This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
248. As a rule, no more woodwork should be placed in a wall than is absolutely necessary. Wooden lintels supporting walls are especially objectionable, because, besides being combustible, they are also liable to shrinkage. Framing lumber is very seldom thoroughly dry, and, when a brick wall is supported in part by a wooden lintel, a crack is almost sure to develop, as shown at a in Fig. 105. It is quite obvious that this crack is caused by the wooden lintel b shrinking, and permitting that part of the brick wall resting on it to settle by an amount equal to the shrinkage of the wood, while the portion of the brick wall c, being carried on the brick pier, does not settle.
At d, Fig. 105, is shown a small crack such as is often seen just above the end of door or window sills. Such cracks usually occur near the bottom of high walls, and are caused by the compression of the mortar in the lower joints of the pier. If slip sills are used, these cracks may be avoided.
249. Sometimes, templets, formed of pieces of studding, are laid under the ends of floor joists to give a level bearing to the joists, and distribute the weight over the walls. Their use is not recommended, because they are very sure to shrink and leave the wall over them unsupported. If used at all, they should not be put in buildings over two stories high, and in walls less than 12 inches thick, and the very dryest, best seasoned timber procurable should be used. Strips of wood are often built in the joints of brick walls, to form a nailing place for wood finish or furring strips. It is not well to use these in buildings over three stories high, because the weight of the wall and the shrinkage of the wood will cause the plaster to crack. If these nailing strips are used, they should not be over 3/8 inch thick, or not wider than the mortar joints. Wooden bricks, that is, blocks of wood corresponding in size to the bricks in the wall, are sometimes built in for nailing places for door and window trim, furring, etc. The great objection to their use is that they generally shrink and become loose, thereby losing their holding power.
Porous terra-cotta blocks may be had, the size of a brick, and should be used as nailing blocks, in all first-class work. These blocks do not shrink, and will hold a nail as well if not better than wood.