There is much diversity in building regulations regarding the thickness of party walls, although they all agree in that such walls should never be less than 12 inches thick. About one-half of the laws require the party walls to be of the same thickness as external walls; the remainder are about equally divided between making the party walls 4 inches thicker or thinner than for independent side walls.
When the walls are proportioned by the rule previously given, the author believes that the thickness of the party walls should be increased 4 inches in each story. The floor load on party walls is obviously twice that on side walls, and the necessity for thorough fire protection is greater in the case of party walls than in other walls.
In buildings of the skeleton type the outer masonry walls are usually supported either in every story or every other story by the steel framework, and carry nothing but their own weight. Such walls may, therefore, be considered as only one or two stories high, and are usually made only 12 inches thick for the whole height of a twelve or fifteen-story building.
As a rule, no more woodwork should be placed in a brick wall than is absolutely necessary. Wooden lintels for supporting brick walls are objectionable not only on account of their being combustible, but also on account of their shrinkage. It is generally impossible to obtain framing lumber that is thoroughly dry, and when a brick wall is partially supported by a wooden lintel a crack is quite sure to develop sooner or later in the manner shown in Fig. 151. The crack is obviously caused by the shrinkage of the lintel, which permits the portion of the wall supported on it to settle by an amount equal to the shrinkage of the wood, while the portion of the wall a, being supported on the brick pier, does not settle.
Bond timbers, or pieces of studding laid under the ends of the floor, joist, are also objectionable, for the reason that they are quite sure to shrink, and thus leave the wall above them unsupported. Bond timbers are very convenient for the carpenters, as they give a level bearing for the floor joist, and they also distribute the weight over the brickwork, but they should never be used in buildings over two stories in height, or in walls less than 12 inches thick. If used at all they should be selected from the dryest lumber that can be obtained.
Strips of wood are sometimes built into walls to form a nailing for the wood finish or for the furring strips. Such strips should not be used in buildings over two stories in height, and should not be over 3/8 inch thick, so that they may take the place of the mortar joint.
Wooden bricks, or blocks of wood of the size of a brick, are also sometimes built into brick walls to provide nailings for furring strips, door frames, etc. These not only tend to weaken the wall, but they generally shrink so as to become loose, thereby losing their holding power. If the bricks are so hard that nails cannot be driven into them, and the mortar is too poor to hold the nails, then porous terra cotta blocks should be used for nailing strips in first-class work.
Porous terra cotta will hold a nail almost like a board, and has none of the objections common to wood.