The stability can be increased by filling them with concrete, but the allowable strength cannot be considered to have been raised. Tests have shown that this filling does not increase the strength, because of the difference in the elasticity of the two materials. TYPES AND CONSTRUCTION There are two types of hollow terra-cotta blocks, one which builds with cells vertically and the other which builds with cells horizontally. This latter is generally an interlocking tile. The strongest wall for vertical-load resistance is built with vertical-cell tiles. [Illustration: 20. Support of floor-joists 18. 12" Hollow-tile wall Cells Horizontal] All hollow-tile should be laid in Portland-cement mortar, and the webs should be arranged so that they build over one another. The bearing of floor beams and girders on walls, built with blocks of vertical cells, should be made by covering the tile with templates of terra-cotta slabs, filling them with concrete or protecting them with plates of steel. Where chases are required for pipes they should not be cut into the wall, but special blocks should be used to build around them. All lintels under 5 feet should be constructed with tile arches, reinforced with concrete and steel rods inside of their webs. [Illustration: 17. Vertical cell Hollow-tile wall] PRECAUTIONS AGAINST DAMPNESS [Illustration: 21. Construction of lintel] [Illustration: Brick Veneered Hollow-tile wall] In order to prevent the penetration of moisture the mason should butter all joints on the inside and outside edges, leaving an empty space between, in order to insulate against the transmission of moisture through the joint. To prevent the collection of mortar in the cells of the tile, due to droppings during construction, the spreading of metal lath over the top of each course of tile will accomplish this and also make the strength of the wall greater. Although it is often recommended that hollow-tile be plastered directly upon the interior, yet this is not safe in those sections of the country where there are driving rain-storms. For this reason it is advisable to fur them on the interior. It is also recommended that a waterproofing compound be added to the stucco applied to the exterior. Another fact should be observed: namely, that all door and window frames, since they are of wood, will tend to shrink and thus open up the joints and permit the leakage of rain-water. Oakum should be stuffed behind all brick moulds to prevent this. Care should also be taken to make drips under all sills, so that no water will leak into the interior of the wall. All belt courses should also have steep washes. Stucco should not be carried down to the grade level, but a course of solid material, like brick, concrete, or stone, should be built at this point. VENEERING