When our custom construction company builds additions, concrete
block is our foundation of choice. All of the plans we bid on
(most of which include full basements) specify block rather
than poured foundations. And to keep our lives simple, we don't
subcontract the work — we do it ourselves.
Such was the case with a two-story addition that we began
building last year. Measuring approximately 15 feet by 26 feet,
the addition expanded the home's existing kitchen, added a new
entry and family room on the first floor, and created space for
a new master bedroom suite above. With easy outside access
through a bulkhead door and 8 inches more headroom than the
existing basement, the new basement gave the homeowner a place
to set up a long-anticipated woodworking shop.
Blockwork doesn't require a large capital investment —
just a few masonry tools and a mixer (we bought our electric
mixer online at a substantial discount because of a slightly
bent wheel). We don't need to own, rent, or store forms, and we
never have to worry about a blowout.
We feel more comfortable building 8-foot-high basement walls
with block than we do forming walls 8 feet high; doing all of
the blockwork ourselves gives us more control over our schedule
and the quality of our work.
Layout and Excavation
After laying out the addition according to the drawings and
spray painting the ground to mark the approximate outside edge
of the footings (Figure 1), we began our excavation. Digging 3
feet beyond the layout lines helped provide the room we'd need
later for laying the foundation drain, parging, and
Figure 1.The spray-painted lines represent the
approximate outside edge of the addition's footing; the
excavator dug 3 feet beyond them to allow room for laying the
foundation drain, parging, and dampproofing. Woodchips were
used to cushion the impact of the excavator on a nearby tree's
To protect a large nearby oak, we consulted a municipal
forester, who recommended cushioning the tree's root system
with a 4-inch layer of woodchips and saw-cutting all exposed
roots 1 inch or larger in diameter (rather than ripping them
out with the backhoe). We also did our best to distribute the
backfill around the property to avoid burdening the root system
with too much weight in any one place.
We established the exact depth of the dig by measuring the
height of the existing basement from the floor to the underside
of the sill, which came to 84 inches. Then we added 8 inches
for additional basement headroom, 4 inches for the new slab,
and 4 inches to allow for the underslab stone.
This put the bottom of the dig 100 inches below our benchmark,
the underside of the existing sill (see illustration).
Once the excavator reached the existing footing, we used that
as our reference point for the rest of the dig, verifying the
depth with a laser level as we went.
Then it began to rain; over the next eight days, 10 inches came
down. Fortunately, we were prepared — we'd extended the
two leaders that served the gutters above the excavation to
drain beyond the hole. The soil drained well, but we still lost
three weeks to the rain and to clearing minor cave-ins.
Pouring the Footings
When things had dried out a bit, we returned to the job and set
up our batter boards and string lines to represent the outside
edges of the new 12-inch block foundation walls.
To create a brick ledge, the top two courses would be built
with 8-inch block, so we also measured 4 inches inside the
lines and ran parallel string lines to represent the last two
courses of the narrower block.
With the lines in place, we plumbed down into the excavated
hole with a plumb bob and marked the outside edges of the
foundation on the soil. Using these markings, we laid out the
24-inch-wide footings, which extended 6 inches out from either
side of the 12-inch-wide foundation wall.
The bottom of the footings needed to be 8 inches lower than the
excavated hole (or 24 inches below the top of the existing
footing), so we hand-dug an 8-inch-deep by 30-inch-wide trench
in which to form our 12-inch-high by 24-inch-wide footing
(Figure 2). This left 4 inches of footing above the excavated
surface, which would accommodate the 4-inch layer of
3/4-inch-diameter stone we'd lay down before pouring the
Figure 2.After excavation, the author's crew dug
the 8-inch-deep footing trench by hand, using the top of the
existing footing as a benchmark.
Besides providing a necessary base for the slab, the stone
created a mud-free surface to work off during foundation
We ripped 12-inch-wide strips of 5/8-inch plywood for the form
walls, and used 30-inch-long form pins to secure them. As we
installed the form walls, we used a spirit level and our
transit to keep the top of the forms at the correct elevation
Figure 3.Placed in an 8-inch-deep trench dug into
the bottom of the excavation (top left), the footings were
reinforced with parallel lengths of 1/2-inch rebar wired
together and held off the ground and away from the form edges
by chairs (top right). For safety, the author's crew makes it
standard practice to cap exposed stakes and rebar
To meet code, we used "chairs" to carry the footing rebar,
keeping the rebar 3 inches inside the forms. (Rebar driven into
the ground or extending outside concrete is subject to
corroding, wicks water, and generally weakens the structure.)
For footings, we usually run two lines of parallel rebar
throughout and space perpendicular pieces every 12 inches, tied
off with wire.
As we proceeded, we took the precaution of capping the pins.
Since caps are cheap and take only a few minutes to install,
we've gotten in the habit of always capping both the pins and
the rebar. Not only does this make for a safer site — it
scores points with inspectors on what is typically their first
visit to the project.
For the footing pour, we constructed a plywood chute to carry
the concrete into the hole (Figure 4). While the truck's own
chute and extensions could reach the hole, they stopped 7 feet
above our footings; more concrete would end up on the ground
than in our wheelbarrows. With two men running wheelbarrows
from the truck to the chute, two men running wheelbarrows in
the hole, and one man working the 3,000-psi concrete into the
forms, the plywood chute allowed for a very organized and
rather calm pour.
Figure 4.A plywood chute made it easier to place
concrete for the footings (left). Later, the chute was used to
slide stone (right) and block into the hole, and to pour the
Later on, we also used it to slide the stone and block into the
Laying the Blocks
After we'd had a little fun sliding close to 600 blocks down
the chute for stacking, we set up our mixer, brought in sand
and Type S mortar, and got down to business laying block. Once
again, we dropped a plumb bob from the lines above, this time
marking the foundation corners on the new footings. Then we
snapped chalk lines between the corners to represent the
outside edge of the block foundation.