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01 Cross-sectional comparison of glazing bars from c. 1689 to c. 1775. Source: Data compiled by David Wrightson from the Brooking National Architectural Museum, published in The Conservation and Renewal of Timber Windows: A Local Public Authority Leaflet, Cygnor Gwynedd Council, Wales: No Date, used with permission.
02 A glazing bar joint allowing the insertion of a 0.5 mm thick feeler gauge blade.
03 Close view on the joint between a glazing bar and the bottom rail of a fixed sash. The timber at the lower extremity of the glazing bar has absorbed adhesive from the joint and thereby locally stiffened by a mechanism similar to chemical impregnation; the bar has fractured just above the glued joint. The fracture location likely coincides with the extent of glue absorption into the timber.
04 Cycling positive and negligible( or negative) wind pressures result in rotation at joints leading to opening and closing action. Illustration by Danielle Hynard.
05 Black staining at joints in Western Red Cedar window joinery due to tannic extractives being transported in solution caused by excess moisture in the framing timber.
06 Water leakage from the bottom corners of glazed panels can be symptomatic of incorrect glazing technique.
07 Sealants perform the function of waterproofing when timber glazing beads are used to retain the glass. Sealant thickness was compromised in an attempt to fit timber beads in rebates intended to receive putty.
08( L-R) The glazing bars, machined to the original windows’ profile could accommodate glass and putty; changing the design to use timber glazing beads and sealant would have required substantially deeper glazing bars; the built glazing detail. Illustration by Danielle Hynard. and correctly claimed by coating products, sounds like it would cope with this issue with ease, the percentage by which these small joints can widen is substantially greater than 300 and is beyond paints’ elastic limits.
Consider a joint or crack measuring 0.1 mm in width; its widening to the insignificant sounding dimension of half a millimetre represents a significant proportional increase in its width of 400 per cent.
Lesson: In large timber-framed windows, the structural job of glazing bars needs to be considered. Glued stub-tenoned joints may not be stiff enough and deflection may lead to joints opening.
HISTORIC GLAZING METHODS
The use of timber glazing beads backed up with sealing compounds in lieu of contiguous puttying likely stemmed from a desire to avoid the time-consuming aspects of puttying-in glass( rebates in timber window frames need to be painted prior to application of putty to prevent premature drying and cracking of the putty, and final painting of completed windows needs to wait until the putty has sufficiently cured). However, simple substitution of beads for putty is ill-advised.
To confirm one of the sources of water leakage in beaded timber-framed windows, as shown in Figure 06, the beading from the perimeter of a glass panel was removed to reveal insufficient thicknesses of back and front applied silicone sealant, as shown in Figure 07.
The rebate in the glazing bars had been machined to match those in the building’ s original windows which had been glazed with putty, so there was insufficient space for the proper application of sealant and the fitting of timber glazing beads to the bars. The sealant, the component which performs the function of excluding rainwater, had been compromised to save space, as shown in Figure 08.
Lesson: There are reasons far beyond appearance which underpin the dimensions and geometry of glazing bars. Be mindful of all aspects of the windows’ construction.
Windows Magazine Autumn 2017 17