Joist Span Finder & More

Joist Span Finder & More is an iOS app for quickly finding the allowable spans for:
  • Roof rafters
  • Ceiling joists
  • Floor joists
and for sizing of:
  • Wood studs
  • Conventional concrete foundations
for buildings conforming to the conventional light-frame wood construction of Section 2308 of 2009 International Building Code (IBC).

Demo video on YouTube (

For better quality video demo, please visit: (runs on Mac or PC only because of the Adobe Flash issue).

Screen Shots: 

 Main Screen  Roof Rafter Sizing  Ceiling Joist Sizing
 Wood Stud Sizing  Footing Sizing  

Instructions for using "Wood Joist Span Finder and More":

1. Tap one of the blue arrow-buttons to size (calculate) the particular member type. Buttons for sizing the following members of conventional light-frame wood construction are available:
    • Roof rafters
    • Ceiling joists
    • Floor joists
    • Wall studs, and 
    • Concrete Footings 

A blue sub-view will appear next to the button, which can be dismissed by tapping 'Done' button at the bottom . All input data shown in green text can be changed by tapping on it. Tapping the green text will display a set of choices (input data selection) relevant to the particular data item. The choices are explanatory. 

No typing is required except for the member IDs, which can be safely left blank or ignored unless the particular calculation needs to be identified. [Tip: Blank member ID is suggested for quick use of the app.]

2. The set of information currently displayed in a blue sub-view (called 'calculation') can be saved, and retrieved for later use; or can be emailed. Tap 'Retrieve' button to see the previously saved calculations, if any. All saved calculations will exist (persist) even if the device is turned off, unless the device memory is cleared. 

3. To dismiss/hide the current sub-view, tap the 'Done' button. The app retains the current contents of the individual sub-views that existed just before the sub-views are dismissed, even if the current contents were not specifically saved. 

[Tip: Before a blue sub-view is dismissed, tapping another dim blue arrow-button anytime (if visible in the background) will hide the current sub-view and display the sub-view specific to that arrow-button. This saves one step. Note: Screen can be scrolled if needed to see other visible dim buttons.]

4. Please read the End User License Agreement (EULA) by tapping the EULA button of the main view before agreeing to use this app.

Important Notes, Assumptions and Limitations:

(i) The results produced by this app are intended to match the joist and rafter span tables, stud table and footing table of Chapters 23 and Chapter 18 of the 2006 or 2009 edition of International Building Code (2006/2009 IBC). International Residential Code (IRC) is smiler. 

[Note: The span tables of 2006/2009 IBC are still based on the allowable design stresses specified in 2001 NDS (not 2005 NDS which is adopted by 2006/2009 IBC). Since some of the allowable stresses of 2001 NDS are slightly less than those of 2005 NDS, particularly for Douglas-Fir-Larch species, the 2006/2009 IBC span tables produce slightly conservative (smaller) spans for some species and grades. To match the span tables of 2006/2009 IBC, which are widely used by construction professionals, this app also uses the 2001 NDS design stresses in computing the allowable spans. If you wish to obtain an app that uses 2005 NDS allowable stresses, please contact this developer for availability.]

(ii) In order to keep this app simple and easy to use, all choices (input selection) available for calculating allowable spans, stud sizes, and footing sizes are limited to those found in the above-mentioned tables of 2006/2009 IBC (except that 2x4s and 2x12s are available for floor joists and ceiling joists respectively which the IBC span tables lack). 

All loads on the rafters and joists are uniform; all spans are simple. All rafters and joists are assumed to be adequately restrained/braced along their full lengths including at supports to prevent lateral-torsional instability, and are repetitive as defined by NDS; no notching or end tapering is assumed. Shear stress factor (CH) of 1.5 is assumed. 

Maximum shear demand is calculated at 'd' distance away from the support center. Limitations mentioned in the above-mentioned tables and the relevant code sections of IBC or the governing building codes shall be considered by the user.

(iii) This app incorporates the following NDS Adjustment Factors in calculating allowable spans for rafters and joists: Load Duration Factors (CD), Size Factors (CF), and Repetitive Member Factors (Cr), which are also the only NDS Adjustment Factors incorporated in the corresponding IBC span tables. Additional NDS Adjustment Factors have not been incorporated, i.e. their default values of 1.0 are used. 

User shall comply with the additional design and construction requirements of NDS and additional requirements of the governing building codes.

(iv) While the allowable spans for rafters and joists are calculated by this app using the principles of structural mechanics and the code provisions, the stud and footing sizes are simply picked from the corresponding IBC tables since these are prescriptive provisions. 

To use the rafter and joist span tables, stud table and footing tables (and hence to use this app), the building must conform to the limitations specified in IBC Section 2308 for conventional light-frame construction, including Section 2308.2, and the soils/foundation related requirements of Section 1805.4.

(v) This app is intended for use by competent construction professionals. Although rigorous testing have been performed to compare the results from this app with the 2006/2009 IBC span tables, stud table and footing table which are found to be in agreement, the results are not guaranteed to be error-free. User must use his or her professional judgment in reviewing the results before accepting them for use in actual construction. A handful of minor discrepancies because of rounding of the computed span values of the order of 1/2" have been detected, but are considered acceptable. Please notify this developer if any significant discrepancy or error is found. 

(vi) Please read the "End User License Agreement (EULA)" for additional limitations.     

Hints about selecting dead and live load magnitudes, and deflection limits:

The following hints apply to conventional light-frame wood construction; and are for general guidance only. As always, professional judgement and code provisions must be followed. Often times, sum of dead and live (or snow) loads can be looked at for appropriateness if the desired individual dead, live (or snow) loads are not available in this app -- again, professional judgement is required. 

Minimum live loads must be according to Table 1607.1 of IBC. Dead loads depend on actual roof, ceiling and floor assemblies. [Tip: If you are a specifier and desire stiffer joists and rafters, select heavier loads than the minimum loads.]

(a) Roofs: 10 psf dead load is common for sloping rafters with composition shingles without ceiling finish directed attached to the rafters, including allowance for one layer of future re-roofing (say 2.5 psf). Select 20 psf dead load if heavier, such as for vaulted ceilings where ceiling finish is directly attached to rafters, or for ordinary flat roofs. 

Minimum roof live load of 20 psf is required by IBC for typical roofs. For snow load, contact your local building department since snow load varies from place to place.

(b) Ceilings: 5 psf dead load is appropriate for typical light ceiling, say with one layer of gypsum board plus typical batt insulation. Select 10 psf if the construction is somewhat heavier. For ceiling live loads, select 10 psf for uninhabitable attics without storage, 20 psf for uninhabitable attics with limited storage (see IBC Table 1607.1).

(c) Floors: 10 psf dead load is appropriate for most regular floors, including hardwood floors. Select 20 psf if somewhat heavier floor finish or soffit finish are anticipated. For floor live loads, IBC requires minimum 40 psf for ordinary floors of residential buildings. Habitable attics and sleeping areas of residential buildings can be 30 psf. 

© CrispiApps 2012