Building components · Roof Carcass & Edges
Chimney Stack
Explains the stack as a penetration through the whole roof build-up and as the most exposed masonry on most buildings, separately from the flue it contains.
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 stack is
The chimney stack is the part of a chimney at and above roof level: the construction that crosses the roof plane and stands clear of it. Below the roof the same construction is enclosed by the building; above it, it is a free-standing element with four exposed faces, an exposed top, and a continuous junction all the way round its base.
It matters for two reasons that have nothing to do with each other. It is a hole through every layer of the roof at once — structure, insulation, air barrier, underlay and covering all stop at it and have to be reconnected around it — and the part standing above the roof is, on most buildings, the masonry most exposed to weather and the least often looked at.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a flue. The flue is the void and its lining, the path that products of combustion travel; the stack is the construction around it. A stack can be entirely redundant, with no flue in use at all, while remaining fully exposed to weather and fully a penetration through the roof.
- It is not a masonry pier. A pier thickens or stiffens a wall within the building and is weathered by the wall it belongs to; a stack passes through the roof plane and is weathered on four faces and a top, which is a different condition from anything a pier experiences.
- It is not a parapet. A parapet continues a wall along a roof edge and presents three exposed surfaces in a line; a stack is an isolated projection that the roof covering has to be returned around on all four sides, with water arriving at it on the upslope face and shed away on the downslope one.
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.
- stack
- chimney above roof level
- chimney breast head
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- How the covering is returned around all four faces, since each face meets the slope at a different relationship and needs a different treatment.
- How the roof's insulation and control layers are continued around a penetration that is neither small nor regular.
- Its own condition above the roof line — the pointing, the capping and any damp-proof course within it — because that part of the masonry is wetted on every face.
- Its restraint and stability where it stands well clear of the roof, which depends on its construction and on what remains connected to it inside the building.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Masonry above a roof is wetted on four faces and on top, dries in wind, and freezes without the shelter that a wall below a roof enjoys, which is why deterioration is commonly further advanced here than anywhere else on a building.
Water reaching the inside near a chimney has frequently entered at the junction around it rather than through it, and the two possibilities call for different work.
Everything about a stack requires access at height, which is why its condition is commonly assessed only when something else has already gone wrong.
A stack is one of the few elements where a combustion route, a structural element and a weathering junction occupy the same construction, which is why work touching any one of the three is commonly found to have touched the other two. Which of them a given stack still serves is established by inspection rather than by appearance, and nothing about the masonry from outside indicates it.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Materials
Commonly incorporates
Materials this element commonly contains as a distinct constituent part, rather than being made of.
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 a disused chimney stops mattering. The stack remains a large exposed masonry element and a full penetration through the roof, and it continues to be weathered whether or not anything passes through it.
- That removing a stack simplifies a roof. It replaces one junction with another and touches structure, covering and whatever the stack was connected to inside the building.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- How is the roof covering returned around each of the four faces, and what forms the junction at the upslope face?
- How do the roof's insulation and control layers continue past this penetration?
- What is the condition of the masonry, the capping and any damp-proof course above the roof line?
- If the flue is disused, what is intended for the stack, and what does that mean for ventilation of the flue below?
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 an element and its junctions. It states no height, no unsupported height, no flue arrangement, no separation distance and no fire rating, and none can be inferred from it.
- Chimney and flue work engages structural, weathering and combustion-safety matters at once; it is work for qualified professionals and is governed by local requirements rather than by any general description.
Roof Carcass & Edges
The battens, boards and edge elements that finish a roof structure and prepare it to receive a covering.
Browse all roof carcass & edges entries →