c a s e st u dy
HERITAGE TIMBER WINDOWS AND WATER LEAKAGE: TWO LESSONS
SIMON OWEN
Associate of Building Diagnostics, Jackson Teece Architecture
WATER LEAKS FROM GLAZING BAR JOINTS
The evolution of the window was marked by a desire to maximise glazed areas and reduce the interference of framing. As glass manufacturing techniques allowed for larger panel sizes, framing members became thinner, despite increases in spans, as shown in Figure 01.
In large timber-framed windows with thin glazing bars, wind loading causes juddering deflection of the glazing bars and consequent brief, rapid and repeated rotational action at the bars’ restraint points. Protective coatings applied to the timber are stressed at these joints and fracture after repeated deformation. This then allows rainwater to bypass the paint system at the glazing bar-to-frame joints- which slightly open and close as the glazing bars are quickly deformed and revert to their original orientation, as shown in Figure 02.
This action resembles the opening and closing of a hinge and its effect could be likened to a mouth opening and closing to drink water when strong wind is accompanied by rain, as per the illustrations in Figure 04. The forces generated in this action even cause the breakage of timber where adhesives have shallowly-penetrated and locally strengthened the timber, as shown in Figure 03.
Once water enters these joints it is given access to the glazing bars’ endgrain allowing rapid moisture transport within the timber. Water-based adhesives at glazing bar junctions break down, weaken the joints and allow a greater degree of opening and closing, and the cycle goes on.
Those who have committed the sin of using fasteners other than silicon bronze or stainless steel in their Western Red Cedar window joinery, for example, will notice dark staining resulting from the reaction of timber extractives with iron.
Tannic timber extractives are water soluble and will travel in solution once the window framing timber is overly moist. They are then deposited at the timber surface at sites of water evaporation – usually at cut ends of the timber members, or at areas of coating loss, as shown in Figure 05.
Historically – bearing in mind that such windows were luxurious and costly features of grand homes and that maintenance staff were employed to quickly attend to tasks such as this, coatings would have been scraped off and more paint forced into the joints as and when water leaks appeared.
Paints marketed for application to timber window joinery often claim impressive-sounding elastic properties which can be misapprehended by the unwary consumer. While a coating with the capacity to accommodate crack widening by up to 300 per cent- as is frequently
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16 Australian Window Association