Simon Owen explains the proper installation of aluminium-framed windows
Despite prescriptive instructions in window system manufacturers’ literature and warnings in entry-level manuals aimed at building investigators, the lack of correct installation of dams at the ends of aluminium-framed window system subsills is a defect that is still encountered so frequently as to no longer be surprising.
Jackson Teece Architecture associate director of building diagnostics Simon Owen says the outcomes of the omission or poor installation of end dams are terrible for industry as well as home owners and says there is merit in applying the principle of connecting the end dam to window head ﬂashings.
In the image to the right of an installation from the 1980s, there is clear evidence of poorly installed sub-sill end dams in the form of dampness and mould growth on the subsill and wall interface. Other symptoms include damp carpet and underlay and corrosion of carpet stretcher fasteners and prongs, particularly in the corner formed by the glazed assembly and wall.
Simon notes that he has encountered numerous creative substitutions for purpose-made components to prevent water collected from spilling over the subsills’ end and wetting ﬁnishes – from deformed and repurposed soft drink cans to pieces of torn fabric – but by far the most popular material has been elastomeric sealant applied ‘pointlessly scant to absurd in its gratuitousness’.
“During recent investigative work, we were asked to justify why elastomeric sealant was not a sound alternative to the ﬁtting of purpose-made end dams,” says Simon.
“We located an assembly where the sealant ‘dam’ appeared intact and on agreement gently hose-tested the window assembly. Within minutes this exercise demonstrated the folly of reliance upon sealant alone [see Figures 1a to 1d].
Manufacturers of window systems that incorporate subsills generally supply purpose-made end dams. When these are properly ﬁtted to subsills, no further work is needed to prevent water leakage from the window system at the ends of subsills. Reliance on sealant caulking between the ends of subsills and adjoining wall construction is reckless because:
- The number of variables which lead to decline in sealant joint performance is large;
2. The success of a sealant joint relies heavily upon substrate preparation and sealant application: the responsibility for this rests wholly with the sealant applicator;
3. Whether a substrate has been adequately prepared to receive a sealant cannot be readily veriﬁed after application of the sealant;
4. The ends of subsills must be fully dammed by abutting wall materials;
5. The damming wall material must be waterproof;
6. The cross-sectional shape of subsills is complex and ensuring that a viscous sealant has fully closed the junction of the subsill and abutting surface is diﬃcult;
7. Sealants require maintenance; the only means of gaining adequate access to the ends of subsills is to disassemble the window or door system.
HEAD FLASHING END DAMS – A DAM LUNATIC TALKING?
British Standard BS 5628 Part 3 – Use of Masonry was revised in 1985 which led the Building Research Establishment [BRE] Housing Defects Prevention Unit in the United Kingdom to revise its own Defect Action Sheet number 15  as DAS 98 in 1987. Both Defect Action Sheets dealt with the penetration of water into buildings around windows. The revised Defect Action Sheet included a requirement from the newly-updated British Standard that cavity trays at window heads [head ﬂashings in OzSpeak] include stop-ends to prevent water which accumulated on them spilling over the ends of the ﬂashings and wetting the inside skins of cavity walls by wetting cavity ties, mortar dags or cavity insulation [not often encountered here], or due to sagging of the head ﬂashing creating a discharge point near or at the vertical face of the inside skin of the wall [see Figures 2a and 2b].
The current Australian Standard does not require such a measure but it also presumes that cavities will be clear and not bridged with mortar or debris and that ﬂexible ﬂashings will be installed and forever keep the shape in which they are shown on architects’ wall sections [which are usually prepared by people who have never had to handle, position, fold, join and clean lengths of bitumen-coated aluminium foil to a deadline] and that weepholes will be installed in the correct course and intelligently positioned.
Given the likelihood that cavities can be bridged, weepholes can be poorly formed and then easily become clogged, and that ﬂexible ﬂashing materials can sag with the passing of time, I present the Brits’ improvement on things for your consideration. It might be about time we stopped waiting to be told how to improve what we do.