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Some of the statements in the recent discussion about wood roofs on FGE reefers show a not total understanding of the challenges faced by builders of freightcars in the late nineteenth and early twentieth century, and maybe some general discussion is in order.
The wood roofs we can see in photos can be one of three general kinds, which are virtually indistinguaishable from their outer appearance. These would be:
1.) Single layer roofs, which were just that, a single layer of T&G boards laid over purlins supported by the carlines. These roofs were not very waterproof; they leaked between the boards, but were suitable for some classes of freight, such as livestock.
2.) Two layer, or "double board" roofs were an improvement. Both layers of boards ran the same direction, from roof peak to eave, the joints being staggered by one half the board width. The boards on the lower layer had channels milled in their surface, their purpose was to collect the water that leaked through the joints in the upper layer and direct it out to the eves. Unfortunately, capillary action would still cause some water to migrate to the joints in the lower layer, where it then dripped on the contents of the car.
3.) Single layer wood roofs applied over an "inside metal roof" that consisted of light gauge sheet metal panels. The wood roof kept the light gauge metal from being damaged by trainmen walking on the roof; the purpose of the sheet metal was to capture the water that leaked between the boards and direct it out to the eaves. These roofs can be spotted by the rather boxy fascia that covered the drainage opening.
The ideal roof would seem to have been a single waterproof membrane, but it wasn't, for one simple reason; the freightcars would "weave" as they moved down the track. Unlike a building, which sits on a solid foundation, freightcars were supported at two points,one near each end. If dips in the track at opposite ends of the car were severe enough, the entire car structure would twist. The effect on the roof can be easily illustrated by taking an Accurail kit box, grasping one end in each hand, and twisting. While each of the five panels that make sides, ends, and floor remain rectangles, the open top of the box becomes a parallelogram, angled alternately one way, then the other. The car builders of the day were unwilling to to add the weight necessary, and expend the headroom, to build a roof structure capable of resisting this, and this weaving motion would literally tear the roof membrane to shreds. Sheet metal would crack and tear, nail holes would wear into slots, interlocking joints would spring open, and the roof would shortly be no more waterproof than a wood roof. The multiple iron clasps one sees in illustrations of metal roofs from the early years of the twentieth century were an unsuccessful attempt to deal with this constant motion, as was the inside metal roof mentioned above, with its floating panels.
By the 1920's, the sucessful outside metal roof systems all had words like "pivoted" and "flexible" in their names; in these systems the panels were designed to accommodate the movement of the roof structure, having pinned connections at relatively few points.
The true solution was the Murphy "solid steel" roof, which was finally strong enough to resist weaving, while being light enough to be acceptable to the railroads. This roof design became the basis for the roofs used on the AAR standard boxcars of the mid century; these were so effective that they drive all competing roof systems from the market.