A Shed Roof Rafter Span Calculator helps you determine the maximum span a rafter can safely cover without needing additional support, based on factors like the type of lumber, the size of the rafter, the roof load (including snow, wind, and roofing materials), and the spacing between rafters. This is crucial for ensuring the structural integrity of your shed roof.
How to Use a Shed Roof Rafter Span Calculator
- Determine the Roof Load:
- Dead Load: This is the weight of the roof materials (like shingles, plywood, insulation, etc.). For most sheds, a dead load of 10 to 15 pounds per square foot (psf) is common.
- Live Load: This includes temporary loads such as snow or people walking on the roof. For areas with heavy snow, a live load of 20 to 50 psf may be required.
- Wind Load: In areas prone to high winds, consider wind loads that can affect rafter sizing.
- Select the Lumber Type and Grade:
- Different types of wood have different strengths. Common choices include Douglas Fir, Southern Yellow Pine, and Spruce-Pine-Fir. The grade of lumber (like No. 1, No. 2, etc.) also affects its strength.
- Choose Rafter Size and Spacing:
- Common rafter sizes are 2×4, 2×6, 2×8, etc. The spacing between rafters is typically 12, 16, or 24 inches on center (OC).
- Input Roof Pitch:
- The pitch of the roof (expressed as a ratio, e.g., 4:12, 6:12) affects the length of the rafter and the loads it must bear.
- Calculate the Maximum Span:
- Input the above details into the rafter span calculator to determine the maximum allowable span for the selected rafter size and spacing.
Basic Span Calculation Formula
The formula for calculating the rafter span is based on building codes and engineering principles. The span (S) is influenced by the maximum allowable deflection, the modulus of elasticity (E) of the wood, and the moment of inertia (I) of the rafter.
For a simple shed roof, the maximum span (S) can be approximated by:
Span (S)=Rafter Length/cos(Pitch Angle)
Where:
- Rafter Length is the length from the top of the wall to the roof peak.
- Pitch Angle is the angle of the roof slope.
However, using an online calculator or local building code tables is much more accurate, as they consider all variables.
Online Span Calculators
There are several online tools available for calculating rafter spans:
- American Wood Council’s Span Calculator – This calculator considers different wood species, grades, and load conditions.
- MyCarpentry’s Rafter Span Calculator – Offers a simpler interface, ideal for basic shed roof calculations.
- Wood Beam Calculator by The Engineering ToolBox – For more detailed engineering calculations.
A useful shed roof rafter calculator to calculate the rafter length for the given dimensions of the roof of the building or a structure. rafter is the beam supporting the building.. Quick look-up shed size & roof pitch. for exact rafter length please use punching calculator. Calculate the length of lean to shed roof rafters for any common roof pitch when the top of the rafters stop into a wall. it includes the rise and works in feet and inches or metric values..
Rafter length calculator . use the calculator below to determine rafter length.. Rafter spans for southern yellow pine, douglas fir, and eastern spruce. rafter span tables – use these tables to determine lengths, sizes and spacing of rafters based on a variety of factors such as, species, load, grade, spacing and pitch.. Douglas fir maximum horizontal roof rafter span for lumber grade selected structural and no. 1 and no. 2, are indicated below. max. dead load (weight of structure and fixed loads) 15 lbs/ft 2..
