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NDS 2024ASD

Wood Roof Tie (ASD, NDS 2018)

Rafter loads link from upstream calculations directly - change span or loading and both the tie tension and all rafter checks update automatically. Designs wood collar ties and rafter ties to NDS 2018 ASD, checking tie tension (ft / F't ≤ 1.0) and all NDS limit states for the rafter.

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What it calculates

Rafter loads link from upstream calculations directly - change span or loading and both the tie tension and all rafter checks update automatically. Designs wood collar ties and rafter ties to NDS 2018 ASD with tension checks for the tie and all NDS limit states for the rafters.

Code standards

  • NDS 2018 (ASD)

How it calculates

Rafter geometry and horizontal thrust

The calculator resolves the pitched rafter geometry from the horizontal span and roof pitch. For a rafter spanning from ridge to wall plate, the horizontal thrust at the top plate that the tie must resist is derived from the tributary load and pitch angle:

H = (w × L / 2) / tan(θ)

where w is the total ASD load per unit length of the rafter under the governing NDS load combination, L is the horizontal rafter span, and θ is the pitch angle. For a collar tie at height y above the wall plate, the tie tension is amplified by the ratio of the full rafter height h to the tie height y:

T = H × (h / y)

Tie tension check

The ASD allowable tension stress F't is derived from the reference design value F_t adjusted for all applicable NDS modification factors:

F't = F_t × C_D × C_M × C_t × C_F

The calculated tension stress ft = T / A_n (net area at connections) must satisfy ft / F't ≤ 1.0. The load duration factor C_D applies based on the governing load type - 1.0 for floor live load, 1.15 for roof live or snow, 1.25 for wind.

Rafter bending and shear checks

The rafter is modeled as a simply supported inclined member. The governing bending moment at mid-span and critical shear at the face of the support are computed for the applied load combination. Design values for bending (F'b) and shear (F'v) include the full set of NDS adjustment factors:

  • C_D (load duration) - governing load type in the combination
  • C_M (wet service) - applied when in-service moisture content exceeds 19%
  • C_t (temperature) - for sustained temperatures above 100°F
  • C_F (size factor) - section-dependent adjustment for sawn lumber
  • C_r (repetitive member) - 1.15 increase for rafters spaced ≤ 24 in where three or more members share load
  • C_L (beam stability) - computed from the slenderness ratio R_B based on the lateral bracing condition

Utilization ratios fb / F'b and fv / F'v must each be ≤ 1.0.

Deflection check

Mid-span deflection under the service load combination is computed from the inclined rafter span. Checks are performed against:

  • Live load (or snow) deflection: Typically L/240, configurable
  • Total load deflection: Typically L/180

Long-term deflection for the sustained dead load component is multiplied by the NDS creep factor (1.5 for dry conditions, 2.0 for wet) and added to the short-term total load deflection.

Adjustment factors and load combinations

The NDS 2018 adjustment factors applied throughout follow the same hierarchy as the wood beam calculators: C_D is governed by the shortest-duration load type in the combination. For combinations mixing dead load and snow (or roof live), C_D is 1.15. Adding wind to the combination raises C_D to 1.25 for that combination only. The calculator evaluates all user-specified load combinations and reports the governing utilization for each limit state.

Related calculators

Wood Beam (ASD, NDS 2018)→Wood Column (ASD, NDS 2018)→Wood Connectors (ASD)→Wood Member (Design Only - ASD)→

Support articles

Wood Roof Ties to NDS 2018 - Usage & Theory Guide→
Browse all docs →

Frequently asked questions

What does this calculator design?
Wood collar ties and rafter ties per NDS 2018 ASD. Collar ties restrain ridge spread near the upper third of the rafter; rafter ties at ceiling level resist the horizontal outward thrust that pitched rafters generate at the top plate. The calculator checks both tie tension and all rafter limit states.
What are the key inputs?
Roof pitch, rafter span (horizontal run), rafter spacing, tie height (collar) or position (rafter), wood species and grade, section sizes for the tie and rafter, and applied loads (dead, live, snow).
What does it check and output?
Tension in the tie member (ft / F't ≤ 1.0), rafter bending (fb / F'b ≤ 1.0), rafter shear (fv / F'v ≤ 1.0), and rafter deflection. Outputs include horizontal thrust at the wall, tie tension force, and utilization ratios for each limit state.
How is this different from a plain wood beam check?
A plain beam check does not account for the geometry of pitched rafters or the horizontal thrust they generate. This calculator derives the tie tension from the rafter's horizontal thrust component - the critical force that must be transferred through the tie to prevent the walls from spreading.
When do rafters need a tie, and what type?
Rafters without a ridge beam or structural ridge need a tie to resist outward thrust. Rafter ties at ceiling level provide the most effective restraint. Collar ties positioned in the upper third of the rafter reduce spread but are less efficient. IRC Section R802 prescribes minimum tie requirements; this calculator lets you design and verify the tie member.
Does this calculator support load linking with connected calculations?
Yes. Dead and live loads applied to the rafters can be linked from upstream calculations in the same project - gravity loads from a load analysis update the rafter and tie design automatically. Rafter reactions at the wall plate link downstream to stud, column, or wall calculations, keeping the full vertical load path connected without manual re-entry.

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