bottom of the rafter where it rests on the wall plate.
Scenario A: Without Ridge (Top Diagram):
* This diagram shows a roof frame where two opposing rafters meet directly at the peak.
* They are connected directly to each other, forming a triangular structure without a central ridge board. This method is often used for simple sheds or outbuildings.
Scenario B: With Ridge (Bottom Diagram):
* This diagram shows the more common and structurally sound method for building a roof.
* The rafters do not meet each other directly. Instead, they are attached to a vertical board at the peak, which is called the ridge board (though it is not explicitly labeled, its function is clear from the connection). The plumb cut of the rafter butts against this ridge board.
* This method provides a continuous, straight ridge line and strengthens the overall roof structure.
Roof Pitch Formula:
At the bottom of the image, a formula is provided to calculate the roof pitch:
* INCH RISE PER FOOT RUN = (RISE x 12) / RUN
This is the standard method for expressing roof pitch in the United States and Canada. It is a ratio that describes how many inches the roof rises for every 12 inches (one foot) it runs horizontally. For example, a "6/12 pitch" means the roof rises 6 inches for every 12 inches of horizontal run.
Summary:
The image is a concise and informative guide to basic roof framing. It clearly defines the essential terms (run, rise, rafter length) and visually compares two common framing techniques (with and without a ridge board). Most importantly, it provides the universal formula for calculating roof pitch, making it a valuable tool for anyone in construction, architecture, or DIY home improvement.
