Siphonic Roof Drainage: The 10 Commandments

Aviation is the safest mode of transportation. It's no wonder since checklists have been successfully used in the industry since the 1930s. Today, checklists are used in many other fields as well. In the field of Siphonic Roof Drainage, I expect the designer to check the pipework before release for construction execution using a checklist. Here is our proposed checklist in relation to our DrainStar(R) Siphonic Calculator software.

Checklist for Siphonic Roof Drainage

Every Pipework has to fulfil the following 10 rules:

1.   Downpipe
2.   Vertical connection pipe
3.   Start-up requirement
4.   Pressure loss Calculation
5.   Vacuum (static pressure)
6.   Minimum flow velocity
7.   Flow velocity in the transition area
8.   Drainage capacity of roof outlets
9.   Distance between the roof outlets
10. Drainage capacity in the planning stage

Commandment 1: Downpipe

In Siphonic Roof Drainage the diameter of the downpipe shall be same or smaller than the adjacent collecting pipe.

Siphonic Roof Drainage pipework in the transition area from the horizontal collecting pipe to the downpipe. — Rule 1 is fulfilled.

Commandment 2: Vertical connection pipe

VDI 3806 – The diameter of the vertical connection pipe may be larger or smaller than the diameter of the vertical roof outlet spigot.

BS 8490:2007 – The diameter of the vertical connection pipe must not be larger than the diameter of the vertical roof outlet spigot.

Siphonic Roof Drainage pipework with roof outlet, vertical and horizontal connection pipe. — In this example, elements 15 and 16 have the same diameter. This rule is thus fulfilled according to VDI 3806 and
BS 8490:2007.

Commandment 3: Start-Up Requirement

The calculation of the start-up requirement must be carried out at start-up height of < 100cm.

Siphonic Roof Drainage pipework in the area of a vertical roof outlet connection. — The lengths of the two elements 15 and 16 add up to 100cm. Rule 3 is therefore fulfilled.

Commandment 4: Pressure Loss Calculation

In the planning stage of a siphonic roof drainge pipework, the variation between available pressure and pressure loss must be +- 100mbar.

Flow path table with calculation values from a siphonic roof drainage pipework. — The red control light clearly shows you a fault in flow path 15 in this pipe system. Rule 4 is not fulfilled.

Commandment 5: Vacuum (Static Pressure) in Siphonic Roof Drainage

In the hydraulic calculations, the vacuum pressure must not be less than -900mbar. The operating pressure limits of the pipe system must be taken into account.

Table with calculation values Static Pressure of section parts of a Siphonic Roof Drainage pipeworks. — The static pressure at the end of each section part (TS) is okay. See the green control light. Rule 5 is fulfilled.

Commandment 6: Minimum flow velocity

Flow velocity of at least 0.7m/s is required according to VDI 3806!

Table with calculation values flow velocity of section parts of a Siphonic Roof Drainage pipeworks. — The flow velocity in all section parts is okay. Rule 6 is fulfilled.

Commadment 7: Velocity of flow in the transition area

In the transition area from a siphonic system to an open drainage system, the flow velocity must not be higher than 2.5m/s.

Table with calculation values flow velocity of flow in the transition area in a Siphonic Roof Drainage pipeworks. — At a higher flow velocity, there is a risk of damaging the downstream components. Rule 7 is fulfilled.

Commandment 8: The drainage capacity of the roof outlet

The drainage capacity of the roof outlet in Siphonic Roof Drainage must not be selected as greater than the maximum drainage capacity of the relevant roof outlet.

Dialog with display of nominal discharge capacity of a siphonic roof outlet and table of calculation values per flow path in a siphonic roof drainage pipe. — Commandment 8 is okay, the highest current drainage capacity in the pipework is less than the nominal drainage capacity of the selected roof outlet. Rule 8 is fulfilled.

Commandment 9: Distance between the roof outlets

The distance between the roof outlets in a shared drainage axis must not exceed 20 meters.

Isometric drawing of a Siphonic Roof Drainage pipework to control the distances between the roof outlets. — Rule 9 is fulfilled.

Commandment 10: Drainage capacity in the planning stage

In the planning stage of Siphonic Roof Drainage pipework, the effective drainage capacity of all the outlets allocated to a roof surface or roof segment is not allowed to fall short of the nominal drainage required by the calculation.

Anzeige Dialog der erforderlichen und der berechneten drainage capacity von einem Siphonic Roof Drainage pipework. — The drainage capacity of this pipework is okay. Rule 10 is fulfilled.

Closing words

At the end of this blog I would like to ask all designers to follow and apply these 10 commandments. It should be second nature to you, as it is to any airplane pilot. There are other important rules, tips and tricks that I will come back to in other blogs. The examples in this blog were created using Keidel’s DrainStar® Siphonic Calculator, the compact software for sizing siphonic roof drainage.

DISCLAIMER – This Information is based on German laws, standards, and regulations (rules). We shall give no guarantee that the information is suitable for use in other countries or that it is in accordance with the rules in place there. Nevertheless, we are unable to guarantee the correctness, completeness and up-to-dateness of the information and therefore exclude any liability in this respect. Subject to change without notice. © Keidel DrainStar GmbH. All rights reserved. V 2110

Dipl. Ing Rolf H. Keidel

Managing Director