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Snow Loads

The Snow Load module calculates snow loads for roof structures based on ASCE 7-16 Chapter 7. The module supports multiple calculation types including flat roof snow loads, sloped roof snow loads, minimum snow loads, unbalanced loads, drift loads, and rain-on-snow surcharges.

Snow Load Overview

Key Features

  • Project-level ground snow and risk category inputs
  • Multiple roof regions per project
  • Nine different snow load calculation types
  • Expandable/collapsible calculation sections
  • Interactive tooltips for code tables
  • Visual diagrams for drift calculations
  • Summary view with all results
  • Professional PDF export

Getting Started

  1. Navigate to your structure in the left sidebar
  2. Click the three-dot menu and select Add Module
  3. Choose Snow Load from the module list
  4. Enter the ground snow load (pg) for your project location
  5. Click Add Roof Region to create your first roof area
  6. Configure exposure and thermal factors for the roof
  7. Enable applicable design cases (drift, sliding, etc.)
  8. Review calculations in the Summary tab

Project-Level Inputs

At the top of the Snow Load module, you'll find project-level inputs that apply to all roof regions:

General Parameters

Ground Snow Input

Parameter Description
Ground Snow pg Ground snow load in psf from ASCE 7-16 Figures 7.2-1 through 7.2-3
Risk Category Building risk category (I, II, III, or IV) set at project level
Importance Factor Is Snow importance factor derived from Risk Category per Table 1.5-2

Roof Region Management

Snow loads are organized by roof regions. Each region represents a distinct portion of the roof with its own geometry and load calculations.

Adding Roof Regions

  1. Click Add Roof Region at the top of the module
  2. A new region appears with expandable calculation sections
  3. Enter a descriptive name in the Region Description field

Add Roof Region

Editing Roof Regions

Modify roof region settings at any time:

  1. Click on the region header to expand the panel
  2. Update the Region Description field to rename
  3. Modify calculation parameters as needed
  4. Enable or disable design cases using the checkboxes
  5. Changes are saved automatically

Region Rename

Deleting Roof Regions

Remove roof regions that are no longer needed:

  1. Click the three-dot menu (⋮) next to the region name
  2. Select Delete Region from the dropdown
  3. Confirm the deletion when prompted

Permanent Action

Deleting a roof region removes all its calculations and design cases. This action cannot be undone.

Naming Regions

Use descriptive names following ASCE naming conventions:

  • "High Roof - Office Wing"
  • "Low Roof - Warehouse"
  • "Canopy - Main Entrance"
  • "Mechanical Penthouse"

Region Rename

Multiple Regions

Projects can have multiple roof regions, each with independent calculations:

Multiple Regions

Region Options

Click the three-dot menu (⋮) next to a region name to access options:

Region Menu Options

  • Duplicate Region - Create a copy with all settings
  • Delete Region - Remove the region permanently

Expanding and Collapsing

  • Click a region header to expand/collapse the entire region
  • Use Expand all / Collapse all buttons to show/hide all sections at once

All Sections Expanded

Calculation Sections

Each roof region contains nine collapsible calculation sections. The first three (Balanced Flat Roof, Balanced Sloped Roof, Minimum Snow Load) are always included. The remaining six are optional design cases.

Section ASCE 7-16 Reference Optional
Balanced Flat Roof Snow Load Eq. 7.3-1 No
Balanced Sloped Roof Snow Load Eq. 7.4-1 No
Minimum Snow Load Eq. 7.3-2 No
Unbalanced Snow on Hip and Gable Section 7.6 Yes
Ice Dams Along Eaves Eq. 7.4-3 Yes
Sliding Snow Load Eq. 7.9-1 Yes
Drift Loads for Roof Steps Section 7.7 Yes
Drift Loads for Projections/Parapets Eq. 7.8-1 Yes
Rain On Snow Eq. 7.10-1 Yes

Including Design Cases

Optional sections have an INCLUDE DESIGN CASE checkbox. Check this box to include the load case in your design:

All Design Cases Enabled

Balanced Flat Roof Snow Load

This section calculates the balanced flat roof snow load per ASCE 7-16 Equation 7.3-1:

\[p_f = 0.7 \cdot C_e \cdot C_t \cdot I_s \cdot p_g\]

Balanced Flat Roof Section

Input Fields

Field Description Range
Exposure Factor Ce Site exposure condition 0.7 - 1.2
Thermal Factor Ct Roof thermal condition 0.85 - 1.3

Exposure Factor (Ce)

Select based on terrain category and roof exposure per ASCE 7-16 Table 7.3-1. Click the info button to view the complete table.

