How Do You Calculate Snow Load on a Roof?

Snow load is the weight of snow sitting on your roof. If you live in a cold region, this number matters a lot. Too much snow weight can stress the structure and may even lead to damage. The good news is that calculating roof snow load is not as complicated as it sounds.

In this guide, we’ll walk through the formulas in simple language and break everything into easy steps, with 2–3 practical examples.

1. What Is Snow Load?

Snow load is the weight of accumulated snow per square foot (psf) or per square meter (kN/m²) on the roof.

Two main things affect snow load:

1. Snow density (light, wet, packed, icy snow)
2. Snow depth (in inches or centimeters)

Once you know these two, the calculation is straightforward.

2. Basic Formula for Snow Load

The general formula for snow load is:

Formula

$\text{Snow Load (psf)}=\text{Snow Depth (in)}\times \text{Snow Density (pcf)}$

Where:

• psf = pounds per square foot
• pcf = pounds per cubic foot

3. Typical Snow Densities

4. Engineering Formula (Used in Many Regions)

Some regions prefer a structural engineering formula using ground snow load (Pg):

Formula

$P_f=0.7\times C_e\times C_t\times I_s\times P_g$

Where:

• $P_f$​ = roof snow load
• $C_e$​ = exposure factor
• $C_t$​ = thermal factor
• $I_s$​ = importance factor
• $P_g$​ = ground snow load

For homeowners, the first simple method (snow depth × snow density) is usually enough.

Examples

Example 1: Light snow, 10 inches deep

• Snow depth = 10 inches
• Snow density = 10 pcf (fresh snow)

$\text{Snow Load}=10\times 10=100\text{ psf}$

So, your roof is holding 100 pounds per square foot of snow.

Example 2: Wet, heavy snow, 12 inches deep

• Depth = 12 inches
• Density = 25 pcf (heavy snow)

$\text{Snow Load}=12\times 25=300\text{ psf}$

This is a high load—heavy wet snow adds a lot of weight.

Example 3: Engineering method using ground snow load

• Ground snow load (Pg) = 40 psf
• Exposure factor (Ce) = 1.0
• Thermal factor (Ct) = 1.1
• Importance factor (Is) = 1.0

$P_f=0.7\times 1.0\times 1.1\times 1.0\times 40$

$P_f=30.8\text{ psf}$

So, the design roof snow load is 30.8 psf.

How Roof Slope Affects Snow Load

A steep roof sheds snow faster, reducing the load.
Flat or low-slope roofs tend to collect snow, increasing load.

However, slope does not change the basic calculation unless you are using the engineering formula, which includes a slope factor in some regions.

When Should You Worry About Snow Load?

You should be careful if:

• Doors or windows suddenly feel stuck
• You hear cracking sounds
• Roof beams appear bowed
• You see interior ceiling sagging
• Snowfall was unusually heavy or wet

If you ever feel unsure, always consult a local structural engineer.

Conclusion

Calculating snow load is not complicated. With just snow depth and density, you can quickly get a good estimate of how much weight is on your roof.

Quick reminder:

$\text{Snow Load}=\text{Depth}\times \text{Density}$

This simple approach helps homeowners understand risk and know when it’s time to clear snow or get professional help.