Building components · Service Zones & Penetrations
Builders Work Opening
Explains the builders work opening as provision made ahead of the service, and why the interval between forming it and using it matters.
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 builders work is
A builders work opening is a hole made in a wall, floor or roof by the trade building that element, for a service that does not yet exist. It is set out from coordination information rather than from the thing that will occupy it, which is why it is commonly more generous than whatever eventually passes through.
Its significance is sequence. Forming an opening while an element is being built is a different operation from cutting one into a finished element, and the difference shows in the material around it: reinforcement arranged for the opening, support provided over it, edges formed rather than broken. The consequence is an interval — the opening exists, open and oversized, for as long as it takes the service to arrive.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a service penetration. The penetration is the crossing described by the service that makes it; a builders work opening exists before any service is there, and some are never used by one.
- It is not a pipe sleeve. A sleeve lines the opening and holds the pipe clear of the structure; a builders work opening is commonly left unlined, and what closes the space around the service is a later and separate provision.
- It is not an opening head. A head is the top of a door or window opening, where the wall above is carried and water in the wall is intercepted; a builders work opening is a route through an element for a service, and it is concealed rather than finished.
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.
- builder's work opening
- BWO
- builders work hole
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Coordination information being right before the element is built, since the opening's position is committed at the moment the element is formed around it.
- The element's own design allowing an opening where one is wanted, which in a structural element is a question answered before construction rather than negotiated after it.
- How the surplus space around the eventual service is closed, because the opening is deliberately generous and what fills it is not the service.
- What happens to it between being formed and being used, since an open hole through an element is also an open route through it.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The opening is set out before the service exists and the service is set out afterwards, so the two rarely arrive concentric; the gap around a service is the working result of that, not a designed allowance.
An opening formed in a structural element changes how load travels around that position, which is why the provision is designed with the element rather than requested from it afterwards.
An opening that has been formed and not yet closed is an open route through the element, and whether that element is one the fire strategy relies on determines how seriously the open interval is treated.
Which openings were formed, which were used and which were abandoned is worth having recorded, because an unused opening covered by a later finish is invisible and still there.
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 builders work opening and the penetration through it are the same thing. One is provision made in advance; the other is the condition created when a service actually crosses.
- That an unused opening is harmless. It remains a gap through the element, and it is far more often covered by a finish than properly closed.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What coordination information were these openings set out from, and when was that information fixed?
- Who designed the openings through structural elements, and what was provided around them?
- How is each opening closed once the service is in place, and who is responsible for that closure?
- Which openings were formed and never used, and how have those been dealt with?
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 opening size, allowance around a service, closure arrangement, fire rating or period.
- Forming an opening in a structural element is design work for qualified professionals; verify against 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 →