Filling the Joint
Where existing mortar has been removed to a depth of greater
than one (1) inch, these deeper areas should be filled first,
compacting the new mortar in several layers.
The back of the entire joint should be filled successively by
applying approximately 1/4 inch of mortar, packing it well into
the back corners. This application may extend along the wall for
As soon as the mortar has reached thumb-print hardness, another
1/4 inch layer of mortar approximately the same thickness
may be applied.
Several layers will be needed to fill the joint flush with the
outer surface of the masonry.
It is important to allow each layer time to harden before the
next layer is applied; most of the mortar shrinkage occurs during
the hardening process and layering thus minimizes overall shrinkage.
When the final layer of mortar is thumb-print hard, the joint
should be tooled to match the historic joint. Proper timing
of the tooling is important for uniform color and appearance.
If tooled when too soft, the color will be lighter than expected,
and hairline cracks may occur; if tooled when too hard, there
may be dark streaks called "tool burning," and good
closure of the mortar against the masonry units will not be achieved.
If the old bricks or stones have worn, rounded edges, it is best
to recess the final mortar slightly from the face of the
masonry. This treatment will help avoid a joint which is visually
wider than the actual joint; it also will avoid creation of a
large, thin featheredge which is easily damaged, thus admitting
After tooling, excess mortar can be removed from the edge of
the joint by brushing with a natural bristle or nylon brush.
Metal bristle brushes should never be used on historic masonry.