This Is How You Calculate The Snow Load
And Who Is The Guy Behind Structural Basics? An Introduction To All New Subscribers
Hello friends, 👋👋
In many parts of the world, the snow load doesn’t exist or is not very often in the governing load combination.
However, there are also regions where the snow load is greater as the wind load by a factor of 3-4.
I grew up in one of these regions. I also did my bachelor’s degree in Innsbruck, Austria, where the snow load is different for almost every village in the region due to its magnitude in some of these villages.
Today, we’ll learn how to calculate the snow load on flat and pitched roofs.
But since we are now over ❗ 5000 people❗ subscribed to the weekly Structural Basics newsletter, I think it’s a good idea to introduce myself. If you are only interested in how to calculate the snow load, then scroll down and skip the next section.
Who Is The Person Behind Structural Basics? Get To Know Me Better
Hi all,
I am Laurin.
Passionate structural engineer by day, blogger and athlete by night.
I am originally from Kempten, Germany. I grew up in Germany close to the Alps, did my bachelor’s in Innsbruck (Civil and Environmental Engineering - University of Innsbruck), Austria and my master’s in Copenhagen, Denmark (Architectural Engineering - Technical University of Denmark). Therefore, I also speak 3 languages fluently - German, Danish and English.
I work these days as a full-time structural engineer in Copenhagen, Denmark.
One of my big passions is backpacking and travelling in general. I use almost all of my vacation days and overtime hours to travel.
Another thing I love is sports. I hit the gym 4 times a week. I ran my first marathon in May 2024 here in Copenhagen and plan to run another one later this year. Triathlons next year. When I was younger, I played football and handball. Growing up close to the Alps, I also like skiing and hiking.
Besides travelling and sports, I love working on Structural Basics. I am very fortunate that I found a profession which I love. At this moment in my life, my work as a structural engineer feels like a hobby. I am very grateful that I get to share my knowledge with you readers.
This has now already been a lot of talk about me. I am sure you are much more interested in the structural engineering insights, which we’ll dive into now. I hope this short introduction has given you an understanding of the person whose guides you read every Wednesday.
To understand the full picture of Structural Basics, 2 things are missing:
My professional background/experience
Why I started Structural Basics
We’ll quickly touch base on these 2 things in the next 2 weeks.
But now, let’s get into the snow load calculation. 🔥🔥
The Snow Load Calculation
Let’s start with the formula of the snow load according to Eurocode. Then it’s a bit clearer which steps we have to follow to calculate the snow load.
Where,
μi = snow load shape coefficient
Ce = exposure coefficient
Ct = thermal coefficient
sk = snow load value on the ground
This is the snow load formula from EN 1991-1-3 (5.1) and it’s used for any roof shape and type.
Alright, let’s look at how we define or calculate these 4 values.
Snow Load Shape Coefficient μi
μi depends on the shape of the roof. Snow accumulates much better on flat roofs than on pitched roofs. On the latter, snow can “slide” down the roof, which results in a smaller snow load.
In this newsletter, we’ll calculate the shape coefficients of a flat roof and a duo pitched roof.
There is, however, another design situation which happens very often. Flat roofs abutting to taller constructions. Like for example the roof of a garage which is lower than the roof of a residential building. We don’t cover this in our newsletter, but I included a very thorough explanation in the book of module 1 about loads on residential buildings. The next image is a screenshot from the book, showing the snow load on the garage roof. The launch of module 1 is in November/December.
Flat roof - snow load shape coefficient μi
The flat roof counts as a monopitch roof and therefore the value for 𝜇1 is given in EN 1991-1-3 Table 5.2. For 𝛼 = 0 which means that the slope of the roof is 0 we get a value of:
Duo pitched roof - snow load shape coefficient μi
It’s a bit more of a calculation for a duo pitched roof. When increasing the angle of the roof slope over 30°, the snow load shape coefficient 𝜇1 can be decreased below 0.8 due to sliding of the snow off the roof.
The value for 𝜇1 is given in EN 1991-1-3 Table 5.2. For 𝛼 = 40 which means that the slope of the roof is 40°, we get:
Exposure Coefficient Ce
EN 1991-1-3 5.2 (7) recommends Ce 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 Ce.
In this post, we assume a normal topography for our design. Therefore:
❗❗Always double-check with your National annex because the 𝐶𝑒 values can be specified there for your country. ❗❗
Thermal Coefficient Ct
EN 1991-1-3 5.2 (8) defines Ct 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:
Characteristic Snow Load Value On The Ground sk
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 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:
Characteristic Snow Load
Now, we can calculate the snow load on both the flat and the duo-pitched roof with the formula we introduced in the beginning of the article.
Flat roof
The snow load of the flat roof is calculated as:
Duo-pitched roof
Inserting all values leads to a snow load of:
For a pitched roof, 3 different cases need to be considered due to drifted load arrangements (EN 1991-1-3 5.3.3 (3) + (4)).
Case 1: s = 0.53 kN/m2 is applied on the whole area
Case 2: s = 0.53 kN/m2 is applied on the left inclination and s/2 = 0.27 kN/m2 on the right inclination
Case 3: s/2 = 0.27 kN/m2 is applied on the left inclination and s = 0.53 kN/m2 on the right inclination
Final Words
The snow load needs to be considered in every structural design. At least in most parts of Europe. If you are from a place where you don’t verify a building for the snow load, let me know in the comments.
As for the wind load, it’s vital to calculate the snow load correctly. We don’t want a building, garage or even a canopy to collapse because we didn’t calculate the snow load correctly.
By following the steps, we taught in this episode, you are able to calculate the snow load of flat and pitched roofs.
Next week, we’ll check out another type of load we need to consider as structural engineers.
If you missed episode #1 - #7 of this new series, then you can find all previous posts → here ←.
Have a great rest of the week.
Until next time. 🙋♂️🙋♂️
Cheers,
Laurin. 😎😎