Yesterday I talked about why one might want to build a bowstring truss; today, the opposite. Per a few questions that came up, I’m going to address the questions of identity – are these “Belfast trusses”? are these “lattice trusses”? – in “the Weird” post, coming soon.
Some of the problems with bowstring truss roofs are generic: for example wood trusses, like all wood structure, are prone to fire damage. But the specific way in which these trusses were used in the US makes the problem worse. I’ve seen several dozen bowstring-truss roofs, and none of them had the slightest attempt at fire protection. No plaster ceilings below, no enclosures around the truss. Since they were generally used here for cheap industrial buildings, that makes sense: if you have to go to the trouble of fire-protecting them, they may very well have no longer been the inexpensive option.
The picture above is what kicked off this whole topic. I was on my way to a project site in Carroll Gardens, Brooklyn, and saw the demolition as I came out of the subway. You can pretty clearly see the trusses on each end of the roof, adjacent to the subway marker (the green and white globe light) and the street sign. This is an area where nearly every building is a rowhosue, but not that building. Here it is in 1904:
“Undertaker & Livery, Coach house” sure sounds like a business that might need (a) a long span roof over a work floor and (b) inexpensive construction. The trusses (and the current demolition) are the left part of the building in the map, which appears to be the shop floor rather than the coach house.
In general, small cross-sections of wood are more vulnerable to fire than large ones, since the char that forms on the surface of a piece of wood as it burns acts as insulation to temporarily protect the wood underneath. Because the 2xs are ganged together in the chords, they don’t have the large ratio of exposed surface area to volume typical of small pieces of wood, so they’re probably better off than than average. The lattice webs, on the other hand, are very vulnerable, and their loss can precipitate overall collapse.
There are three specific issues (two linked) with US bowstring trusses that are not unique to the type but rare elsewhere. The first concerns the most common damage, which is rot at the truss ends, not where they are embedded in the brick walls, but just inboard of the walls. The curve of the upper chord provides natural drainage, and the logical thing to do is to simply let the water run off the roof edges. In most of these buildings, there’s a parapet surrounding the roof, which means that there is a natural gutter between the parapet and the roof curve. If that gutter were properly drained, this detail would be more or less okay. (Snow build-up will still be an issue, but a less frequent and less urgent one.) If. Every building we investigated had some degree of rot where water in that gutter found gaps or seams in the roofing and led to rot in the upper chord.
Every building of this type that I’ve seen had truss lower chords that originally were isolated, not fastened to any structure but the rest of the truss. Those flat lower chords seem to exhibit a gravitational pull for do-it-yourself expansion, and most of the buildings have over the years acquired mezzanines, which is the second issue. You put wood joists spanning truss to truss, supported on the lower chord, and voila! free space. This is in itself a problem because it greatly increases the load on the trusses, to more than they were designed for. The third issue is linked: the truss webs prevent you from walking between segments of your new mezzanine, and those webs are only 2x4s, and you have a saw handy… We saw portions of the lattice web cut away to provide access between mezzanine segments in all but one of the buildings that had mezzanines added.
In short: these truss roofs have all the general problems of wood construction, and also a built-in rot problem, and also are apparently magnets for people acting stupidly.