The short answer
The details that matter most are the slab edge (brackets, fire stopping and movement), the head and sill where the wall meets roof and ground, the joints between transoms and mullions, and the interfaces with other cladding. Most leaks, cold spots and fire gaps start at these junctions, not in the middle of the glass.
On this page
A curtain wall is a repeating grid, and the middle of that grid is the easy part. The proprietary system has been tested, the profiles are standard and the glass is made to order. The hard work is at the edges and junctions, where the wall meets the structure, the roof, the ground and other trades. Those are the details worth reading closely on any set of drawings.
The slab edge
Every floor slab is a junction. Three things happen here.
Brackets and movement
Brackets fix the wall to the slab and carry its weight and the wind load. They need adjustment in three directions so the wall can be set true even if the slab edge is out of line. Each floor usually also has a movement joint, often a spigot sleeve inside the mullion, so the wall can take up slab deflection and thermal expansion without stressing the glass.
Ask the designer what movement each joint is designed for and how it is sealed.
Fire stopping
The gap between the back of the curtain wall and the edge of the slab is a route for fire and smoke to pass between floors. Approved Document B expects that gap to be closed with fire stopping that keeps the compartment intact. The detail must be compatible with the wall’s movement, so it is usually a tested system of mineral wool and a flexible seal rather than a rigid infill.
Spandrel zone
In front of the slab and the ceiling void, the wall usually has an opaque, insulated spandrel panel. On residential and some other buildings of 18 m or more, regulation 7(2) applies to the materials in the external wall. Glass, frames, gaskets and seals are exempt, but the insulation and backing boards in spandrels are not, so the build-up has to be checked against the rules.
Head and sill
At the top, the wall meets a roof, parapet or soffit; at the bottom, it meets the ground, a podium or a lower roof. These are where water collects and where membranes from different trades must overlap correctly.
| Location | What to check |
|---|---|
| Head | Membrane from the roof dressed over and sealed to the wall; coping or flashing that throws water clear |
| Sill at ground | A threshold or upstand high enough above finished ground; a drained sill so water in the frame can get out |
| Sill at a lower roof | Roof membrane turned up behind the wall and sealed, with enough upstand height |
| Soffit | Insulation continuous with the wall, and a drip to stop water running back |
Transom to mullion joints
In a stick system, each transom meets a mullion, and every one of those joints is a potential leak. Systems differ in how they do it, but most rely on:
- end seals or gaskets where the transom meets the mullion,
- drainage paths so water in the transom runs into the mullion or out through slots,
- an inner line of seals that must be continuous around each pane.
Good workmanship matters more here than anywhere. A missing end seal or a cut gasket will leak, and the water can appear some distance away.
Stack joints in unitised walls
Unitised panels meet their neighbours along split mullions and at a horizontal stack joint at each floor. The joints are formed by gaskets as the panels are lowered into place. They depend on the panels being made accurately and the brackets being set in the right place. Check the joint’s movement capacity and how the seal is completed at panel corners, where four panels meet.
Interfaces with other trades
A curtain wall nearly always meets something else: brickwork, rainscreen cladding, roof systems, doors or louvres. Each interface needs a drawing that shows:
- Who is responsible for each part of the junction.
- How the air and vapour control layers connect.
- How water is drained to the outside.
- How fire is stopped where compartment lines meet the wall.
- How the two systems move relative to each other.
An interface that falls between two packages, with each trade assuming the other is sealing it, is a common source of leaks.
Testing the details
Laboratory tests to the CWCT standard prove the system’s design. On-site hose tests check the joints that were actually built. Test areas are best chosen to include the risky details: slab edges, corners, interfaces with other cladding and the top quarter of the building. See CWCT testing for how the tests run.
When details go wrong
Leaks at curtain walls usually trace back to one of the junctions above. If an existing facade is leaking, a specialist will map where water appears, then water test suspect joints from the outside, working upwards. Read more in curtain wall repair.
Whether you are reviewing drawings or chasing a leak, a curtain walling specialist can look at the details for you. Use the form below and we will look for one that covers the building.