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ASCE 7-22ASCE 7-16

Snow Loads (ASCE 7-16)

US structural engineers calculating roof snow loads to ASCE 7-16 for framing design. Covers ground-to-roof conversion, balanced and unbalanced loads, drift loads on lower roofs, and rain-on-snow surcharge flagging in a single calculation.

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What it calculates

Get balanced and unbalanced snow loads for sloped and flat roofs to ASCE 7-16 Chapter 7. Inputs cover ground snow load, surface roughness, importance, exposure, pitch, and thermal condition. Drift loads on lower roofs and projections are returned automatically.

Code standards

  • ASCE 7-16, Chapter 7

Who uses this calculator

US structural engineers calculating roof snow loads to ASCE 7-16 for framing design. Covers ground-to-roof conversion, balanced and unbalanced loads, drift loads on lower roofs, and rain-on-snow surcharge flagging in a single calculation.

Apply ground-to-roof snow load factors in one step, reviewed by an external engineer for defensible use.

How it calculates

The Snow Loads (ASCE 7-16) calculator computes balanced and unbalanced roof snow loads, drift loads, and rain-on-snow surcharges per ASCE 7-16 Chapter 7.

Flat roof snow load

The base calculation converts ground snow load (p_g) to flat roof snow load (p_f) using three adjustment factors:

p_f = 0.7 × C_e × C_t × I_s × p_g

  • C_e - roof exposure factor (0.9 fully exposed, 1.0 partially exposed, 1.2 sheltered)
  • C_t - thermal factor (1.0 for heated buildings, up to 1.3 for cold structures)
  • I_s - importance factor (0.8 to 1.2 based on risk category)

A minimum roof snow load p_m = I_s × p_g is also enforced for low-sloped roofs.

Sloped roof (balanced) snow load

The balanced design snow load on sloped roofs applies the roof slope factor (C_s):

p_s = C_s × p_f

C_s is determined separately for warm roofs, cool roofs, and cold roofs based on roof pitch and thermal condition. The governing design load is max(p_s, p_m).

Rain-on-snow surcharge

For roofs with p_g not exceeding 20 psf and slopes below the minimum threshold, a rain-on-snow surcharge of 5 psf is added to the balanced snow load per ASCE 7-16 Cl. 7.10.

Unbalanced snow loads

Unbalanced loading is checked for gable roofs with pitches between 0.5:12 and 7:12. The windward side carries a reduced load while the leeward side carries an increased load. For rafter-supported roofs the leeward surcharge magnitude is h_d × gamma / sqrt(S) where gamma is snow density and S is roof slope run per unit rise. Unbalanced loads are not required outside the 0.5:12 to 7:12 pitch range.

Snow drift loads

Leeward and windward drifts on lower roofs and at roof projections are calculated from the ASCE 7-16 Figure 7.6-1 parabolic drift height curves. The drift height h_d is a function of the upwind fetch length and ground snow load. Drift surcharge pressure is h_d × gamma, tapering over a width of 4 × h_d from the step. Drift calculations are not required where the projection height is less than 15 ft.

Outputs

The calculator reports p_f, p_s, p_design, rain-on-snow surcharge flag, unbalanced leeward and windward pressures, and drift heights and pressures for each lower roof or projection entered. All values reference the governing ASCE 7-16 clause.

Related calculators

Wind Loads (ASCE 7-22)→Seismic Analysis (ASCE 7-16)→

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Frequently asked questions

What design standard does this calculator follow?
The calculator applies ASCE 7-16 Chapter 7 - Snow Loads. It calculates flat roof snow load (pf) from the ground snow load (pg) using the roof exposure factor (Ce), roof thermal factor (Ct), and importance factor (Is). It then derives sloped roof snow loads, unbalanced loads, and drift surcharges from the base pf value.
What are the key inputs?
Key inputs are ground snow load (pg) from ASCE 7-16 Figure 7.2-1 or the authority having jurisdiction, roof geometry (span, eave height, slope angle), surface roughness category, roof exposure condition (fully exposed, partially exposed, sheltered), thermal condition, and occupancy category for the importance factor.
What outputs does the calculator return?
Outputs include flat roof snow load (pf), balanced sloped roof snow load (ps), unbalanced snow load distribution for gable and hip roofs, drift loads on lower roofs and at obstructions (hd), sliding snow load where applicable, and a rain-on-snow surcharge flag for low-slope roofs where pg does not exceed 20 psf.
Does the calculator handle drift loads on lower roofs?
Yes. The calculator computes leeward and windward drift heights using the ASCE 7-16 Figure 7.6-1 parabolic drift curves. You input the upper roof width and the lower roof width separately, and the calculator returns the governing drift surcharge height and peak pressure for each drift condition.
When should I use ASCE 7-16 instead of ASCE 7-22 for snow loads?
Use ASCE 7-16 when your project is governed by IBC 2021 or earlier. ASCE 7-22 made relatively minor adjustments to snow load procedures compared to the wind load provisions, so the edition distinction matters most when the AHJ specifies a particular code year. Check your local building code adoption schedule before selecting the edition.

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