Rafter Tie Calculator

Allows calculation of thrust at rafter ties if desire is to increase rafter tie height beyond code prescriptive bottom third.

Main Engineering Principle: Horizontal Thrust Resolution in Pitched Roofs

This calculator addresses a fundamental structural engineering concept: when a pitched roof carries a vertical load, it creates horizontal thrust forces that must be contained.

The Core Principle in Simple Terms:

A pitched roof creates two types of forces:

1. Vertical forces from gravity loads (dead weight and live loads like snow)
2. Horizontal thrust forces that push outward at the walls

The rafter tie is a critical structural member that resists these horizontal forces through tension. Without sufficient ties, the walls would be pushed outward, potentially causing structural failure.

The calculator specifically determines:

• The total roof load based on dead and live loads
• How this load translates to horizontal thrust forces
• The tension force in the rafter tie needed to resist this thrust
• The required connection strength (number of nails) to transfer these forces

The key formula shown in your spreadsheet calculates the tie force (T) using geometric relationships between the roof dimensions and the applied loads. The formula accounts for:

• The position of the tie relative to the ridge (h)
• The roof pitch (rise/run ratio)
• The total load on the rafter (WR)

This is essentially an application of structural statics - analyzing a triangular structure to ensure equilibrium of forces.

The calculation also demonstrates practical construction requirements by converting the theoretical force to an actual number of nails needed for the connection.