Glazing bars

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THE TROUBLE WITH GEORGIAN GLAZING BARS

Independent glazing consultant Philip Rougier collects his thoughts on Georgian glazing bars
(he doesn't like them very much).

January 1998, Glass and Glazing Products, "Tech Talk"
Copyright © Philip Rougier 1998

In the bad old days, someone had to find a way to make up big window panes from the little bits of glass then available, so invented glazing bars - and created a nest of problems that have intrigued the industry for 300 years. Now big panes are available, so we don't need glazing bars - but for some strange reason people still want them. So, we cut big panes into little pieces, then reassemble them to make big panes again. IG units are forced into 12mm rebate upstands, and sometimes even puttied in, against all the best advice. Increasingly, we don't even do it for real - we simulate Georgian glazing bars by stuffing aluminium pressings into IG unit voids, or we bond plastics or wood mouldings direct to outside of the glass. In many ways we are still in the bad old days.

Architects and Planners talk in terms of "rhythm of the elevation", "interest" and "integrity" - I called Richard Pierpoint RIBA of RJP Architects to check the latest architectural jargon. "People like them because they break up a window" he said, and had the grace to laugh when I suggested a simpler way to do that. Georgian bars are suitable for single glazed applications, of which precious few remain. Double glazing in these components (just like other frames) needs deep upstands that result in chunky sections, offending the aesthetes. IG's need drainage and ventilation, unless you really want them to fog up, that in these little glazing beads requires the Building equivalent of micro-surgery. The only use I have for Georgian bars is the profitable litigation of defects that haunt them. I object to them, in any material. But feel free !! I don't mind if the industry continues to specify and install them, if the industry doesn't mind my giving evidence to Judges on why they've failed. Please, carry on forcing IG units into the things !! Go on, solid bed them!! Don't drain or ventilate the rebates !! Form 12mm upstands that expose the spacer bars and edge seals, and allow them to bake in the sunshine !! Come to think of it, if the industry stops doing these things, I'1l be out of work.

But there are genuine cases of need. Someone telephoned a while ago asking why some original and reproduction 18th century Georgian windows leaked. He explained that a big country house, heavily Listed and bristling with Royal connections, had been severely fire-damaged a few years before. It had now been reconstructed. Some of the original 1700's window frames survived intact, others were fastidiously copied right down to the concealed joints. The world and his wife attended the site visit - the joinery and glass manufacturers, the sealant people, several conservation types, members of committees, all but the typist from the contractor's shed - and everyone had a theory. I didn't ask, but it seemed there'd been quite a lot of discussion about this problem, I was the last resort and instant answers were wanted - apparent when one of them opened with "So what's the answer ?", while I was still slipping into my white coat. The answer was simple, and it was this : no matter how precisely you make a set of timber glazing bars, the wood shrinks across the grain. And at a cruciform joint it shrinks in four directions at once, all away from each other. Of course the joint leaks. People forget, but in the old days it was necessary to repeatedly scrape off the surface paint and force fill the joints, sometimes for several years after construction, until the timber shrinkage stabilised and the leaks stopped. And with modern kiln-dried timbers, which don't get the years of natural seasoning that some of the 18th century timbers did, the problem is worse. "But that doesn't explain why the original windows leak now !" piped up one my army of theorists in rising inflection. Oh indeed it does, I replied - and went into that too. A fire raged through the building for about a week while fire-fighters did things with hose-pipes, then the whole edifice was left to the mercy of the elements for a few months while everyone decided what to do with the ruin. When building work started, the contractors began by spraying everything with water to clean it up. The structure went virtually unheated for a couple of years during refurbishment, then, when it was buttoned up, they turned on the heating to dry it out. In there somewhere is a classic imitation of the kiln-drying process, with a bit of randomised natural seasoning thrown in for good measure. Of course the old windows leaked in just the same way as the replicas, and for precisely the same reason - the mitred cruciform joints were shrinking in four directions simultaneously. The windows are made of wood, and all wood, even 18th century wood, does that ! It's only a building - just bricks, timber and so on, and old or brand new, all the materials react in the way they always do. The conservators were the only ones who didn't laugh when I said they had some absolutely authentic 18th century leaks here. I went on to explain that these bars were really too slender for local wind conditions, and that their flexing would tend to open the joints. I raised an eyebrow to the joiner, who nodded back. "They"ve been fine for hundreds of years" muttered a conservator, which sounded like one of many litanies that had guided the rebuilding process. Well, they're sensitive, delicate things these glazing bars - and the cruciform joints are one of the main weaknesses. This time the joiner nodded without prompting. These days we limit deflection to L/125 for single glazing, and (doing some rough mental arithmetic) here that amounts to about 12mm. In full storm conditions these very slender bars will deflect 25mm or so, each way, and quiver to stop after each gust. The contractor said "ouch" and the joiner looked at his shoes. "You tell us the problem but what's the answer ?" grilled another theorist, giving me a sense of what it must have been like during the Spanish Inquisition, 100 years before this place was built.

I offered two solutions, take your pick. The first involved exactly what the original owner's 18th century minions would have done - where you notice a leak, scrape off the surface decoration and force fill the joint with paint. Keep it up for as many years as it takes, until all the leaks stop. I drew a deep breath before offering the second solution. Now, this could offend some of the more purist conservators, but the contractors could sort it out right away with modern materials - stabilise and (incidentally) strengthen the joints, injecting them with a low-viscosity structural epoxy wood repair system. I suggested the main contractor call Nickerson Chemicals (01924-461341) whose products have been used in many historic buildings. They'll probably recommend drilling deep into the joint from both sides at unobtrusive locations within the enclosed rebate areas, and injecting "Timbabuild ERC" (Epoxy Rapid Cure) used with a special 2-part applicator gun. Nickerson tell me they can't make any serious money from jobs like this because they hardly use any material, so they just bask in the publicity a little.

Don't get me wrong, I have nothing against timber window frames - quite the reverse in fact. Trees are our only renewable resource for these products - everything else (plastics, steel, aluminium, fibreglass) irretrievably consumes a lump of Planet Earth. Gone for good. Properly designed, installed and maintained (they don't need much), timber frames make a fine building component, and farmed wood is healthy ecology. But with the single exception of preserving museum pieces, I have only one piece of advice - don't even think about Georgian glazing bars.