# Mullion and Transom: The Grid That Carries a Curtain Wall

> Mullions are the vertical members of a curtain wall and transoms are the horizontal members that span between them. Mullions carry wind and self-weight back to the building's slabs through brackets; transoms carry the glass weight to the mullions and divide the facade into panels, and their joints are where most of the drainage design happens.

## Why it matters on a project

The mullion and transom grid is the skeleton of a curtain wall. The glass gets the
attention, but the grid decides three things the client will notice later: whether
the facade stays dry, whether it moves with the building without cracking anything,
and how the elevation reads from the street.

## Who does what

| Member | Direction | Main job | Where it fails |
| --- | --- | --- | --- |
| Mullion | Vertical | Spans between slab brackets, resists wind, carries the grid | Undersized for the span, stack joint without movement allowance |
| Transom | Horizontal | Supports the glass above, divides panels, collects drained water | Unsealed end joints, blocked weep slots |
| Bracket | At slab edge | Transfers mullion load to the structure, absorbs site tolerance | Adjustment used up by poor slab-edge accuracy |

## How load travels

Wind pushes on the glass. The glass passes that load to the mullions and transoms
through its supports, the transoms pass theirs to the mullions, and the mullions
carry everything to the slab through brackets. Glass self-weight sits on setting
blocks on the transom and is carried to the mullion through the transom
connection.

The design wind pressure for that chain comes from **IS 875 (Part 3)** for the
specific site and building height. Structural performance is commonly proven by
test to **ASTM E330**, and the curtain wall as a product is described by
**EN 13830**.

## How water travels

In a drained system, some water always crosses the outer seal. The transom is the
gutter: it catches that water and leads it sideways to the mullion or out through
weep slots. That only works if:

- the transom-to-mullion junction is sealed exactly as the system supplier details it,
- weep slots are present, the right size and kept clear, and
- the cavity behind the glass can equalise with outside pressure.

Water penetration is proven by test, commonly to **ASTM E331**, on a mock-up
before production.

## Common mistakes

- **Skipping the transom end seal** because it is hidden once the glass goes in.
- **Locking the stack joint** with sealant or fixings, so floor movement loads the glass.
- **Choosing sections from a catalogue** instead of from a calculation for the actual span and wind pressure.
- **Mixing system components** from different suppliers, so drainage paths no longer line up.
- **Leaving the mock-up test until late**, when changing a profile means re-ordering extrusions.

## Standards referenced

**EN 13830** for curtain walling; wind loading to **IS 875 (Part 3)**; structural
and water testing commonly to **ASTM E330** and **ASTM E331**. Section sizes,
bracket positions and movement allowances for a specific building must be set by
the project's facade engineer.

## Frequently asked questions

### What is the difference between a mullion and a transom?

Orientation and job. A mullion runs vertically, usually floor to floor, and is the primary member that spans between slab brackets and resists wind. A transom runs horizontally between two mullions, supports the glass or panel above it, and closes the grid. In most stick systems the mullion is the deeper, stronger section and the transom is fixed into it.

### Why is the transom joint so important?

Because water that gets past the outer seal collects on transoms. A stick or unitized system relies on the transom to gather that water and lead it to the mullion or out through weep slots, so the way the transom meets the mullion, and how that junction is sealed, decides whether the drainage path actually works. Most curtain wall leaks trace back to a transom end that was not sealed as detailed.

### How is the size of a mullion decided?

By structural calculation for the specific building. The design wind pressure comes from IS 875 (Part 3) for the site, height and exposure, and the mullion section is chosen so that stress and deflection stay within the limits set in the project specification. Span, bracket positions and glass type all feed into it, so the section cannot be read off a generic table.

### Do mullions and transoms carry the building's load?

No. A curtain wall is non-load-bearing: it carries its own weight and the wind on it, and passes those loads to the structure through brackets. It must not pick up slab or beam loads, which is why mullion joints include movement allowances so the floor above can deflect without loading the glass.

### Why do some mullions have a split or stack joint?

To let the building move. Floor slabs deflect, the frame sways in wind and aluminium expands with heat. A joint at each floor, often a spigot sleeve or, in unitized systems, a split mullion where two half-sections interlock, lets each floor's section move independently without stressing the glass or tearing the seals.

## Sources

- [EN 13830](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Curtain walling — product standard
- [IS 875 (Part 3)](https://standards.bis.gov.in/website/published-standards/department-wise) — Wind loads on buildings and structures
- [ASTM E330](https://www.astm.org/products-services/standards-and-publications.html) — Structural performance of windows, doors and curtain walls by uniform static air pressure
- [ASTM E331](https://www.astm.org/products-services/standards-and-publications.html) — Water penetration of exterior windows and curtain walls

---

Source: https://hagerstone.com/glossary/mullion-and-transom