A poorly designed drainage system can significantly shorten the life of the roof sheathing and cause damage in the order of hundreds of thousands of crowns. In doing so The correct solution for the drainage of a flat roof is not difficult — you just need to know the basic principles, be able to calculate the required capacity and select the appropriate system.

In this guide you will learn how to drain flat roofs including calculation formulas and comparison of gravity and vacuum system. Here you will find practical information for the design of drainage of logistics halls as well as administrative buildings.

What to drain the roof: basic principles and systems

The service life of the roof sheathing of a flat roof depends fundamentally on the method of drainage of rainwater. The accumulation of water leads not only to damage to the waterproofing, but can also cause static problems of the entire structure.

Roof drainage can be realized internally (through drains and drains into sewers inside the building) or outside (to the under-roof gutters with a duct along the facade). For industrial halls, internal drainage prevails due to simplicity, for apartment buildings, a combination of both systems is often chosen.

The choice depends on the size of the building, climatic conditions and architectural requirements. The key is to follow the basic rules for the placement and sizing of components.

What helps to drain the roof

When draining a flat roof, first of all, high-quality system elements and their correct installation are essential:

Roof gullies They represent the heart of the whole system. Industrially manufactured parts with an integrated cuff made of the same material as the waterproofing, which can be welded perfectly, are recommended. The minimum diameter is DN 70, but at least DN 100 is recommended to reduce the risk of clogging.

Correct location of ports is critical — it must be at the lowest point of the drained area and at least 500 mm away from attics and penetrations. The body of the gate is always attached to the supporting structure.

Protective baskets lets in prevent clogging of the drain with larger debris (such as leaves, etc.). For roofs with a duck or other aggravating composition, they are used special perforated baskets.

Gutter heating is an important measure against freezing. There are self-regulatory systems to 230V or low-voltage variants with thermostatic control.

Safety insurance overruns include emergency drainage in the event of a blockage of the main drainage system or a sewage accident.

For one internally drained roof, they are designed at least two ports with separate rain wastes. Alternatively, one inlet may be replaced by an overflow, emergency drainage or other appropriate technical measure (see CSN 73 1901-1 - 7.2.3.2.5).

For all coated roofs, emergency drainage must be designed according to ČSN 75 6760, ČSN EN 12056-3 and ČSN EN 752, or drain through the eaves edge.

Inlet flow and gargoyles we calculate individually for each element separately, according to its respective effective area of the roof.

Calculation of drainage of a flat roof

How to size well? The correct design of the drainage of a flat roof is the basis of a functional system. The basic formula is:

Q = r × A × C

Where:

• Q: Outflow in liters per second (l/s)
• r: Rain intensity 0,03 l/s.m² (according to ČSN 75 6760:2014)
• AND: Effective roof area in m²
• C: Drainage coefficient (for impermeable roofs C = 1.0)

Practical example: Hall with an area of 1200 m²: Q = 0.03 × 1200 × 1.0 = 36 l/s

Based on the calculated drain and flow capacity of roof drains, we determine their number,

vertical drainage (drains) inside the building is required

For our hall (1200 m²) we need at least 5 drains DN 100 or 4 drains DN 125.

For a clearer work, we have prepared for you a simple calculator for calculating drainage.

Gravity vs. Vacuum Roof Drainage

Gravity System: A Proven Solution

The gravity system utilizes pipeline gradients and gravity. It is suitable for smaller buildings up to 1000 m² and is characterized by:

• Simplicity of design and maintenance
• Availability of components
• Lower initial costs
• The need for more leads

Drainage of a flat roof into the gutter is carried out by external drainage into the underground gutters (recommended slope min. 0,5%). Inter-channel gutters carry a high risk of breakdowns and are recommended only exceptionally.

Vacuum roof drainage: Modern solutions for large buildings

Vacuum drainage uses the height of the building and the negative pressure in the pipes. During heavy rain, the pipe fills up and a special inlet prevents air access — a siphon effect is created.

Advantages of the vacuum system:

• Smaller pipe dimensions — savings of up to 80% on vertical downspouts
• Freedom of projection — horizontal pipe without gradient
• Fewer temptations — usually one drainage point
• Self-cleaning due to high flow rate
• Space savings at the disposal of the object

The system is ideal for logistics halls, shopping centers, sports complexes and industrial buildings over 1500 m².

Special cases and practical tips

Drainage of the green roof requires a special approach. Although the substrate can retain up to 70% of precipitation, C = 1 is used for the calculation due to safety.

When calculating the main drainage of roofs with a substrate, taking into account the retention capabilities, the drain coefficient could be 0.5. However, it is recommended that it be not less than 1, since not only at the time of construction the drainage would be insufficient, but the investor can decide at any time and replace the roofing substrate with a non-accumulative finish.

Critical is the use of protective baskets or manhole against clogging with substrate.

Drainage of a flat roof over an atique is not recommended because of the risk of freezing. If necessary, gargoyles must be heated. But the security holdup can be dealt with gargoyle through the atique.

Emergency drainage — is mandatory for new roofs with attics and provides drainage of excess water in case of clogging of the main drainage system or sewage accidents..

The most common mistakes:

• Underestimation of rainfall intensity
• Bad location of gullies
• Neglect to use emergency drainage
• Lack of maintenance (inspection and revision at least 2 times a year)

Quality drainage as an investment in the future

Properly designed drainage of a flat roof is a key factor for the long-term functionality of the roof sheathing. Whether you choose a gravity or vacuum solution, it is important to follow the basic principles: correct calculation, quality components and expert assembly.

Saving on drainage can pay off with several times the cost of repairing damage. Investing in a quality system with redundancy always pays off.

Need advice on choosing a system? TOPWET experts will help you with a custom drainage design for your project. Contact us for a free consultation and learn how to design functional and efficient drainage.

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