# Imposed Roof Loads Due to Snow, Wind and Occupancy: BS 6399-3:1988 Explained

## What is BS 6399-3:1988?

If you are involved in the design or construction of roofs, you may have come across the term BS 6399-3:1988. But what does it mean and what does it entail? In this article, we will explain what BS 6399-3:1988 is, why it is important, how to use it, where to download it, and how to update it.

## bs6399part31988pdfdownload

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BS 6399-3:1988 is a British Standard that provides a code of practice for imposed roof loads. It specifies the minimum values and methods of calculation for roof loads due to snow, wind and occupancy. It applies to roofs of any shape, slope and material that are supported by walls or other structural elements. It does not cover roofs that are subject to dynamic effects or special loadings such as blast or impact.

BS 6399-3:1988 was published in May 1988 by the British Standards Institution (BSI) as a replacement for CP 3:Chapter V:Part 2 (1970), which dealt with wind loads on roofs. It was also intended to complement BS 6399-1 (1984), which covered dead and imposed loads on buildings other than roofs. It was one of the three parts of BS 6399, which also included BS 6399-2 (1997) for wind loads on buildings.

## Why is BS 6399-3:1988 important?

BS 6399-3:1988 is important because it provides guidance and recommendations for designing and constructing roofs that can withstand the effects of imposed loads. Imposed loads are external forces that act on a roof due to environmental or human factors. They can vary in magnitude, direction and duration depending on the location, shape and use of the roof. If they are not properly accounted for, they can cause damage or failure of the roof structure or its components.

By following the code of practice in BS 6399-3:1988, you can ensure that your roof is safe, durable and fit for purpose. You can also avoid unnecessary costs and risks associated with overdesigning or underdesigning your roof. Moreover, you can comply with the relevant regulations and standards that apply to your project, such as the Building Regulations, the Health and Safety at Work Act, and the Eurocodes.

## How to use BS 6399-3:1988?

BS 6399-3:1988 provides general principles and methods of calculating roof loads for ultimate limit state design. Ultimate limit state design is a method of designing structures that ensures that they do not collapse or become unstable under the most severe combination of loads that they are likely to face during their lifetime. It also ensures that they have adequate strength and stiffness to resist deformations and vibrations that could impair their serviceability or appearance.

To use BS 6399-3:1988, you need to follow these steps:

Determine the minimum imposed roof loads for your roof type and building category.

Determine the variable imposed roof loads for your roof due to snow, wind and occupancy.

Combine the different types of roof loads according to the rules given in the standard.

Check that your roof structure and its components can resist the resulting roof loads without exceeding the allowable stresses, deflections and vibrations.

In the following sections, we will explain each of these steps in more detail.

### Minimum imposed roof loads

The minimum imposed roof loads are the lowest values of roof loads that you need to consider for your design. They are based on the type of roof and the category of building that you are working on. They are given in Table 1 of BS 6399-3:1988, which is reproduced below:

Roof type Building category Minimum imposed roof load (kN/m) --- --- --- Flat roofs (slope 10) A: Residential, institutional, educational, commercial 0.75 B: Industrial, storage 0.6 Pitched roofs (slope > 10) A: Residential, institutional, educational, commercial 0.6 B: Industrial, storage 0.4 Note that these values are for roofs that are not accessible or used for any purpose other than weather protection. If your roof is accessible or used for other purposes, such as gardens, terraces, parking, sports or maintenance, you need to increase the minimum imposed roof load accordingly. You also need to consider any additional loads due to fixtures, fittings, equipment or services on your roof.

### Variable imposed roof loads

The variable imposed roof loads are the values of roof loads that vary depending on the environmental or human factors that affect your roof. They include snow loads, wind loads and occupancy loads. You need to calculate them based on the location, shape and use of your roof. You also need to take into account the probability and duration of these loads occurring on your roof.

#### Snow loads

Snow loads are the forces exerted by snow accumulation on your roof. They depend on the amount and type of snow that falls on your roof, as well as the shape and thermal properties of your roof. To calculate snow loads, you need to use the following formula:

$$S = \mu_i C_e C_t S_k$$ where:

S is the snow load on the roof (kN/m)

$\mu_i$ is the shape coefficient for the roof (dimensionless)

Ce is the exposure coefficient for the site (dimensionless)

Ct is the thermal coefficient for the roof (dimensionless)

Sk is the characteristic ground snow load for the site (kN/m)

The shape coefficient $\mu_i$ accounts for the effect of the roof slope, geometry and obstruction on the snow accumulation. It ranges from 0.8 for flat roofs to 0 for vertical walls. It also varies for different types of pitched roofs, such as duo-pitch, mono-pitch, curved and saw-tooth roofs. You can find the values of $\mu_i$ for different roof shapes in Table 2 of BS 6399-3:1988.

of BS 6399-3:1988.

The thermal coefficient Ct accounts for the effect of the roof temperature on the snow melting or freezing. It ranges from 1 for cold roofs to 0 for warm roofs. You can find the values of Ct for different roof types and materials in Table 4 of BS 6399-3:1988.