Thermal Factor (Ct)

Select from the dropdown based on roof thermal condition per ASCE 7-16 Table 7.3-2:

Thermal Factor Dropdown

Ct Value Condition
0.85 Continuously heated greenhouse
1.0 All structures except below
1.1 Structures kept just above freezing
1.2 Unheated/open air structures
1.3 Freezer buildings

Calculated Results

Output Description
Snow Importance Factor Is From project risk category
Ground Snow Load Factor Pg From project-level input
Balanced Flat Roof Load pf Final calculated value (psf)

Balanced Sloped Roof Snow Load

This section calculates the balanced sloped roof snow load per ASCE 7-16 Equation 7.4-1:

\[p_s = C_s \cdot p_f\]

Balanced Sloped Roof Section

Input Fields

Field Description
Roof Obstructions/Friction Surface condition (Obstructed or Unobstructed & Slippery)
Roof Slope θ Roof slope angle in degrees

Slope Factor (Cs)

The slope factor is automatically calculated based on:

  • Roof slope angle
  • Thermal factor Ct
  • Roof surface condition (obstructed vs. slippery)

Per ASCE 7-16 Figure 7.4-1 (warm roofs) or Figure 7.4-2 (cold roofs).

Calculated Results

Output Description
Thermal Factor Ct Inherited from Balanced Flat Roof section
Slope Factor Cs Calculated from slope and conditions
Balanced Sloped Roof Load ps Final calculated value (psf)

Minimum Snow Load

This section calculates the minimum snow load per ASCE 7-16 Equation 7.3-2. This represents the minimum roof snow load regardless of other calculations.

Minimum Snow Load Section

Formula

\[p_m = \begin{cases} I_s \cdot p_g & \text{for } p_g \leq 20 \text{ psf} \\ 20 \cdot I_s & \text{for } p_g > 20 \text{ psf} \end{cases}\]

Calculated Results

Output Description
Ground Snow Load pg From project-level input
Snow Importance Factor Is From project risk category
Minimum Snow Load pm Final calculated minimum (psf)

Unbalanced Snow Load on Hip and Gable Roof

This section calculates unbalanced snow loads for hip and gable roofs per ASCE 7-16 Section 7.6.1. Unbalanced loads occur when wind redistributes snow to one side of the roof.

Hip and Gable Section

Applicability

Unbalanced loads are required when:

  • Roof slope is between 2.38° (½:12) and 30.2° (7:12)
  • Eave-to-ridge distance W > 20 ft (for simplified method)

Input Parameters

Field Description
Ground Snow Load pg From project inputs
Balanced Sloped Roof Load ps From previous calculation
Horizontal Projection/Rise S Derived from roof slope
Density of Snow γ Calculated per Eq. 7.7-1

Adding Drift Rows

Click Add Row to add drift load calculations for specific roof locations. Click Diagrams to view visual representations.

Ice Dams Along Eaves

This section calculates loads from ice dams and icicles along eaves per ASCE 7-16 Section 7.4.5 and Equation 7.4-3:

\[p_{ice} = 2.0 \cdot p_f \text{ (at eaves)}\]

Ice Dams Section

When Required

Ice dam loads are required for:

  • Unventilated roofs with R-value < 30 ft²·hr·°F/Btu
  • Ventilated roofs with R-value < 20 ft²·hr·°F/Btu

Calculated Results

Output Description
Balanced Flat Roof Load pf From flat roof calculation
Ice Dams Along Eaves Load pice Double the flat roof load (psf)

Design Consideration

The load on the overhang shall be based upon the flat roof snow load for the heated portion of the roof upslope of the exterior wall. No other loads except dead loads shall be present on the roof when this uniformly distributed load is applied.

Sliding Snow Load

This section calculates loads from snow sliding off an upper roof onto a lower roof per ASCE 7-16 Section 7.9 and Equation 7.9-1.

