Using the Module
The wood roof ties calculator is capable of designing the ties and rafter for a gable roof having any combination of collar ties, rafter ties, or ridge straps. All load combinations are checked, and both uplift and downward forces are considered. Using the calculator involves three essential steps, and one optional step in which the design can be fine-tuned.
In the design of a gable roof, ties serve two purposes, as illustrated in the graphic on the right:
- Ties prevent the rafters from separating at the ridge in wind uplift loads; and
- Ties prevent the walls from being pushed outwards by the roof under downward or gravity loads.
1. Geometry Inputs

- Rafter Plan Length: The horizontal length of a single rafter. Usually, this will be half the width of the roof, from eave to ridge. This is the horizontal plan length, which will only equal the total rafter length if the roof pitch is zero (flat).
- Roof Pitch: The rise over run pitch of the roof, given in the form of βx:12β.
- Ridge Strap Present?: A ridge strap is a metal strap which runs over the top of the roof and connects rafters on either side of the rafter. Note that this module does NOT design the ridge strap itself; it is simply assumed that the ridge strap is of sufficient strength to keep the rafters connected at the ridge under any uplift conditions.
- Number of Ties Present: This refers to collar ties or rafter ties. If both are present, then select βTwoβ, otherwise select βOneβ.
- Depth of First/Second Tie: The depth of the collar and/or rafter tie from the roof ridge to the tie. Note that, if there are two ties, it does not matter which tie is the βfirstβ or βsecondβ, as long as it remains consistent throughout
- Eaves & Rafter Supports: The length of the eave, equal to the distance between the edge of the rafter and the wall, as well as the bearing length of the wall, are set in one table. For the purpose of bearing calculations, the supports are assumed to be eclusively on the rafter, not any of the ties.
2. Enter Loads

3. Member Selection

Worked Examples
Two worked examples are provided here: one of gravity forces only, and one of a wind uplift scenario. Both are simplified examples, but should be illustrative of the usage of this module.Gravity Loading Only Example

- Total roof width = 17 ft, with no eaves
- Total roof height = 6 ft
- Tie depth = 3 ft (mid-height)
- Dead load = 20 psf on a horizontal plan area, including self-weight (equals 16.3 psf on a sloped area)
- Snow load = 35 psf on a horizontal plan area (equals 28.6 psf on a sloped area)
- Tributary width = 16 in


Wind Uplift Example

- Rafter plan length = 11.5 ft, with 1.5 ft eaves
- Roof pitch = 10 : 12
- Tie depth = 2 ft
- Dead load = 10 psf on a sloped plane, including self-weight,
- Wind uplift load = 40 psf
- Tributary width = 16 in

Theory Guide
The module consists of two major portions: analysis and design.Design
The design of the rafter is exactly the same as that used in our regular Wood Beam calculators; details on this methodology are described in detail in the Wood Beam article, and will not be repeated here. For ties, because the module does not allow load to be applied directly to the ties, the ties are definitionally in pure tension (ignoring self-weight, which is negligible for the purpose of design). Tie design therefore adjusts the base tension strength with the following adjustment factors:- Duration Factor (for ASD), or Time Factor and Format Conversion Factor (for LRFD)
- Wet Service Factor
- Incising Factor (for dimensioned lumber and timber only)
- Size Factor (for dimensioned lumber and timber only)
Analysis
There are two methods that could be used for analyzing a tied roof: either a full truss analysis, or an advanced beam analysis with oriented roller supports. While a full truss analysis would be able to handle more loading scenarios, it also introduces an unnecessary level of complexity, and this module therefore uses the latter method of an advanced beam analysis. That beam analysis looks at a single rafter in isolation (assuming that the left and right rafters are identical), with two possible support conditions depending upon whether the total loads result in a net uplift or net downward force:Net Gravity / Downward Force

Net Uplift Force


A Few More Notes
- Of course, ties donβt magically appear or disappear depending upon whether the net force is upward or downward. If there are two ties present, then two ties will be present in both uplift and and downward configurations, and if only one tie is present, then that tie going to resist loads regardless of the direction of net force, even if itβs inefficient in one of the directions.
- Internally, both uplift and downward configurations are always run; two FEA models will always exist in the background. In compiling the analysis results, the module will check whether the total force on the rafter is upwards (negative total load) or downwards (positive total load), and pull results from the correct model accordingly. If there is no load combination in which a net uplift occurs, then no results from the uplift model will be included in the final demands.
- A keen observer may note here that it is possible to have an upwards movement of the ridge if an applied moment load is applied, or if a particularly high downwards load is applied on the eave. It is for this reason that, unlike other ClearCalcs modules, this module forbids any applied moment loads from being added. It is further listed as an assumption that eave loads are not sufficient to cause uplift at the ridge, though it is left to the user to verify this very rare possibility does not occur.