The characteristic ground snow load Sk is the value of snow load on the ground that has a probability of being exceeded once in 50 years. It varies depending on the geographical location and altitude of your site. You can find the values of Sk for different snow zones and altitudes in Table 5 and Figure 1 of BS 6399-3:1988. Alternatively, you can use online services such as Dlubal or Lisa.blue to determine the characteristic value of snow load for your site based on a map .

#### Wind loads

Wind loads are the forces exerted by wind pressure or suction on your roof. They depend on the speed and direction of the wind, as well as the shape and exposure of your roof. To calculate wind loads, you need to use the following formula:

$$W = q_p C_p$$ where:

W is the wind load on the roof (kN/m)

qp is the peak velocity pressure at the roof height (kN/m)

Cp is the external pressure coefficient for the roof (dimensionless)

The peak velocity pressure qp is calculated by multiplying the basic wind speed Vb by a series of factors that account for the site altitude, terrain, season, direction and probability. The formula is:

$$q_p = 0.613 \times 10^-3 S_3 S_2 S_1 V_b^2$$ where:

S3 is the seasonal factor (dimensionless)

S2 is the directional factor (dimensionless)

S1 is the probability factor (dimensionless)

Vb is the basic wind speed at 10 m above ground level (m/s)

The seasonal factor S3 accounts for the variation of wind speed due to seasonal effects. It ranges from 1 for winter to 0.8 for summer. You can find the values of S3 for different months in Table 6 of BS 6399-3:1988.

The directional factor S2 accounts for the variation of wind speed due to wind direction. It ranges from 1 for the most unfavourable direction to 0.8 for the most favourable direction. You can find the values of S2 for different directions in Table 7 of BS 6399-3:1988.

The probability factor S1 accounts for the variation of wind speed due to probability of occurrence. It ranges from 1 for a return period of 50 years to 0.87 for a return period of 500 years. You can find the values of S1 for different return periods in Table 8 of BS 6399-3:1988.

The basic wind speed Vb is the value of wind speed at 10 m above ground level that has a probability of being exceeded once in 50 years. It varies depending on the geographical location and altitude of your site. You can find the values of Vb for different wind zones and altitudes in Table 9 and Figure 2 of BS 6399-3:1988.

The external pressure coefficient Cp accounts for the effect of the roof shape and orientation on the wind pressure or suction. It ranges from positive values for wind pressure to negative values for wind suction. It also varies for different types of roofs, such as flat, pitched, curved and saw-tooth roofs. You can find the values of Cp for different roof shapes and angles in Table 10 and Figures 3 to 9 of BS 6399-3:1988.

#### Occupancy loads

Occupancy loads are the forces exerted by people or objects on your roof. They depend on the use and function of your roof, as well as the frequency and duration of occupancy. To calculate occupancy loads, you need to use the following formula:

$$O = q_k \psi_0 \psi_1 \psi_2$$ where:

O is the occupancy load on the roof (kN/m)

qk is the characteristic value of occupancy load (kN/m)

$\psi_0$ is the occupancy factor (dimensionless)

$\psi_1$ is the reduction factor for frequent combination (dimensionless)

$\psi_2$ is the reduction factor for quasi-permanent combination (dimensionless)

The characteristic value of occupancy load qk is the value of occupancy load that represents a realistic expectation of the maximum load likely to occur during the lifetime of the structure. It varies depending on the category of use of your roof, such as residential, educational, commercial, industrial or storage. You can find the values of qk for different categories of use in Table 11 of BS 6399-3:1988.

The occupancy factor $\psi_0$ accounts for the probability of occurrence of the occupancy load. It ranges from 1 for roofs that are permanently occupied to 0 for roofs that are never occupied. You can find the values of $\psi_0$ for different categories of use in Table 12 of BS 6399-3:1988.

The reduction factors $\psi_1$ and $\psi_2$ account for the duration and frequency of occurrence of the occupancy load. They range from 1 for roofs that are frequently or permanently occupied to 0 for roofs that are rarely or exceptionally occupied. You can find the values of $\psi_1$ and $\psi_2$ for different categories of use in Table 13 of BS 6399-3:1988.

### Combination of imposed roof loads

The combination of imposed roof loads is the process of adding or subtracting different types of roof loads to obtain the most unfavourable load effect for your design. You need to consider both ultimate limit state and serviceability limit state combinations. Ultimate limit state combinations are used to check the strength and stability of your roof structure, while serviceability limit state combinations are used to check the deformations and vibrations of your roof structure.

To combine imposed roof loads, you need to use the following rules:

For ultimate limit state combinations, you need to consider both favourable and unfavourable effects of each type of roof load. Favourable effects are those that reduce the load effect, while unfavourable effects are those that increase the load effect.

For serviceability limit state combinations, you need to consider only unfavourable effects of each type of roof load.

You need to apply appropriate partial factors to each type of roof load to account for uncertainties in their values and variations in their effects. The partial factors are given in Table 14 of BS 6399-3:1988.

You need to apply appropriate combination factors to each type of roof load to account for their probability and duration of occurrence. The combination factors are given in Table 15 of BS 6399-3:1988.

