Probably the most frequent cause of the cracking of masonry walls is the settlement of the foundations, either from their being improperly designed or from the settlement of the ground caused by wet. A strict observance of the principles laid down in Sections 24, 29 and 30 will generally prevent cracks starting from the foundation.
The effect produced on certain soils when they become saturated with water is described in Section 9.
Next to faulty foundations, the most common cause of cracks in brick walls is probably the use of wooden lintels, as described in Section 255.
It is very common to see a small crack just above the end of a door or window sill, as shown in Fig. 152. Such cracks generally occur near the bottom of high walls, and are caused by the compression of the mortar in the lower joints of the pier. They may be avoided by using slip sills, as described in Section 191.
Another place where cracks produced by the settlement of mortar joints sometimes occur is where a low wall joins a very high one. To prevent such cracks the walls should be joined by a slip joint, as described in Section 206.
Generally cracks are more likely to occur in walls that are broken by frequent openings than in a plain, unbroken wall.
The use of plenty of anchors and thorough bonding will do much toward preventing cracks.
When buildings are built on ground that is continually moist or wet the moisture is very apt to soak up into the walls from the foundations, rendering the building unhealthy and often causing the woodwork to rot. To prevent the moisture rising in this way a horizontal damp-proof course should be inserted in all walls below the level of the first floor joist. It should be at least 6 inches above the highest level of the soil touching any part of the outer walls, and should run unbroken all around them and at least 2 feet into all the cross walls; and on very wet ground, where the water is but a few feet below the surface, it should be continuous through all the walls. In buildings finished with parapet walls it is also desirable to insert a damp-proof course just above the flashing of the roof or gutter to prevent the wet from soaking down into the woodwork of the roof and into the walls below.
Materials. - These damp-proof courses may be formed of either of the following materials :
Asphalt. - A layer of rock asphalt § of an inch thick makes the best damp-proof course, and should be used for all first-class buildings. The surface to receive the hot asphalt should be quite dry and should be made smooth to economize material, and all the joints should be well flushed up with mortar. The best asphalts for this purpose are the natural rock asphalt from Seyssel, Val de Travers or Ragusa, which are imported into this country in the shape of blocks and cakes. When used the cakes are melted in large kettles, mixed with a small proportion of coal tar and applied hot. One or two layers of tarred felt imbedded in the hot asphalt may also be used with good results.
"Roofing slates, or even hard vitrified bricks, two courses breaking joint, laid in half cement and sand mortar, or such bricks laid without any mortar in the vertical joints, form an inexpensive damp course." Glass is also sometimes used for this purpose.
Portland Cement. - A ½-inch layer of Portland cement mortar, mixed in the proportion of 1 part cement and 1 of sand, will often answer the purpose, but is not as desirable as the materials mentioned above.