CASE STUDY | WINDOWS and – correctly – riveted only on one side of each paired mullion, intended to prevent water from entering mullions through their otherwise open top ends, and weep slots machined into sills, suggesting that water in mullions was intended to seep into sills and then away. For the curtain wall units above those on the lowest level of this building, this water penetration management strategy was effective( figure 03).
At the bottom of the façade, outside observation suggested the presence of another stack joint – but gave no clues as to what supported the apparently continuous lower sill member( image 04). However, looking inside the lower floor mullions with a rigid borescope( inserted in carefully drilled holes) showed that the mullions extended to floor level – about a metre below the‘ stack joint’. The sill member was not continuous.
APPEARANCES DECEIVE
The lower floor’ s façade appeared to match the building’ s remainder, however, its actual construction was markedly different.
03
05
Figure 05 is the curtain wall manufacturer’ s crosssectional detail through the sill in question. It shows that the lowest curtain wall unit mullions in fact punctuate the lowest sill members, negating the possibility of draining unit frames via sills as was done effectively elsewhere on the building.
Instead, the mullions punch through the lowest sills and extend downwards to bear the units’ weight onto the reinforced concrete floor slab’ s upwardfacing surface.
As is typical of such drawings, the intended water drainage mechanism is illustrated with sequences of teardrop shapes indicating the assumed drainage pathways of collected water. However, in this drawing, there is a hole shown drilled or punched in the side of the mullion – it is highlighted in green in figure 05. The hole was annotated‘ drain hole’.
Holes in the mullions, in positions where one would expect to find them in generic aluminium shopfront framing to allow drainage into mullions, – simply provided this very pathway- a means for water trapped in sill extrusions and under wind pressure to be pushed into the mullions and to then be deposited onto the office’ s floor slab within the spandrel zone.
Just how the shop detailer thought that water within the mullions would leap through the drain holes against wind pressure and then drain away is a mystery.
LESSON
Things are not always what they seem to be; the right method of investigation is key to diagnosis. Don’ t stop until the problem is understood.
04
06
01 Diagram illustrating the concepts of the unitised and stick curtain wall systems: unitised( left) and stick( right).
02 Downward view within the spandrel zone, taken via an access port cut into the plasterboard spandrel zone lining. Cold-formed steel stud framing on the right of the image exists to support and position interior plasterboard wall lining and MDF sill trims. Bottom-centre of the image is an extruded aluminium pocketed shopfront section, abutting a powdercoated aluminium vertical member. The finish of this vertical member matches that of the curtain wall framing members.
03 Extract from the curtain wall manufacturer’ s shop drawing of the project’ s typical stack joint. The detail shows water, illustrated with classic teardrop-shaped icons, exiting the framing system via weepage slots machined into the sills’ upstanding legs [ coloured in blue ]. Water in unit mullions is discharged into sills which punctuate mullions and are made effectively contiguous with local and close-fitting sleeves. Mullions are coloured in red.
04 The base of the curtain wall intersects with a nearhorizontal ledge. This image was captured during our investigation into leakage via the ledge due to incorrectlysealed joints between aluminium-faced composite cladding panels.
05 Extract from the curtain wall manufacturer’ s shop drawing of the lower extremity of the curtain wall. Highlighted are; red- mullions, extending through‘ stack joint’ zone; blue- weep slots machined into sill extrusions’ upstanding legs; green- drilled or punched‘ drain hole’ in mullions.
06 Three-dimensional visualisation showing water draining from the sill into the mullion, defying gravity and challenging common sense. Illustration by Danielle Hynard.
Simon Owen is Associate Director of Building Diagnostics at Jackson Teece Architecture.
WINDOWS MAGAZINE 19