How To Calculate The Snow Load Of A Flat Roof {2024}

Are you building or designing a flat roof, and now you are wondering how to consider the snow load?

Understanding the snow load calculation is crucial to ensure the building structure doesn’t collapse.

In my job as a structural engineer, I’ve calculated the snow load of many roof structures. So in this post, I’ll show you step-by-step how to calculate the snow load of flat roofs according to Eurocode. 💡💡

You can also watch our video tutorial. 👇👁️

Alright, let’s get into it. 🚀🚀

Example Structure

We’ll calculate the snow load for the flat roof of the below canopy structure.

Examples always help understand the formulas better. 👍👍

1. Snow Load on Roofs

For a persistent / tansient design situation EN 1991-1-3 (5.1) is used to calculate the characteristic snow load:

Where

• $\mu_{i}$ = the snow load shape coefficient
• C_e = the exposure coefficient
• C_t = the thermal coefficient and
• s_k = the characteristic snow load value on the ground

Let’s have a closer look how we get those parameters.

2. Snow Load Shape Coefficient

The flat roof we are looking at in this example counts as a monopitch roof and therefore the value for $\mu_{1}$ is given in EN 1991-1-3 Table 5.2. For $\alpha$ = 0 which means that the slope of the roof is 0 we get:

$$\mu_{1} = 0.8$$

3. Exposure Coefficient C_e

EN 1991-1-3 5.2 (7) recommends C_e to be taken as 1.0. However, this value depends on the topography of the location. EN 1991-1-3 Table 5.1 categorizes the topography in windswept, normal and sheltered with different values for C_e. In this blog post, we assume a normal topography for our design. Therefore

$$C_{e} = 1.0$$

4. Thermal Coefficient C_t

EN 1991-1-3 5.2 (8) defines C_t as 1.0. However, this value can be reduced if the roof is covered by glass, which would lead to melting of the snow. In our case, we are not using any glass. Therefore:

$$C_{t} = 1.0$$

5. Characteristic Snow Load Value on the Ground s_k

The characteristic snow load on the ground is found in the national annex of the country your roof is located in. However, there is a great online tool made by Dlubal Software GmbH which calculates the snow load according to location and national annex. Click on the link. Enter the location and the national annex.

If we do that for Copenhagen, Denmark, we get a value of

$$s_{k} = 1.0 \frac{kN}{m^2}$$

So now we finally have all 4 values required to calculate the characteristic snow load of a flat roof. The snow load for a flat roof located on Copenhagen, Denmark is calculated as

$$s = \mu_{i} \cdot C_{e} \cdot C_{t} \cdot s_{k} $$

$$s = 0.8 \cdot 1.0 \cdot 1.0 \cdot 1.0 \frac{kN}{m^2} = 0.8 \frac{kN}{m^2}$$

This load is now applied perpendicular to the roof surface. 👇👇

Conclusion

Now, that you got an overview of how to calculate the snow load for flat roofs, you can learn about the snow load for pitched roofs and other loads such as:

1. Dead load
2. Wind load on a flat roof
3. Live load

Because there are always multiple loads acting on structures. Considering these different loads in the structural design is done by setting up Load Combinations with safety factors.🦺

Once all load cases and combinations are set up, the structural elements can be designed. We have already written guides on how to design structural elements. Check it out!

• Timber truss roof design
• Steel beam design
• Timber beam design

I hope that this article helped you understand the snow load and how to go further from here. In case you still have questions.

Let us know in the comments below. ✍️✍️

Laurin Ernst

Hi! I am Laurin and I started Structural Basics in January 2022 with the aim to improve the online knowledge about structural engineering. I am a full-time structural engineer with a passion for content creation and digital tools to improve the AEC industry. I have a background in BIM too.

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Snow Load FAQ