Building components · Facade Support
Transom
Explains the transom's dual role of supporting glazing weight and forming the horizontal drainage line.
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 transom is
Transoms run horizontally between mullions in a glazed facade. They carry the weight of the glass above them through setting blocks, and they divide the facade into panels.
They are also the horizontal collection line in the facade's drainage. Water that passes the outer seals runs down the glazing and arrives at the transom, where channels collect it and take it out to the mullion drainage.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a mullion. Mullions are vertical and carry wind load to the structure; transoms are horizontal and carry glazing weight to the mullions.
- It is not a rail in a rainscreen. A rainscreen rail carries opaque panels; a transom carries glazing and drains.
- It is not a lintel. Nothing of the building is carried across an opening; the transom carries glass within a facade.
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.
- transoms
- horizontal framing member
- rail in a glazed facade
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Its span between mullions and the weight of glazing it carries.
- The setting blocks positioning the glass, which determine where the glass load actually arrives.
- The drainage channels within it, and whether they connect to the mullion drainage.
- Its thermal break, for the same reason as the mullion's.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Glazing weight arrives at the transom through discrete setting blocks rather than along its length, which makes their position significant.
The transom is where water within the facade is collected horizontally before being taken out, so any interruption in that channel traps water.
Like the mullion, a continuous metal member from outside to inside would be a heat path and a condensation surface without a break.
Transom-to-mullion junctions are where the drainage of the two directions must connect, and they are the detail curtain walling depends on.
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.
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.
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Facade
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That transoms are visual divisions. They carry the glazing and form the facade's horizontal drainage.
- That the glass rests along the transom. It sits on setting blocks at defined positions, which is a different load path.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What glazing weight does each transom carry, and over what span?
- Where are the setting blocks, and does the transom support them?
- How does the transom drainage connect to the mullion drainage?
- Is the thermal break continuous through the transom-to-mullion junction?
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
- No section, span, load, setting block position or drainage arrangement is stated here.
- Curtain wall design and glazing support are specialist engineering matters.
Facade Support & Panels
The brackets, rails, subframes and framing members that hold a facade off a backing structure, and the panels they carry.
Browse all facade support entries →