Snow Load Calculator
ConstructionEstimate your roof's design snow load in psf from ground snow load, roof pitch, exposure, and thermal condition using a simplified ASCE-7-style method.
Design Roof Snow Load
15
Flat Roof Snow Load
21
What is a Snow Load?
A Snow Load Calculator estimates the design roof snow load — the weight per square foot that a roof structure must be engineered to support — from your local ground snow load, roof pitch, exposure condition, and building thermal status, using a simplified approximation of the ASCE 7 methodology used in US building codes. This figure is a critical input for structural engineers sizing trusses, rafters, and roof sheathing.
Snow load requirements vary dramatically by region and by a roof's specific characteristics, which is why the same ground snow load can translate into very different design requirements for a steep, exposed roof versus a flat, sheltered one. Pair this with the Roof Truss Calculator and Rafter Length Calculator for the framing decisions that snow load informs.
How to use this Snow Load calculator
- Enter your Ground Snow Load in psf — find this value from your local building code or ASCE 7 hazard maps for your region.
- Enter your Roof Pitch in degrees.
- Select your Exposure Condition — fully exposed, partially exposed, or sheltered — based on your site's surroundings.
- Select your Thermal Condition — heated or unheated structure.
- Review the Design Roof Snow Load result, and verify it with a licensed structural engineer before finalizing any actual construction plans.
Formula & Methodology
The calculator applies a simplified ASCE 7-style approximation: Flat Roof Snow Load = 0.7 × Exposure Factor × Thermal Factor × Ground Snow Load Slope Factor = max(0, 1 − Roof Pitch ÷ 70) Design Roof Snow Load = Flat Roof Snow Load × Slope Factor Worked example: For a 30 psf ground snow load, 20° roof pitch, partially exposed site (1.0), and heated structure (1.0): Flat Roof Snow Load = 0.7 × 1.0 × 1.0 × 30 = 21 psf Slope Factor = max(0, 1 − 20 ÷ 70) ≈ max(0, 0.714) ≈ 0.714 Design Roof Snow Load = 21 × 0.714 ≈ 15 psf This simplified estimate is useful for preliminary planning — final structural design should always be verified against your local building code's exact methodology.
Frequently Asked Questions
Roof snow load is the weight of accumulated snow that a roof structure must be designed to support, typically expressed in pounds per square foot (psf). Building codes require roofs to be engineered for a specific design snow load based on the local ground snow load and the roof's own characteristics like pitch and exposure.
Ground snow load is a baseline measurement, published by local building codes or ASCE 7 standards, representing the expected weight of snow accumulation on flat, unobstructed ground in a given region. It's the starting point for calculating the actual design load a specific roof needs to withstand, which is then adjusted for exposure, roof shape, and thermal conditions.
Steeper roofs naturally shed snow more easily than flat or low-slope roofs, so building codes typically apply a reduction factor to design snow load as pitch increases. This calculator applies a simplified slope reduction that decreases the design load as roof pitch angle increases.
Exposure factor accounts for how wind affects snow accumulation on a roof — fully exposed roofs in windswept areas tend to have less accumulated snow (wind blows it off), while sheltered roofs surrounded by trees or taller structures can accumulate more snow, hence a higher exposure factor for sheltered conditions.
Thermal factor accounts for whether a structure is heated — heated buildings melt some snow from below, reducing accumulation, while unheated structures (like detached garages or storage buildings) retain more snow and therefore use a higher thermal factor in the design load calculation.
No, this calculator uses a simplified approximation of the ASCE 7 methodology for educational and preliminary estimation purposes — a full code-compliant snow load calculation for actual construction should be performed or verified by a licensed structural engineer familiar with your local building code amendments.
Ground snow load values and required design methodologies vary significantly by jurisdiction and are often amended from the base ASCE 7 standard by local building codes to reflect regional conditions — always verify the correct ground snow load value for your specific location from your local building department.
Design snow load is one of the key inputs structural engineers use when sizing roof trusses and rafters, since the structure must support this additional weight without excessive deflection or failure — see the [Roof Truss Calculator](/roof-truss-calculator/) and [Rafter Length Calculator](/rafter-length-calculator/) for related framing planning, though truss engineering itself requires this snow load figure as an input.
Also known as