What size should my mechanical smoke shaft be?

After you've decided on your flow/extract rate, your next step is to decide on the size of the builder’s work shaft that is needed to transport the smoke to the external atmosphere.

Smoke shafts are essentially a simple ventilation system designed to extract any smoke leaking into a common lobby to protect the escape stairs. Typically a vertical builders’ work duct rising through the building would be used to extract smoke from the lobbies and each lobby would have a damper connected to the builders work duct.


What size should my mechanical smoke shaft be?

Once again, a set of industry standards have evolved and the most commonly used shaft area is 0.6m2, which caters for both the ‘means of escape’ and ‘firefighting duty’ with a maximum duct velocity of 10m/s which is not unreasonable for smoke extract purposes. The aspect ratio of the shaft should not be less than 2:1 and a reasonable rule of thumb would be to go for internal dimensions of 800mm x 800mm. This assumes that the shaft is vertically straight and of smooth and airtight construction with maximum leakage of 3.85m3/hour at 50Pa pressure.


To ensure you have all the information about smoke shaft systems you need, go through the steps we've outlined below and read all the information that is available to you:

  1. Get a detailed understanding of what smoke shafts are and how they operate by reading our blog about the inner workings of smoke shaft systems
  2. Find out what you need to keep in mind when choosing the right smoke shaft for your system in our blog about the specifications that apply to smoke shaft wiring.
  3.  Determine the flow/exact rate you need to use for your mechanical shaft system. The common method of doing that is by constructing a computational fluid dynamic (CFD) model of the building, which you can read more about in our step-by-step guide to smoke shaft systems. 
  4. Download the guide to smoke shaft systems to see what your next steps are!

Download your Guide to Smoke Shaft Systems

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