To secure proper strength in the wall it is necessary that the two portions of the wall shall be well bonded together, so that neither may buckle or get out of plumb. Until within a few years this bonding was usually accomplished by placing brick headers across the air space, with the ends slightly built into the two portions of the wall, as shown at a, Fig. 154.
Brick bonding, however neutralizes much of the benefit gained by the air space, as it permits of the passage of moisture through the wall wherever it is bonded. The moisture not only passes through the bond bricks, but also through the mortar droppings that invariably collect upon them.
The best method of bonding, and the only one which retains the full benefits of the air space, is by means of metal ties provided with a drip in the centre. Either of the metal ties shown in Fig. 154 may be used. That shown at b is the "Morse" tie, which is made of different sizes of galvanized steel wire and from 7 to 16 inches in length. The other ties are not patented, and may be made by any blacksmith.
That shown at e is probably the best shape where both walls are 8 inches thick, as it gets a firm hold on the walls and is also much stiffer than the wire tie. The iron ties should either be galvanized or dipped in hot asphalt or coal tar.
If either of the ties b, c or d are used they should be spaced every 24 inches in every fourth course. The tie e, being stronger, need be used only in every eighth course.
Wherever door or window openings occur in hollow walls it is necessary to build the wall solid for 8 inches at each side of the opening, and also to carry the relieving arch entirely through the wall. It is almost impossible to prevent some moisture passing through the wall at these points, but much may be done by covering the top of the relieving arch with hot tar and laying the connecting brickwork in cement mortar. The top of the relieving arch is obviously the most vulnerable point, and should be protected in some way and kept as free as possible from mortar droppings. Ventilation of Air Space. - There seems to be some difference of opinion as to whether or not the air space should be connected with the outer air. American writers, however, appear to be generally of the opinion that the air space should be ventilated to carry off the moisture that collects on the inside of the outer portion of the wall.
It is recommended that the bottom of the air space be ventilated through openings into the cellar, and that openings be left in the inner portion of the wall just under the coping of a parapet wall, or above the attic floor joist if the wall is covered by the roof. If the air space cannot be ventilated into the attic, then ventilation flues should be carried up and topped out like a chimney, or built in connection with a chimney. It is also recommended that a U-shaped drain tile be laid at the bottom of the air space to collect any moisture that may run down the outer wall.