Curtain wall

Frequently asked questions

What is the difference between stick and unitised?

Where the assembly happens, and it changes the programme, the tolerance regime and the cost base. A STICK system arrives as components — mullions, transoms, gaskets, glass — and is built up in place on the building, working off a scaffold or a mast climber. It is cheaper in material, forgiving of an irregular building, and slow, weather-dependent and dependent on site workmanship for the seals that keep water out. A UNITISED system arrives as completed storey-height panels made in a factory, lifted into place and hooked onto brackets, with the joints between units designed as split mullions that engage as each panel lands. It is faster on site, needs no external access, and is far more consistent because the sealing is done in a controlled environment — at the cost of a long lead time, a large factory order committed early, and tolerances at the brackets that the structure has to actually meet. Tall buildings go unitised for the programme; irregular and low-rise ones stay stick.

How does a pressure-equalised joint keep water out without a seal?

By removing the force that drives water through a gap rather than by closing the gap. Water crosses a joint when three things coincide: water at the opening, a gap for it to pass, and a pressure difference pushing it. Sealing everything attacks the second and is fragile — one failed bead, one shrinkage crack, and the water is inside with nothing to stop it. A pressure-equalised design attacks the third instead: the outer face is deliberately open, a chamber behind it is vented to the outside so the air pressure inside the joint matches the wind pressure outside, and the airtight line is set well back at the inner face. With no pressure difference across the outer opening, water has nothing pushing it inward; the little that enters by gravity or momentum runs down internal gutters and out through weep holes at the transoms. The consequences on site are the ones that surprise people: weep holes must never be sealed, water appearing inside the frame may be the system working, and a repair that simply caps the outer joint with sealant converts a drained wall into a trap.

Why is the movement joint at the slab the detail that decides everything?

Because the wall and the building move differently and the brackets have to allow it without letting the facade carry load. The frame shortens elastically as floors are added, concrete creeps and shrinks for years, floors deflect under their own imposed loads, and the whole building sways in wind — while the curtain wall expands and contracts with temperature on an entirely different cycle. If the connections do not permit that relative movement, the facade starts to carry load it was never designed for: glass goes into compression and cracks, gaskets roll out of their channels, and the joints between units close up until they bind. So the brackets are slotted in the directions movement is expected and fixed in the direction load is transferred, and the joint width between units is a calculated figure rather than a constructional gap. It is also the reason a curtain wall cannot simply be surveyed once at design stage: the structure's tolerances at every bracket are the input, and discovering them after fabrication is the expensive order of events.