- Concrete Block Foundation Walls
blocks are available in various sizes and shapes, but the most widely
used come in modular sizes 8 in. (200 mm) high, 16 in. (400 mm) long,
and 6, 8, 10 or 12 in. (150, 200, 250 or 300 mm) wide. The actual size
is 3/8 in. (10 mm) less than the modular size to
allow for the mortar
Block courses (rows)
start at the footings
and are laid up with 3/8 to ½ in. (10 to
12 mm) mortar joints. No joint should exceed 3/4
in. (20 mm). All joints should be tooled smooth to resist water seepage.
Full bed and head joints
should be used in the bottom course. Succeeding courses may be laid with
mortar applied to the contact surfaces of the block. Pilasters
projections that normally protrude into the basement
space. They are sometimes required by building codes to strengthen a wall
or support a beam.
Ensure that they are placed at a height where they can properly support
beams if necessary. In these situations, they will often need to be at
a height lower than the top of the foundation.
Special concrete blocks, such as universal, pier
or sash blocks, should
be used to frame the sides of openings for basement doors and windows.
For example, sash blocks (
18) have a keyed face or recess
into which the frames are connected, thus providing rigidity and preventing
Proper sill and lintel
details should also be used to achieve the same effect.
Block walls should be capped either with 2 in. (50 mm) of solid
masonry or concrete, or with a mortar filling in the top course of
blocks. Alternatively, where termites are not a problem, a wood plank
2 in. (38 mm) thick and the same width as the wall may be used. At
grade, another separation should be introduced to prevent convection
currents in the cores
of hollow masonry walls. This separation can be achieved with a strip
between the top two courses, by filling the top course with mortar, or
by using a solid masonry
In all cases, the siding should overlap the foundation wall by at least
½ in. (12 mm) so that the rainwater cannot reach the top of the
foundation. Pilasters supporting beams should be capped with 8 in. (200
mm) of solid masonry.
Freshly-laid block walls should be protected from below-freezing temperatures.
Freezing of the mortar before it is set will result in low adhesion, low
strength and joint failures. Mortar mix proportions should conform to
those shown in Table 5.
Concrete-block walls should be parged on the outside with at least ¼
in. (6 mm) of Portland
A cove should be formed on the outside perimeter joint between the footings
and the wall (
19). The wall should then be dampproofed by applying at least one
heavy coat of bituminous material over the parging
up to the proposed ground level. For added protection where quantities
of water accumulate in the soil,
two layers of bitumen-saturated
membrane may be mopped on and coated overall with a heavy coating of bituminous
material. This covering will prevent leaks if minor cracks develop in
the blocks or joints between the blocks.