You need to consider both normal and exceptional combinations of roof loads. Normal combinations are those that represent realistic situations that may occur during the lifetime of the structure, while exceptional combinations are those that represent extreme or accidental situations that may occur rarely or exceptionally.

You need to consider both frequent and quasi-permanent combinations of roof loads. Frequent combinations are those that represent situations that may occur frequently during a year, while quasi-permanent combinations are those that represent situations that may persist for a long period during a year.

You can find examples of normal and exceptional combinations of roof loads for ultimate limit state and serviceability limit state in Table 16 of BS 6399-3:1988.

## Where to download BS 6399-3:1988 PDF?

If you want to download BS 6399-3:1988 PDF, you have several options. You can purchase the official PDF version of the standard from the BSI website or other online platforms that sell standards. You can also access the PDF version of the standard for free from some academic websites or libraries that have subscriptions to standards databases. However, you need to be careful about the quality and validity of the PDF versions that you download from unofficial sources, as they may be outdated, incomplete or inaccurate.

Here are some links where you can download BS 6399-3:1988 PDF:

BSI website

Idoc.pub

## How to update BS 6399-3:1988?

BS 6399-3:1988 is no longer the current standard for imposed roof loads in the UK and Europe. It has been superseded by Eurocode 1 Part 1-3 (EN 1991-1-3:2003), which provides a harmonized and updated approach to roof load calculations. Eurocode 1 Part 1-3 is part of the Eurocodes, which are a set of European standards for the design of buildings and civil engineering works. The Eurocodes have been adopted by all EU member states and many other countries around the world.

If you want to update BS 6399-3:1988 to Eurocode 1 Part 1-3, you need to be aware of the main differences and similarities between the two standards. Here are some of the key points to consider:

Both standards use similar methods and formulas to calculate roof loads due to snow, wind and occupancy. However, Eurocode 1 Part 1-3 uses more detailed and refined parameters and coefficients that reflect the latest research and data on roof load effects.

Both standards use ultimate limit state and serviceability limit state combinations to check the roof load effects. However, Eurocode 1 Part 1-3 uses different partial factors and combination factors that are consistent with the other parts of Eurocode 1 and Eurocode 0 (EN 1990:2002), which provides the basis of structural design.

Both standards use maps and tables to provide values of ground snow load and basic wind speed for different locations and altitudes. However, Eurocode 1 Part 1-3 uses more accurate and updated maps and tables that cover a wider range of countries and regions in Europe.

Both standards allow for national annexes that provide additional or alternative information for specific countries or regions. However, Eurocode 1 Part 1-3 has more national determined parameters (NDPs) that allow for more flexibility and adaptation to local conditions and practices.

You can find more information about Eurocode 1 Part 1-3 on the Eurocodes website or on the Wikipedia page . You can also download Eurocode 1 Part 1-3 PDF from various sources, such as Docserv or Scribd.

## Conclusion

In this article, we have explained what BS 6399-3:1988 is, why it is important, how to use it, where to download it, and how to update it. We have learned that BS 6399-3:1988 is a British Standard that provides a code of practice for imposed roof loads due to snow, wind and occupancy. We have also learned that BS 6399-3:1988 has been superseded by Eurocode 1 Part 1-3, which provides a harmonized and updated approach to roof load calculations.

We hope that this article has been helpful and informative for you. If you have any questions or comments, please feel free to contact us. Thank you for reading.

## FAQs

Here are some frequently asked questions and answers about BS 6399-3:1988:

What is the difference between flat roofs and pitched roofs in BS 6399-3:1988?

Flat roofs are roofs that have a slope of less than or equal to 10 degrees, while pitched roofs are roofs that have a slope of more than 10 degrees. Flat roofs and pitched roofs have different values of minimum imposed roof loads and shape coefficients for snow loads in BS 6399-3:1988.

What are the snow zones and wind zones in BS 6399-3:1988?

Snow zones and wind zones are geographical areas that have different values of characteristic ground snow load and basic wind speed in BS 6399-3:1988. They are based on historical data and climatic conditions of the UK and Ireland. They are shown in maps and tables in BS 6399-3:1988.

What are the categories of use in BS 6399-3:1988?

Categories of use are classifications of roofs according to their function and occupancy. They affect the values of characteristic value, occupancy factor and reduction factors for occupancy loads in BS 6399-3:1988. They are divided into two main groups: A for residential, institutional, educational and commercial roofs, and B for industrial and storage roofs.

What are the partial factors and combination factors in BS 6399-3:1988?

Partial factors and combination factors are numerical factors that are applied to each type of roof load to account for uncertainties in their values and variations in their effects. They are used to combine different types of roof loads for ultimate limit state and serviceability limit state combinations in BS 6399-3:1988.

What is the difference between normal and exceptional combinations in BS 6399-3:1988?

Normal combinations are combinations of roof loads that represent realistic situations that may occur during the lifetime of the structure, while exceptional combinations are combinations of roof loads that represent extreme or accidental situations that may occur rarely or exceptionally. They have different values of