Building A Block Foundation - Continued
Project manager Danny DoCouto started the blockwork by setting
the corners, laying the blocks in a full bed of mortar. Then we
strung masonry lines between the corner blocks and laid the
rest of the first-course blocks, using the strings to keep the
courses straight and leveling each block with a 2-foot spirit
level (Figure 5). The elevation at the top of the first course
measured 88 inches — exactly 11 courses of block —
below the top of the existing foundation. If we matched the
existing sill, joist height, and subfloor, the floor of the
addition would match that of the existing house.
Figure 5.The bottom block course was set in a full
bed of mortar, creating a strong bond between the footing and
blockwork. The corners were done first, using the string lines
During the first-course run, we had to install a
3-inch-diameter PVC pipe that would later connect to the
foundation drain and bring water into the basement sump well.
I've never been comfortable with the idea of allowing water to
drain into a basement, but on a flat site there's no
alternative: Water that's 8 feet down won't gravity-flow
At the building inspector's suggestion, we installed the PVC
pipe in the foundation wall rather than through the footing,
where it can cause weakening and cracking. We notched the
first-course block with a diamond-blade-equipped grinder,
locating the notch high enough in the block that the pipe would
clear the poured slab. After setting the first and second
courses, we filled the block cores with concrete along the
The plans called for a set of basement stairs on the south side
of the addition, with one side abutting the existing foundation
wall. Therefore, we needed to build a section of block wall
next to the existing foundation to help carry the bulkhead
door. First, though, we had to underpin the part of the old
footing that was exposed during excavation and form up a new
footing. (Eventually, we'd have to underpin the length of the
exposed existing foundation wall, but for the moment we wanted
to keep our focus on the block, so we concentrated on the stair
To keep on course with the rest of the foundation, we formed up
a 12-inch footing (Figure 6), then used 4-inch solid block to
bring us up to 16 inches above the new footing, the equivalent
of two block courses.
To support a new block
wall for the bulkhead door capping the basement stairs, the
existing footing was underpinned and a new 12-inch-high footing
formed and poured.
A story pole marked off in 8-inch increments (the height of
each course) helped keep us on track for succeeding courses.
Each day, we'd set up the transit and mark our story pole,
working off the existing footing. As a second check, we also
measured down from the existing sill, exposed earlier.
Our daily ritual consisted of setting up the transit and
marking the story pole, setting corners, running lines,
leveling blocks, checking walls for plumb — with a level
and plumb bob from the lines above — and checking the
distance to the sill, making sure it was divisible by 8. If we
were off, we'd adjust the 3/8-inch-thick mortar joint to
correct the wall height.
As we laid up the block, we used Dur-O-Wal galvanized truss
reinforcement (Dayton Superior, 800/323-0090,
www.dur-o-wal.com) on every other course to
laterally strengthen the foundation. At the corners, we dropped
in rebar and filled the cores solid with grout all the way to
the top. Where new block adjoined old, we fastened ties to the
existing foundation with a .27-caliber Remington
For additional lateral reinforcement, we built a pilaster
midway through the longest wall; we crisscrossed the blocks,
installed rebar, filled the cores, and ran the Dur-O-Wal
through the pilaster (Figure 7).
Figure 7.To strengthen the block foundation,
workers placed rebar in all the corners, grouted the cores, and
used Dur-O-Wal wire mesh on every other course. A pilaster of
interlocking block reinforced with rebar at the midpoint of the
longest wall provided additional lateral strength.
For the last two courses, we switched from 12-inch to 8-inch
block to create the brick shelf for the brick faade
that would cover the foundation, installing brick ties between
the two courses in preparation for the brickwork (Figure 8).
The brick shelf ended at the entrance to the mudroom door,
where a poured platform stoop would provide a step into the
Except for at the
mudroom entrance (in foreground), the top two courses were
built with 8-inch block to leave room for a 4-inch-wide brick
Brick would also be used to frame the basement window
Parging and Water Management
We parged the below-grade portion of the walls using the same
basic 21/2-to-1 sand/mortar mix that we use to lay our block,
spread evenly over the entire foundation 1/2 inch thick (Figure
9). Adhesion is always tricky, and the exact mix always needs
adjustment for temperature, humidity, and the wetness of the
Parging — applying
a 1/2-inch-thick layer of mortar with a trowel and screeding it
smooth with a straightedge — helped seal the porous
surface of the concrete-block foundation (left), while a
brushed-on coat of cold asphalt (right) dampproofed the
Before applying the mortar, we used a mason's brush to splash
and spread water on the block. Then we troweled on the mortar,
screeding it smooth with a straight board. A lot of mortar
falls during this process, but we pick most of it up and reuse
it. As we proceed, we keep an eye on the work that's already
been done, watching for the right time to wet-sponge-float the
We gave the parging a week to cure, then brushed on a coat of
cold asphalt. The parging fills the pores in the block, and
dampproofing provides one more layer of defense against
moisture entering the basement walls.
Because the asphalt makes such a sticky mess, we like to give
the bituminous coating time to dry before installing the
foundation drain. But the sooner you get the drain in, the
sooner you can backfill. To keep sediment from building up in
the pipe over time, we put filter fabric down before the stone
went in, and then wrapped the stone and pipe with it.
We pitched the pipe from the corners of the addition to the
sump well 1/2 inch to 3/4 inch for every 10 feet, with the
perforated side facing down so that rising water could enter
and gravity-flow to the well.
To prepare for the basement slab, we installed Flexcell
expansion-joint filler (Knight-Celotex, 800/596-9699,
www.aknightcompany.com) around the
foundation's interior perimeter, fastening the 4 inch-high
material to the wall with our Remington. The expansion joint
provided us with a thickness reference and screed point for
pouring the slab (Figure 10).
Before pouring the
basement slab, workers attached lengths of expansion joint
filler to the interior perimeter of the block wall (top), then
laid down a 6-mil poly vapor barrier and wire mesh (middle).
Sheets of plywood and plastic protected the walls from
splashing concrete (bottom).
We also cleaned up the existing footing with a chipping gun in
preparation for underpinning, mixing the debris in with the
loose stone. Then we placed 6-mil poly in the hole as a vapor
barrier, with wire mesh on top.
We set up our chute again, but this time it didn't matter if
concrete ended up on the ground. We didn't want it splashing on
the basement walls, though, so we used plywood and plastic to
protect them during the pour.
At first we used wheelbarrows to move the concrete to the far
corners, and then we just let the material pour into the hole,
working it with a jigger bug to vibrate the stones down and
away from the surface. We bull-floated the slab and, after the
concrete had cured slightly, continued to work the slab with
The following day we backfilled the foundation in 2-foot lifts,
dumping in 2 feet of dirt at a time and compacting it with a
jumping jack until we reached the brick shelf. We compacted by
hand, intentionally keeping the backhoe away from the hole for
fear of putting too much lateral pressure on the foundation
The wall was backfilled
and hand-compacted in 2-foot lifts to avoid putting too much
lateral pressure on the foundation.
To set the anchor bolts, we filled the cores of the top two
courses, using busted block to create a barrier and keep the
grout from dropping all the way to the bottom. Then we set our
12-inch anchors 9 inches into the grout, leaving 3 inches
exposed (Figure 12). The anchors were spaced 18 inches on
center, doubled up at the corners.
The anchor bolts were
set into grout-filled cores so that only 3 inches were left
exposed (top). Then the bolt holes were counterbored to accept
the nuts and washers (bottom).
We counterbored the bolt holes to accept the washers and nuts
holding down the sill, which made it easier to install the
joists (no notching required) and safer to walk the
sills.Rob Corbo is a building contractor in