Incompatibility: Cavity Walls and Veneer

by Don Friedman on October 17, 2016

How McKim, Mead and White handled a solid wall, with brick shown as diagonal hatching and stone as stipple hatching:

mckmw-monograph-v2-dragged


I previously talked about the problem of providing support in cavity walls and how that is inherently different from the way in which masonry walls evolved. That’s actually the lead-in to a bigger problem, which is how we’ve divorced appearance from construction.

Stone veneer of a traditional, bearing-wall building is thick and cannot move relative to the back-up. The veneer is part of the body of the wall, differing in appearance but not function or construction. Stone veneer in a cavity wall is, by definition, thin and fastened to the body of the wall by flexible metal ties. A lot of modern cavity walls have thermal insulation in the cavity, meaning that the veneer is subject to temperature swings not experienced by the back-up masonry; even when the insulation is on the inside face of the back-up, the back-up is restrained against thermal movement in a way that the veneer is not. This is all a long-winded way of saying that stone veneer in cavity walls moves differently than the body of the walls.

How do engineers handle differential movement? We put in expansion joints, with one logical location being where the movement changes direction. Left to ourselves, we would put straight, vertical expansion joints right at every plan corner in the veneer…which would completely destroy the vital illusion that the veneer was part of a traditional wall. The expansion joints can be moved off the corner, and can step around through the regular joints, but this makes them less efficient without greatly improving their appearance.

If only flat veneer stones are used, their lack of thickness is exposed at the corners. The work-around is to use stones that are L-shaped in plan, to make it look like the veneer actually consists of large stone blocks. The L stones are difficult to support using the ordinary details and may move peculiarly with temperature swings, further damaging the illusion of solidity.

I’m not arguing for “structural honesty,” largely because I don’t think such a thing exists. I’m arguing that when you use a specific architectural style with construction technology other than that for which it was developed, you have two options: live with some funny-looking details the expose the fakery, or watch it fail. In other words, the architectural appearance of am ashlar masonry wall is incompatible with thin-veneer cavity construction.

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