Measuring roof trusses primarily involves determining their length or span, which is the crucial dimension for ordering and ensuring a proper fit for your structure. This measurement, typically taken along the top plate of the house, defines the horizontal distance the truss will cover.
Measuring Roof Truss Length (Span)
To accurately measure the length (span) of your roof trusses, you'll generally need assistance and specific equipment. The span refers to the horizontal distance the truss will bridge, typically from the outside edge of one top plate to the outside edge of the opposing top plate.
Step-by-Step Guide to Measuring Truss Span
Here's how to measure the truss size by determining its length, as well as other important considerations:
- Gather Your Equipment: You will need a long, accurate tape measure, two sturdy ladders, a notebook, and a pencil.
- Enlist a Helper: You'll typically need a buddy to help you measure truss size. Measuring long distances, especially at height, is much safer and more accurate with two people.
- Position Ladders: Place two ladders at each end of the area you'll be measuring. Ensure they are stable and secure for safe access to the top plates of your walls.
- Measure the Top Plate: Position one end of the tape measure at the outside edge of the top plate on one end of the building. While your assistant holds the other end, pull a tape measure along the top plate of the house to determine the length. This ensures you're measuring the actual bearing surface the truss will rest upon.
- Record the Measurement: Once the tape is taut and straight, read the measurement and write down the measurement in feet. Always double-check your readings to avoid costly errors.
This span measurement is fundamental for designing or ordering the correct trusses, as it dictates the primary dimension of the roof structure.
Why Accurate Truss Measurement is Crucial
Accurate measurement is not just a recommendation; it's a necessity for any construction project involving roof trusses. Incorrect measurements can lead to a cascade of problems, impacting safety, budget, and project timelines.
Key Considerations for Truss Measurement
- Safety First: Always prioritize safety when working at heights. Ensure ladders are secure, and you are using appropriate personal protective equipment (PPE).
- Double-Check All Measurements: Take each measurement at least twice, and ideally have both individuals confirm the reading. Small discrepancies can lead to big problems.
- Consider Bearing Points: The span measurement should be taken from the center of one bearing wall to the center of the opposite bearing wall, or, as often done on site, from the outside edge of the top plate to the outside edge, as this is where the truss will sit.
- Account for Overhang: While the span is the distance between bearing points, remember that trusses often have an overhang that extends beyond the exterior walls. This dimension is typically specified separately by the truss manufacturer based on your design.
- Consult Blueprints: Always cross-reference your on-site measurements with the architectural blueprints or engineering plans.
Other Important Roof Truss Dimensions
While the span is a primary measurement for "truss size," other dimensions are vital for a complete truss design and understanding.
Common Truss Dimensions Explained
Understanding these additional dimensions ensures that the trusses are designed to meet specific roof pitch requirements, aesthetic preferences, and structural loads.
| Dimension | Description | How to Measure (General) |
|---|---|---|
| Span (Length) | Horizontal distance between the outside edges of the top plates (bearing points). | Measured along the top plate from outside edge to outside edge. |
| Overall Height | Vertical distance from the bottom chord to the peak (highest point) of the truss. | Measured vertically from the lowest point of the bottom chord to the highest point of the top chord. |
| Pitch (Slope) | The angle of the roof, expressed as a ratio of "rise" (vertical) over "run" (horizontal), e.g., 6/12. | Calculated from the truss's rise and half its span, or measured with a digital angle finder. |
| Heel Height | The vertical distance from the bottom of the bottom chord to the top of the top chord at the outside wall. | Measured vertically at the end of the truss where it rests on the wall. |
| Overhang | The horizontal projection of the truss beyond the exterior bearing wall, forming eaves. | Measured horizontally from the outside edge of the bearing wall to the end of the truss. |
Accurate measurement of all relevant dimensions is critical for successful roof construction, ensuring structural integrity, proper drainage, and the desired aesthetic of your roofline.
