We’ve talked about joists before, but let’s go a little bit further and discuss how they’re laid in a typical rowhouse. In buildings framed exclusively or predominately of wood, joists tie in to other wood members; they’re usually either nailed or fastened with a joint, and they generally sit on a sill plate or another horizontal member.
When rowhouses were built, however, they did not feature extensive vertical framing along the walls, so there would have been no horizontal member to fasten the joist to. Enter the joist pocket.
Joist pockets are recesses built into masonry walls that allow for joists to sit snugly inside. During construction of the walls, bricklayers would interrupt the bond to accommodate these recesses. These pockets were often crude affairs, with bits of brick thrown together with gobs of mortar to create the desired opening. Joists were then inserted and set atop one wythe of brick, with the rest of the wall built around the newly created level.
Rowhouse joists do more than just offer support for floors and ceilings- because they are tied into the masonry, they act as rigid structural members between brick walls that would otherwise be teetering towers of hardened clay. The weight of the joists (and the flooring, fixtures, furniture, humans they support) bears on the walls and prevents them from leaning in either direction.
Since walls were built around joists once they’d been laid in a pocket, it’s tough to remove the entire joist without disturbing the bricks around it. For this reason, we cut our joists as close to the wall as possible, leaving a little joist stub in the pocket.