Sliding Snow Section

Formula

\[p_{sliding} = 0.4 \cdot p_f \cdot W_s + 15$$ $$w_{sliding} = 15 - s\]

Where:

  • Ws = Width of upper roof (eave to ridge)
  • s = Gap between buildings

Adding Sliding Snow Locations

Click Add Row to define sliding snow scenarios. Click Diagrams to visualize the load distribution.

Drift Loads for Roof Steps

This section calculates drift loads at roof elevation changes per ASCE 7-16 Section 7.7.

Drift Roof Steps Section

Key Parameters

Parameter Description Formula
Snow Density γ Density of drifted snow 0.13·pg + 14 ≤ 30 pcf
Balanced Snow Height hb Height of balanced snow ps / γ
Clear Height hc Available space for drift h - hb
Drift Height hd Maximum drift height 0.43·lu(⅓)·(pg+10)(¼) ≤ hc
Drift Width w Width of triangular drift 4·hd
Drift Load pd Maximum drift surcharge hd · γ

Adding Drift Locations

Click Add Row to add calculations for each roof step location.

Drift Loads for Projections and Parapets

This section calculates drift loads at roof projections and parapets per ASCE 7-16 Section 7.8 and Equation 7.8-1.

Drift Projections Section

Key Difference from Roof Steps

For projections and parapets, the drift height is reduced by 75%:

\[h_d = 0.75 \cdot (0.43 \cdot l_u^{1/3} \cdot (p_g + 10)^{1/4}) \leq h_c\]

Input Table

Field Description
Location Descriptive name for the drift
lu upper Length of roof upwind of drift (ft)
lu lower Length of roof downwind of drift (ft)
h Height of projection or parapet (ft)

Rain On Snow

This section calculates the rain-on-snow surcharge per ASCE 7-16 Section 7.10 and Equation 7.10-1.

Rain On Snow Section

Applicability

The 5 psf rain-on-snow surcharge applies when:

  • Ground snow load pg ≤ 20 psf
  • Roof slope θ < W/50

Formula

\[p_{rain} = p_s + 5 \text{ psf}\]

User Note

Rain on snow surcharge is not required if θ < W/50

Summary View

Click the Summary tab to view a comprehensive report of all snow load calculations:

Summary Report

Report Contents

General Snow Parameters:

  • Ground Snow Load pg with code reference
  • Building Risk Category
  • Snow Importance Factor Is

Roof Region Summaries:

For each region, the report includes:

  • Design Summary with key load values
  • Design Detail sections with complete calculations
  • Code equation references for each value

PDF Export

Click the Export PDF tab to generate a professional PDF report.

PDF Contents

  • Project Header: Project name, engineer, ID, and design code
  • Company Branding: Prose Engineering logo
  • General Parameters: All project-level snow load inputs
  • Roof Region Calculations: Complete calculation details
  • Summary Tables: Key results in professional format
  • Page Numbers: Footer with pagination

Best Practices

See Also

Region Organization

  • Create separate regions for each distinct roof area
  • Use ASCE-style descriptive names (e.g., "High Roof - Office Wing")
  • Include all applicable design cases for each region

Design Case Selection

  • Check INCLUDE DESIGN CASE for applicable optional loads
  • Hip and gable loads: Required for slopes 2.38° to 30.2°
  • Ice dams: Required for low R-value roofs
  • Drift loads: Required at elevation changes and obstructions
  • Rain on snow: Required when pg ≤ 20 psf and low slopes

Drift Analysis

Drift loads at roof steps often govern design. Always check both leeward and windward drift heights, and verify the clear height (hc) allows for the calculated drift.

Quality Control

  • Verify ground snow load with local building official
  • Review calculated values against typical ranges
  • Check that drift heights don't exceed clear heights
  • Use Diagrams buttons to visualize load distributions
  • Export PDF for documentation and review

Code Reference

Snow load requirements per ASCE 7-16 Chapter 7:

  • Section 7.2: Ground snow loads shall be based on mapped values or site-specific studies
  • Section 7.3: Minimum roof snow loads apply to all roofs regardless of slope
  • Section 7.6: Unbalanced loads must be considered for gable and hip roofs within slope limits
  • Section 7.7: Drift loads at steps require both windward and leeward drift analysis
  • Section 7.10: Rain-on-snow surcharge applies only to low-sloped roofs in areas with pg ≤ 20 psf
  • Section 7.4.5: Ice dam loads apply to inadequately insulated roofs in cold regions