Snow Load

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Attributes

Complete reference for snow load calculation input parameters.

Required Attributes

These attributes must always be provided:

exposure

Roof exposure category per ASCE 7 Table 7.3-1.

Value	Description
"Fully Exposed"	No obstructions within 10h_o of roof
"Partially Exposed"	Partial shelter from wind
"Sheltered"	Roof fully sheltered from wind

Note: Per ASCE 7 Table 7.3-1, the "Sheltered" exposure is not valid when roughnessCategory is "Above tree line" or "Alaska no trees".

thermalCondition

Roof thermal condition affecting snow melt.

ASCE 7-16 (numeric codes per Table 7.3-2):

Value	Description
"1"	All structures except as indicated below
"2"	Structures with unheated portions maintained just above freezing
"3"	Unheated and open air structures
"4"	Structures intentionally kept below freezing
"5"	Continuously heated greenhouses with roof R < 2.0
"6"	Structures with high thermal resistance roof

ASCE 7-22 (descriptive conditions per Table 7.3-2):

Value
"All other structures"
"Structures kept just above freezing"
"Unheated and open air structures"
"Structures intentionally kept below freezing"
"Continuously heated greenhouses"
"Freezer buildings"

eaveToRidgeDistance

Horizontal distance from eave to ridge in feet.

• Type: number
• Unit: feet
• Example: 30

roofSlope

Roof slope as a ratio to 12 (rise:12 run).

• Type: number
• Unit: ratio
• Example: 4 (for 4:12 slope)

Optional Attributes

overrideDefaults

When true, allows specifying site-specific parameters instead of using project defaults.

• Type: boolean
• Default: false

When enabled, you should also provide:

• riskCategory
• groundSnowLoad
• roughnessCategory

riskCategory

Risk category per ASCE 7 Table 1.5-1. Required when overrideDefaults: true.

Value	Description
"I"	Low risk to human life
"II"	Standard occupancy
"III"	Substantial hazard to human life
"IV"	Essential facilities

groundSnowLoad

Ground snow load (p_g) in psf. Only used when overrideDefaults: true.

• Type: number
• Unit: psf
• Example: 35

roughnessCategory

Surface roughness category. Only used when overrideDefaults: true.

Value	Description
"B"	Urban and suburban areas
"C"	Open terrain with scattered obstructions
"D"	Flat, unobstructed coastal areas
"Above tree line"	Mountain terrain above treeline
"Alaska no trees"	Alaska areas without trees

roofSurface

Roof surface type affecting slope factor (C_s).

Value	Description
"Slippery surfaces"	Metal, slate, glass, or similar
"Obstructed or non-slippery surfaces"	Asphalt shingles, wood shakes, etc.

Default: "Obstructed or non-slippery surfaces"

comments

Optional text comments to include on the calculation sheet.

• Type: string
• Example: "Warehouse roof - main building"

Drift Attributes

lowerRoofDrifts

Array of lower roof drift configurations for step/setback conditions.

Each entry is an array: [label, lu_upper, lu_lower, step_height]

Element	Description	Unit
label	Description of drift location	string
lu_upper	Upwind fetch distance (upper roof)	feet
lu_lower	Upwind fetch distance (lower roof)	feet
step_height	Height difference between roofs	feet

Example:

Code
 
{
  "lowerRoofDrifts": [
    ["North step", 50, 30, 8],
    ["South step", 40, 25, 6]
  ]
}

projectionDrifts

Array of projection drift configurations for rooftop units, parapets, etc.

Each entry is an array: [label, upwind_distance, projection_height]

Element	Description	Unit
label	Description of projection	string
upwind_distance	Upwind fetch distance	feet
projection_height	Height of projection	feet

Example:

Code
 
{
  "projectionDrifts": [
    ["RTU-1", 60, 4],
    ["Parapet", 100, 3]
  ]
}

ASCE 7-22 Specific Attributes

These attributes are only available for ASCE 7-22 calculations.

roofInsulationRValue

Roof insulation R-value for thermal factor calculation.

• Type: number
• Unit: h-ft²-°F/BTU
• Example: 30

Used in conjunction with thermal condition to determine the thermal factor (C_t).

widthFactor

Width factor (W₂) for unbalanced snow load calculations per ASCE 7-22 Section 7.6.

• Type: number
• Range: 0 to 1 (dimensionless ratio)
• Example: 0.5

Complete Examples

Basic Calculation (Use Project Defaults)

Code
 
{
  "projectId": "your-project-id",
  "name": "Warehouse Roof",
  "attributes": {
    "exposure": "Partially Exposed",
    "thermalCondition": "1",
    "eaveToRidgeDistance": 30,
    "roofSlope": 4
  }
}

Override Project Defaults

Code
 
{
  "projectId": "your-project-id",
  "attributes": {
    "overrideDefaults": true,
    "riskCategory": "II",
    "groundSnowLoad": 40,
    "roughnessCategory": "C",
    "exposure": "Fully Exposed",
    "thermalCondition": "1",
    "eaveToRidgeDistance": 25,
    "roofSlope": 6,
    "roofSurface": "Slippery surfaces"
  }
}

With Snow Drifts

Code
 
{
  "projectId": "your-project-id",
  "attributes": {
    "exposure": "Partially Exposed",
    "thermalCondition": "1",
    "eaveToRidgeDistance": 40,
    "roofSlope": 3,
    "lowerRoofDrifts": [
      ["Office wing step", 60, 35, 10]
    ],
    "projectionDrifts": [
      ["Mechanical unit", 80, 5]
    ]
  }
}

ASCE 7-22 with R-Value

Code
 
{
  "projectId": "your-project-id",
  "attributes": {
    "exposure": "Partially Exposed",
    "thermalCondition": "All other structures",
    "eaveToRidgeDistance": 35,
    "roofSlope": 4,
    "roofInsulationRValue": 30
  }
}

Attribute Summary Table

Attribute	Type	Required	ASCE 7-16	ASCE 7-22
exposure	enum	Yes	Yes	Yes
thermalCondition	enum	Yes	Numeric (1-6)	Descriptive
eaveToRidgeDistance	number	Yes	Yes	Yes
roofSlope	number	Yes	Yes	Yes
overrideDefaults	boolean	No	Yes	Yes
riskCategory	enum	When overriding	Yes	Yes
groundSnowLoad	number	When overriding	Yes	Yes
roughnessCategory	enum	When overriding	Yes	Yes
roofSurface	enum	No	Yes	Yes
lowerRoofDrifts	array	No	Yes	Yes
projectionDrifts	array	No	Yes	Yes
roofInsulationRValue	number	No	No	Yes
widthFactor	number	No	No	Yes
comments	string	No	Yes	Yes