WINDOWS | CASE STUDY
NOT ALL JOINTS STACK UP
A CASE STUDY IN WATER LEAKAGE FROM AN
ALUMINIUM-FRAMED CURTAIN WALL
SIMON OWEN
Associate Director,
Jackson Teece Building
Diagnostics
T
here are several generic types of curtain
wall systems, each of which has a purposely
simplistic name:
• The stick system (figure 01 - right) - this first
requires the installation of vertical framing
members into which infills of panelled or
framed materials are installed.
• The unitised system (figure 01 - left) - this is
essentially a façade made up of discrete framed
window and spandrel units which mate with
adjoining units at heads, sills and jambs.
• Further systems - which are essentially hybrids
of the stick and unit system, such as the stick
and panel system.
It is essential to recognise the type of curtain
wall under assessment to be able to diagnose
causes of its failure, as each type employs
particu lar mechanisms for the management of
water penetration and differential movement of
materials.
The term stack joint is used to describe an
interface between (typically) storey-tall portions
of curtain wall cladding (where the cladding of one
storey meets the cladding of the next). Usually,
storey-tall cladding incorporates both vision
glazing and spandrel cladding zones. When dealing
with a unitised curtain wall system, it can be useful
to think of the stack joint as simply a head-to-sill
junction between separate and discrete window
and cladding units.
WATER PENETRATION MANAGEMENT
In unitised curtain wall systems, each panel
– which may comprise of vision glazing and
spandrel cladding, along with thermal and acoustic
insulation, needs to perform in much the same way
as a discrete window unit does. It needs to prevent
the unwanted transmission of air, pollutants, noise
01
18 Winter 2017
and water. And, by means similar to those used
in the manufacture of discrete framed windows,
water which bypasses outer seals and joints is
allowed to flow down through framing members
to be diverted to the outside through carefully-
considered drainage provisions.
Such drainage provisions may take the form of
holes or slots in sills, or they can be as simple as
leaving the ends of mullion extrusions open.
Mullions are generally the curtain wall unit’s
framing members which do most of the structural
work and it normally makes sense for these
members to extend for the full height of the unit
with the head and sill members fitted between
them – especially when the unit’s vertical members
are halves of paired or nested mullion sections
which need to be fitted together to achieve
necessary stiffness.
UNUSUAL SYMPTOMS
A reconnoitre of a Sydney fringe commercial
building showed that during and immediately after
heavy rainfall, water leaked from the spandrel zone
of the lowest commercial floor into tenanted areas.
We created access ports in the spandrel lining and
conducted water testing of the curtain wall and
adjoining ledges. The ledges leaked voluminously
and in a manner which would account for the
pattern of water penetration (image 02). Once
repaired and water tested, the ledges were
confirmed to be waterproof. During a storm, water
again leaked onto the commercial floor.
THE DANGER OF NOT CONSIDERING
THE UNLIKELY
The curtain wall fabricator’s shop drawings
included the detail shown in figure 03 as the
typical stack joint detail for this façade. It shows
folded aluminium plate ‘sleeves’ bedded in sealant
02