February 6, 2023


February 6, 2023


Carrying out the correct roof calculation does not require in-depth technical knowledge, however, it is usually a stage of the work that raises many doubts among professionals.

Several factors can influence the roof calculation, such as the total area, the type of tile or even the functionality of the roof, so we must be very careful in this process.

If you have doubts regarding which slope to use, the type of tile and the types of materials most used, this publication is for you!

Follow up!



The purpose of the roof calculation is to guarantee savings in materials, safety of use and comfort, all of these without forgetting the aesthetics. After all, who doesn't want a house with a beautiful roof?

A correctly dimensioned roof, in addition to offering comfort and safety, must meet technical standards established by specific regulations per country.    

As it is a robust structure, it must be correctly dimensioned to avoid overloading the building structure. Even though it is a very simple process, we must pay attention to some important points, such as the inclination.



Each type of tile requires a specific inclination, which impacts the quantity of tiles, each manufacturer must indicate the average consumption of tiles per m² of roof.

For example, colonial model ceramic tiles usually have a yield of 15/m², which for a 75m² roof, we must multiply 15 x 75, resulting in the required amount of 1.125 tiles.

As it is a relatively fragile material against impacts, it is common for parts to break during transport, delivery and installation, so it is recommended to buy a surplus of tiles of at least 5%.



Choosing the type of roof for a house is a very important step for any work, considering that in addition to aesthetics, we have to worry about issues of comfort, safety and,last but not least, the materials that will be used.

There are several models of coverings and materials that can be used, so it is important to know the characteristics of each one of them before choosing the best option.

Let's get to know some of the main models and materials used in roofing projects.

  • Ceramic tile - most used option in residential projects, offering great thermal comfort and finishing;
  • Fiber cement tile - light, inexpensive and does not need reinforced support, in addition to enabling installation with a minimum inclination of 15%;
  • Metallic tiles - resistant, light and durable, allow to overcome large spans, also found in thermo-acoustic versions;
  • Polycarbonate tiles - light, resistant and have up to 90% transparency, providing greater use of natural light;
  • PVC tile - relatively durable and with low maintenance, which, being very light, allows installation on small slopes;
  • Concrete tile - are durable and offer good thermal comfort, offering a wide variety of shapes and colors.

Each type of tile has its technical specifications, which in many cases can be consulted in specific regulations per country. 

After choosing the type of tile that best meets the needs of your project, we continue with the step that demands the most attention: the roof calculation

So let's learn how to do it with a very simple step by step.



The calculation of the slope of a roof is a simple process, but it must be carried out with care, as in addition to being an element that has a robust structure, errors can generate unnecessary expenses.

The first step is to identify the total roof area, which can be easily identified through the roof plan or even a floor plan, remembering to include the eaves (if any).

The second step is to define the type of tile that will be used, as this choice affects both the structure that will be used and the necessary inclination for the roof, which is normally specified by  the tile manufacturer who determines the minimum and maximum values.

According to the required roof slope, we need to use the correction factor to determine the area in m² that will be covered by the tiles, which is different from the roof area, after all it is an inclined plane.

This is easily solved just by multiplying the roof area by the correction factor. To make the calculation easier you can refer to the table below.


With the total area of the inclined plane we can go to the third step, which is to calculate the amount of tiles. Each manufacturer usually provides an average consumption of tiles per m², which may vary according to the type of tile that will be used.

An important recommendation is to purchase a surplus quantity of tiles, something around 5%, as breakages can occur during transport, delivery and installation. In addition, it is also important to have some spare parts.

For a better understanding, let's imagine an area of 9.00 x 4.00 m, where I need to dimension a roof.


As a first step I need to identify the total roof area, which according to the dashed line we have a projection of 60 cm, which gives us an area of 9.00 x 4.60 m, resulting in 41.4 m² of total roof area.


The next step is to identify the type of tile that will be used which, for the example, for this case I will use colonial model ceramics.

According to the manufacturer, it requires a minimum inclination of 30% for a length of up to 3.00 meters and an increase of 2% of inclination for each excess meter. As per our example, let's choose the smallest measurement for the slope direction, where we have 4.60 meters.

As the length exceeds 3.00 meters, I need to add 2% of inclination for each excess meter, which to simplify our calculation, I will round it to 35%.


Once we have defined the necessary inclination, we need to identify the area of the inclined plane. It is necessary for us  to use the correction factor, which according to the table presented, for the 35% inclination is 1.059.

Multiplying the total area of 41.4 m² by the correction factor of 1.059 we have as the total area of the inclined plane the value of 43.84 m².


Now that we have the total area of the inclined plane, we can calculate the amount of tiles, which in the case of ceramic tiles in the colonial model, we have the value of 15/m², where it is enough to multiply this value by the area of the inclined plane.


As we have to consider an excess value of 5%, the amount goes to 691, which we could consider 700 tiles to execute this covering.

Thus, it is clear that the roof calculation process is quite simple, but a simple error can generate unnecessary costs or even a lack of material during the roof execution.



The inclined plane of a roof is called water since they are used to drain rain and it is very common for a project to have two or more gable roofs. 

The choice of the amount of water takes into account architectural, structural and also aesthetic aspects, as it is an element of great influence on the final look of the building.

The amount of water in a roof is a determining factor for the complexity of its construction; the more water, the more difficult it will be to build.

The calculation of a roof with two or more slopes is the same as for roofs with a single slope, and it is necessary to perform a calculation for each of the roof's slopes.

The biggest impact of roofs with multiple waters is on the structure, which needs to accommodate all the roof waters, respecting their respective slopes, in addition to requiring extra care in the seals.

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The roof calculation is a very simple process but it requires a lot of attention since, in addition to ensuring the protection of the building's internal space, it has a strong visual appeal in any project.

Every professional must know how to size a roof correctly, specifying the type of tile and the most appropriate slope, ensuring safety, watertightness and comfort for the building.

Another factor that can be a difference for any professional is using quality families, so get ahead by using Blocks Revit's family library, which offers a huge catalog with weekly updates!

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The author of the article

Luis André
Master of architecture, product designer and professor. Sharing professional content from leading architectural software for 10 years.
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