Building with Common Rafters for Strong Roofs

Common rafters are a crucial part of any roof, providing the structural support that keeps your home safe and dry. They are typically spaced 16 or 24 inches on center, depending on the design and local building codes.

The width of common rafters is usually between 2x6 and 2x12 inches, with the choice of size depending on the weight of the roof and the type of roofing material used. A wider rafter can support more weight, but may also require more expensive materials and labor.

In addition to their structural role, common rafters also provide a nailing surface for the roof sheathing, which helps to distribute the weight of the roof evenly. This is especially important for roofs with heavy roofing materials like clay tiles or slate.

To calculate the length of a common rafter, you'll need to know the building width, ridge width, overhang, and roof pitch.

The building width is the distance from the center to the outside of the wall, which is half the total width of the building. For example, if the total width is 30 feet, the building width would be 15 feet.

The ridge width is the thickness of the ridge beam, usually 1-1/2 inches.

The overhang is the distance from the wall to the end of the rafter, including the subfascia. This can vary, but in one example, it's 8 inches.

The roof pitch is the ratio of the rise to the run, usually expressed as a fraction with 12 as the denominator. For example, a 6/12 roof rises 6 inches for every 12 inches of run.

To calculate the run, you'll need to subtract half the ridge width from the building width and include the exterior plywood wall sheathing in the dimensions.

Here are the steps to calculate the run:

1. Mark the center line and the edges of the ridge.

2. Measure from the outside of the building to the inside of the ridge to get the run.

3. Make sure to include the exterior plywood wall sheathing in the dimensions.

For example, if the building width is 30 feet and the ridge width is 1-1/2 inches, the run would be half the building width minus half the ridge width, which is 14 feet, 11-3/16 inches in one example.

Once you have the run, you can use a roofing ready reckoner or a construction calculator to find the rafter length. A ready reckoner is a book that contains tables for working out rafter lengths quickly and easily. Each page has a different table depending on the pitch of the roof.

Here's an example of how to use a ready reckoner:

For example, if the total span is 3.265m, the ridge thickness is 45mm, and the pitch is 30°, you would:

Alternatively, you can use a construction calculator to find the rafter length. These calculators can quickly figure out the rest of the right triangle, including the rise, run, and diagonal measurements.

For example, if you know the run and the pitch, you can enter the values into the calculator to find the rafter length and other measurements.

Here's a summary of the steps to calculate the rafter length:

1. Determine the building width, ridge width, overhang, and roof pitch.

2. Calculate the run by subtracting half the ridge width from the building width and including the exterior plywood wall sheathing.

3. Use a roofing ready reckoner or a construction calculator to find the rafter length.

By following these steps, you can accurately calculate the length of a common rafter and ensure a successful roofing project.

A scientific calculator and trigonometry can be used to work out rafter lengths, but it's complicated unless you're a math whiz.

For larger roofs, a roofing ready reckoner is a must-have tool. This book contains tables for working out rafter lengths quickly and easily, with each page having a different table depending on the pitch of your roof.

You can also use a ready reckoner like Goss's Roofing Ready Reckoner, which costs around £20 and has paid for itself a hundred times over. It takes all the complicated trigonometry calculations out of the way and provides loads more information, like diminishing jack measurements and hip rafter lengths.

To use a ready reckoner, you'll need an adjustable Stanley quick square to take precise measurements. This will save you a ton of time and trips up and down the ladder.

Here's a list of tools you might need to get started:

To design a common rafter, you need to perform a structural analysis to determine the internal forces and moments it will experience under the applied loads. This involves calculating the bending moments, shear forces, and axial forces along the length of the rafter.

The span, roof slope, and additional design constraints must be considered in this analysis. The deflection limits for the rafter must also be evaluated to ensure it meets the required serviceability criteria.

To determine the required spacing and size of rafters, the anticipated roof loads, including dead loads and live loads, must be calculated. This includes snow loads, wind loads, and any other applicable loads.

Connection Design is a crucial aspect of roof construction. It's essential to design the connections between the rafter and other structural elements, such as the ridge beam, ceiling joists, and supporting walls.

To ensure efficient force transfer and overall stability, connections should be designed to withstand various external forces, including wind and snow loads. The building code's guidelines for fastening and bracing requirements should be followed to prevent structural instability and damage.

Proper connection design involves considering the span of the rafters and selecting an appropriate material based on the desired strength, load-carrying capacity, and structural requirements. For example, the required spacing and size of rafters should be determined based on the anticipated roof loads, including dead loads and live loads.

Connections should be designed to transfer forces efficiently, maintaining the overall stability of the roof system. This includes tie-down connections for uplift forces. Don't forget to incorporate appropriate connections, such as hangers or brackets, to ensure proper load transfer between the rafters and other structural elements.

Here are some key considerations for connection design:

Load Combinations are a crucial step in designing rafters that can withstand various external forces. Load Combinations consider different load scenarios to ensure the rafter is designed to withstand the most critical conditions.

To determine Load Combinations, you need to combine the different loads, including dead loads, live loads, snow loads, and any other applicable loads, such as wind loads. Dead loads are the weight of the roof materials, while live loads can include anything from furniture to people.

The most critical Load Combinations can be obtained by consulting local building codes or standards. These codes and standards provide specific design loads for your location, taking into account factors like climate and geographical conditions.

Performing a structural analysis of the rafter is crucial to determine the internal forces and moments it will experience under the applied loads. This involves calculating the bending moments, shear forces, and axial forces along the length of the rafter.

Consider the span, roof slope, and any additional design constraints when performing the analysis. The span of the rafter, roof slope, and design constraints all play a significant role in determining the forces it will experience.

The analysis will help determine the loads acting on the structure, which is essential for ensuring the structural integrity, safety, and performance of the design. Engineers will use this information to assess the internal forces and moments, and make necessary adjustments to the design.

The structural analysis will also help identify any potential issues with the rafter, such as excessive deflection or stress. By addressing these issues early on, engineers can ensure that the final design meets the required serviceability criteria.

To perform the analysis, engineers will need to consider both immediate and long-term effects, including creep and shrinkage of wood. This will help ensure that the rafter can withstand the anticipated loads over time.

Structural Design Considerations are crucial when it comes to ensuring the stability and safety of a roof. The required spacing and size of rafters depend on the anticipated roof loads, including dead loads and live loads.

To determine the rafter spacing, you need to consider the span of the rafters and the material used. Wood, steel, and cold-formed steel are common materials used for rafters, each with its own strengths and load-carrying capacities.

Incorporating appropriate connections, such as hangers or brackets, is essential to ensure proper load transfer between the rafters and other structural elements. This includes tie-down connections for uplift forces.

Follow the building code's guidelines for fastening and bracing requirements to ensure structural stability and resistance to lateral forces. This includes considering factors such as wind loads and snow loads.

Here are some key considerations for rafter design: