Historic Structural Detail: Diagonal Sheathing

by Don Friedman on February 29, 2016

Every once in while, when you strip the outer sheathing (clapboard or shingles) or the inner finish (plaster) off a wall, you find that the exterior sheathing is a bunch of diagonal planks.

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Why?

A topic that I keep returning to is that people in the past had to be clever because they had fewer tools than we did. That’s “tools” in the broad sense, meaning all of the technologies we use in construction today, from computerized analysis to manufactured lumber. In this case, the modern technology that is missing is plywood.

The traditional, pre-industrial method of providing lateral bracing to wood-frame building was to use diagonal braces between exterior-wall posts and the floor-level girts within the walls. This method works fine, but requires large-dimension timbers and a lot of fancy carpentry to create the mortise-and-tenon connections. The introduction in the mid-1800s of the houses framed with 2x and 3x “sticks” required a different system. One possible solution, used mostly before 1900, was to “let in” 2x braces running diagonally in the same plane as the 2×4 or 2×6 wall studs. This still required a lot of relative difficult work, notching each stud on an angle to create the space for the brace to fit. (The braces couldn’t be simply nailed to the stud faces or they would interfere with the sheathing.) These braces continued the previous idea of discrete braces providing stability.

A different idea was to use what we would now call a diaphragm: a planar structural element loaded in its strong direction, in-plane. When we use plywood as exterior-wall sheathing, that’s what we’re doing, creating a lateral-load diaphragm. To do it without plywood took some thought. Horizontal plank is extremely inefficient as a diaphragm, as its lateral-load resistance is limited to the moment that can be developed by a pair of nails through a plank-to-stud connection. It’s true that there are a lot of those connections and the resistance is additive, but the resistance at each connection is so small that the totaling it from all of the connections is still weak. Diagonal sheathing, on the other hand, connects the studs to the floor structure (either rim joists, or the top and sole plates, depending on the frame type) and creates a whole lot of the triangles that engineers love for bracing. This form of bracing works so well that it is still in the codes as an acceptable method of bracing a building, and since the diagonal planks are a continuous plane of sheathing, they can just be nailed to the outer face of the studs.

That’s why.

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