Snow load is often the governing structural load for post-frame buildings in northern climates. A 40-by-72-foot pole barn at 40 psf ground snow load carries roughly 57,600 pounds on its roof — nearly 29 tons.
Ground snow load values from ASCE 7 represent the 50-year return period. Over a 30-year building life, there is a 45% probability of experiencing at least one event that reaches the design load. Designing for less means accepting a significant probability of structural failure.
Ground Snow Load and ASCE 7 Requirements
ASCE 7-22 provides ground snow load maps and tables. Values range from 0 psf in southern states to over 100 psf in mountain regions. Local jurisdictions may adopt higher values based on historical data.
Case study regions (Rocky Mountains, Sierras, Cascades) require site-specific studies by a structural engineer. Do not assume a value in these areas.
Common agricultural region values: central Iowa 35 psf, southern Minnesota 50 psf, central Wisconsin 40 psf, northern Michigan 60 psf, western New York 50 psf. Always use the locally adopted value.
1. Check with county building department
2. ASCE 7-22 Table 7.2-1 and Figure 7.2-1
3. State extension snow load maps
4. Local lumber yards often know the value
Never use less than the locally adopted value.
Pole Barn Snow Load Calculator
Calculate roof snow loads for pole barns, post-frame buildings, and metal buildings using ASCE 7 methods. Accounts for ground snow load, exposure, thermal factor, roof slope reduction, and leeward drift loads.
Converting Ground Snow to Roof Snow Load
Pf = 0.7 × Ce × Ct × Is × Pg. Ce: 0.8 (windswept), 1.0 (partial), 1.2 (sheltered). Ct: 1.0 (heated), 1.1 (unheated), 1.2 (cold-ventilated). Is: 1.0 (standard), 1.1 (essential).
For a typical unheated pole barn, partially exposed: Pf = 0.7 × 1.0 × 1.1 × 1.0 × 40 = 30.8 psf. Sheltered barns use Ce = 1.2, increasing the load by 20%.
Most pole barn roof slopes (3:12 to 6:12) do not receive significant slope reduction. Steeper roofs (8:12+) start shedding snow and get modest reduction.
Pf = 0.7 × Ce × Ct × Is × Pg
Example (unheated, partial exposure, 40 psf):
Pf = 0.7 × 1.0 × 1.1 × 1.0 × 40 = 30.8 psf
Ce: 0.8 windswept / 1.0 partial / 1.2 sheltered
Ct: 1.0 heated / 1.1 unheated / 1.2 cold-ventilated
Snow Drift Loads
Drifts form on the leeward side of ridges, against taller walls, and in valleys between roof sections. They are triangular surcharges added to balanced snow load.
A 72-foot barn at 40 psf ground snow has a leeward drift surcharge of roughly 14 to 18 psf at the ridge, tapering over 10 to 15 feet. Combined load at the ridge can reach 45 to 50 psf versus 30.8 psf balanced.
Trusses near the ridge and at drift zones must be designed for the higher combined load or spaced more closely.
How Snow Load Affects Truss and Post Sizing
A 2x6 purlin spanning 8 feet at 30.8 psf is near its allowable limit. At 45 psf in a drift zone, it is overstressed. Solutions: reduce truss spacing to 6 feet, upgrade to 2x8 purlins, or add bracing.
Truss design is handled by manufacturers using proprietary software. Your responsibility is to specify the correct loads. Providing 30 psf when your jurisdiction requires 40 psf results in trusses under-designed by 33%.
Posts act as columns resisting combined axial and lateral loads. A typical 6x6 post at 8-foot spacing carries 12,000 to 16,000 pounds at full snow load. Undersized posts or weak connections are the most common failure mode.
1. Using ground snow load directly as roof snow load
2. Ignoring thermal factor for unheated buildings (Ct = 1.1)
3. Not accounting for drift loads near the ridge
4. Using outdated ground snow load values
5. Spacing purlins based on balanced load only