Loads on roof structures

Loads on roof structures dead load snow load wind load live load and seismic load load distribution and direction

Last updated: December 31st, 2022

Were you also thrown into the first design class at uni, having to apply loads and load combinations without having seen it before and without a good guide?

Well, welcome to the club 😉

That was also me a couple of years ago 😁

But this blog post is also for every architect, builder, technician or someone who wants to learn about structures and how to apply loads.

We’ll explain the concept of loads as simple as possible.

In this post we’ll show you, step-by step, what types of loads need to be applied to roofs, how to apply them and give you references to calculation guides.

💡 Info:
This post is focused on roof loading, but loads on other structures are applied and calculated the same way.

So let’s get into it.

What are area, line and point loads?

So before we are getting into the different loads, we need to talk about some basics first.

Area loads

Area loads are – as the name says – applied on areas, and the unit is kN/m2. Area loads are usually applied on floor slabs, walls and facade elements.

An area load applied to a vertical wall, presenting how the wind load is applied to walls and facades.
Area load on wall

Line loads

Line loads are applied on beams, columns, walls, rafters, purlins and probably a few more

A uniformly distributed line load is applied to a horizontal beam.
Line load on beam

Point loads

Point loads can be applied on all structural members. For example, the vertical support force of a beam can be applied as a Point load on the column that supports the beam.

A vertical point load is applied to a vertical column.
Point load on column

For simplification of the structural calculation, the 3D system and area loads are often transformed in a 2D system and line loads. But let’s look at an example to explain it better 😊

Example of load distribution

We have an area load of 1.0 kN/m2 that is applied on a floor slab. The floor slab is supported by 2 beams. The span of the slab is 4.0 m.

Example of how an area load is applied to a horizontal slab which is supported by 2 beams to emphasize how the area load is transferred into line loads onto the beams.
Example load distribution slab

The area load travels through the slab to the 2 beams. Each beam is taking half of the area load. The line load that is acting on 1 beam is calculated like:

$$1.0 \frac{kN}{m^2} \cdot \frac{4m}{2} = 2.0 \frac{kN}{m} $$

This line load can now be applied to the beams.

The resulting line load of 2.0 kN/m is applied on a beam.
Example load distribution beam

1. Dead load | self-weight

So let’s look at a Purlin roof and its roof layers to see how we calculate the area and line dead loads.

The elements of a purlin roof are demonstrated such as rafter, purlin, concrete ceiling, knee brace and column.
Purlin roof

The roof layers in the following table are simplified and not all necessary layers are shown.

ElementThickness [mm]Density [kg/m3]Area load [kN/m2]
OSB board206500.13
Insulation3001000.3
Roof tiles  0.65
Sum  1.08

The Area load is calculated as:

Density/100 * Thickness = Area Dead load

For the example of the OSB board:

$$\frac{650}{100} \frac{kg}{m^3} * 0.02 m = 0.13 \frac{kN}{m^2} $$

Now the sum of the dead load (value) can be applied to advanced 3D structural analysis models which can automatically calculate the line loads on the rafters. The loads are applied in 3D, like in the following picture.

The area dead load of 1.08 kN/m2 is applied to an inclined roof. The direction of the load is vertical, while the distribution follows the slope of the roof.
Area dead load | Purlin roof

Now if we want to do hand calculations then we need to transform the Area load into a Line load

Area load * Rafter spacing = Line load

$$1.08 \frac{kN}{m^2} * 0.8 m = 0.864 \frac{kN}{m} $$

And add the self-weight of the rafters

Density Rafter/100 * height * width = Dead load rafter

C24 Structural wood is used in this example. Just google something like “C24 structural wood density” to find producers and their technical data sheets.

$$\frac{350}{100} \frac{kg}{m^3} * 0.2 m * 0.1 m = 0.07 \frac{kN}{m} $$

Those 2 line loads can now be added up

$$ 0.864 \frac{kN}{m} + 0.07 \frac{kN}{m} = 0.934 \frac{kN}{m} $$

Which then can be applied to a 2D statical system.

The static system of the rafters of a purlin roof together with a line load is shown.
Line dead load | Purlin roof

2. Wind load

The calculation of the wind force according to Eurocode is too extensive for this post.

We have written extensive guides with examples on how to calculate the wind load and areas for

Make sure to check them out if you need a step-by-step guide.

Wind load in 3D system

The wind load in the following figure is very simplified. Usually a roof has different areas with different values of the load, but the purpose of the picture is to emphasize the load direction perpendicular to the rafters.

We assume a wind load of 1.0 kN/m2 that is equally distributed.

The Area wind load of 1.0 kN/m2 is applied perpendicular to the inclined timber roof surface.
Area wind load | Purlin roof

Wind load in 2D system

Now if we want to transform the area load into a line load again, then we multiply the area load with the spacing of the rafters:

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

And this line load can be applied in a 2D system:

Line wind load | Purlin roof

3. Snow load

The snow load is calculated with EN 1991-1-3.

Here are step-by-step guides with examples on how to calculate the snow load of a

Check them out if you want to learn more about the snow load.

But now⌛: Let’s look at how the snow load is applied to structures.

Snow load in 3D system

As an example we use a snow load of 1.0 kN/m2 which is used for some constructions in Denmark.

Snow area load | Purlin roof

Snow load in 2D system

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

And this line load can be applied in a 2D system:

Snow line load | Purlin roof

4. Live load

The values of the live load can be taken from EN 1991-1-1 Table 6.2 (and National Annex❗) for the different categories of loading areas such as office, roof, balcony, staircase and many more.

I advise you to read up on it in the code to get a better understanding. 📖

So, how is the live load applied to structures?

Live load in 3D system

In this example, we use 1.0 kN/m2 as characteristic live load on the roof. This value can be looked up in the Eurocode National Annex and differs from country to country.

Live area load | Purlin roof

Live load in 2D system

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

And this line load can be applied in a 2D system:

Live line load | Purlin roof

5. Seismic load

Unfortunately, I do not know exactly how you apply seismic loads to roofs because I have so far only lived in regions that had minimal or no seismic activity, where the leading lateral force has always been wind.

But here is a good YouTube video that explains the seismic loads very well.

Conclusion

Now, that you got an understanding of what loads act on roofs and how to apply them, it’s time to understand how to calculate the loads

  1. Snow load on a flat roof
  2. Wind load on a flat roof
  3. Wind load on walls

Because there are always multiple loads acting on a structural element. 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 a lot of guides on how to design structural roof structures. Check them out.

I hope that this article helped you understand the live load. In case you still have questions. Let us know in the comments below ✍️.

Roof Loads FAQ

What are typical loads on roofs?

– Dead load
– Wind load
– Snow load
– Live load
– Seismic load

How do you calculate roof loads?

Roof loads are calculated from formulas given in Standards. Roof loads in Europe – for example – are calculated according to Eurocode. Every load (snow, wind, dead, live & seismic) is calculated according to its specific Standard. Every country in Europe has furthermore its own National Annex which specifies parameters.

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