It’s hard to overstate the importance of a solid roof to a structure, but let’s try: the roof is the most important thing in the history of buildings, ever. Ok, so that might be a stretch, but it’s not that much of a stretch.
The basic function of the roof is to protect the building (and its inhabitants) from the elements, be they rain, snow, wind, sun, dust, you name it. In our work, we often see what happens when a roof is compromised. As soon as water is able to penetrate the building envelope, the condition of the interior deteriorates rapidly: water trickling down party walls causes bricks to erode and plaster to crumble. These bits then accumulate on soaked floorboards, which themselves sit on water-logged (and therefore rot-prone) joists. After a decade or so, you’ve got a pile of mush surrounded by brick walls.
Having established the importance of a roof, let’s turn our attention to the way that late 19th and early 20th century rowhouse builders built these things. There’s more variation than you might expect, so we’ll tackle this subject by looking at different blocks of houses.
First up is the 1500 block of E Federal Street, built around 1888.
In this framing system, 2.5×8 rafters spanning the width of the house are pocketed into opposing party walls every five to eight feet. Purlins, 12-16′ long 2x4s spaced every two feet run front to back, spanning three rafters. In other words, the purlins span from one rafter to another, bridging and, because of the long span of a relatively flimsy 2×4, bearing on a third rafter in the middle of the span.
Atop and perpendicular to the purlins, 1″ thick roof decking spans from party wall to party wall.
What’s notable about this framing system is the significant spacing between the beefy rafters. The long purlins are being asked to do a lot of work holding up the roof decking, tin, and tar. Supporting this weight, the purlins flex downwards. If the purlins are supple, they’ll bend and transfer weight to the rafters. If the purlins are brittle, they’ll crack.
If I had to bet my ducats, I’d say that the builders of these houses thought they could get away with using as few 2x8s as possible, relying on cheaper 2x4s to carry the load. The results are mixed: true, most of the roofs in the houses we worked in were intact, but all of them showed seriously stressed rafters or purlins. Even if the initial framing was sturdy enough to support the original roof coverings, it’s clear that it was not up to the task of supporting decades’ worth of additional layers of tar.