Building components · Service Zones & Penetrations
Duct Void
Explains the duct void as a bounded horizontal route, distinct from the general spaces above ceilings and below floors that services also occupy.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a duct void is
A duct void is a horizontal space formed and enclosed so that services can run along it. It may be built into a floor construction, boxed out below a ceiling, or formed as a bulkhead along a corridor, but in every case it is bounded on all sides and follows a route rather than covering an area.
That boundedness is what separates it from the spaces a building already has. A duct void is constructed around what it carries, so it runs where the route goes, it stops where the route stops, and it takes its requirements from the elements it passes through rather than from the room it happens to be in.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a ceiling void. A ceiling void is the whole space between a ceiling plane and the structure above, present across a room whether services use it or not; a duct void is a bounded route formed for a run and ends where that run ends.
- It is not a riser shaft. A riser is vertical and its central problem is how it is closed at every floor it crosses; a duct void is horizontal and its central problem is which walls it passes and what happens where it passes them.
- It is not a service void system. That system is the reserved and coordinated distribution zone taken as an assembly, with its depth, its occupants and its access provisions; the duct void is the bounded space itself, and one route may be enclosed differently along different parts of its length.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- horizontal service duct
- duct enclosure
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The elements it passes through, since a bounded route that crosses a wall or a floor makes a crossing at every one of them.
- Being closed where it passes a line the building relies on for separation, because continuity along its length is the whole point of the void and the whole difficulty at that line.
- Access along its length rather than only at its ends, since whatever it contains is reached through the enclosure that hides it.
- Being coordinated in section as well as on plan, because its depth is set by what has to cross within it and by what it has to pass beneath.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A bounded route running between areas links those areas, and where it passes a line the fire strategy relies on it is interrupted there by provision determined by qualified designers and by local requirements.
A route running from one room to the next past the wall between them is a path around that wall, which is why the route and the separating line are considered together rather than in turn.
A route enclosed on all sides is maintainable only where access has been provided, and a long run reachable only at its ends is sealed along most of its length.
The enclosure is commonly built by one trade around work installed by several others, so the void's contents and its formed size are settled at different moments by different people.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Clarifications
Commonly misunderstood
- That any space services happen to run in is a duct void. A ceiling void, a subfloor void and a wall cavity all carry services in some buildings without having been formed as routes, and each exists for reasons of its own.
- That enclosing a route deals with it. The enclosure bounds the space; whether the space is interrupted where the building requires separation is an entirely separate provision.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What route does this void follow, and which walls and floors does it cross along the way?
- Where does it pass a line the building relies on for separation, and how is it closed at that line?
- Where is access provided along the route, and does it reach everything that will need maintaining?
- What determined the void's depth, and what has to cross within it?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes a position. It states no void size, route, closure arrangement, fire rating or period.
- Where a service route crosses a separating or fire-resisting element, what is required there is determined by qualified designers and by local requirements.
Service Zones & Penetrations
The voids, shafts, chases and openings left in the fabric so services can pass through it and be reached again afterwards.
Browse all service zones & penetrations entries →