I run a design-build company in Chatsworth, a neighborhood in
greater Los Angeles. We specialize in large-scale remodels and
custom luxury homes. In this area, some municipalities allow a
new home built as part of a teardown to be permitted as a
remodel rather than as new construction, provided it’s
built on the existing foundation. This can make a big
difference in a project’s cost and scheduling, because
the regulatory track for new construction is often cumbersome
and costly, involving more layers of site engineering and soil
analysis than are required for a remodel.
We were able to take advantage of this distinction with the
project discussed in this article. By the time the clients
hired us, they had already spent nearly a year in preliminary
permitting for the teardown and were understandably ready to
get on with the actual work. The design for the new home would
increase the existing footprint substantially, and all things
being equal, it might have been more practical to completely
remove the existing concrete and start from scratch.
To be absolutely sure that it made sense to keep the old slab,
I evaluated the pros and cons of each approach. For example,
while tying into the existing concrete would take more time
than simply pouring a new slab, I was sure it would be less
time-consuming than the permitting process for a new
I also weighed the cost of demolition — breaking up and
hauling away the concrete — against the cost of leaving
it in place, including having to cut off several feet along the
front edge to meet new city setback requirements. Another
factor was that, with a remodel, the owners would pay a school
tax based on the additional square footage only; with new
construction, the tax would be assessed on the entire
All things considered, it would save several thousand dollars
— as well as speed the work — if we kept the
The old slab foundation is pinned to the new with a rigorous
schedule of epoxied rebar dowels. A self-leveling concrete
topping smooths over irregularities and provides a flat surface
for finished flooring.
Slicing and Dicing the Slab
Besides being set back a few more feet, the new footprint was
oriented slightly askew of the original (see Figure
1), which meant that few of the existing footings
would align with the new work. This wasn’t really a
problem, however, because the original foundation was designed
for single-story loads and would require underpinning in any
case to support the new two-story structure.
Figure 1. The placement of the new building, marked in
spray paint, runs askew of the original home’s layout,
rendering most of the subslab footings obsolete (top). Footings
and pads for the addition are laid out on the sloping lot,
ready for the excavator (bottom).
The layout process differed little from new construction,
except that in some areas we were working with an existing
slab. We cut the concrete to accommodate new interior footings,
column pads, and plumbing and electrical lines (Figure
2). We also cut out a few sections that had gotten
damaged during demolition and excavation, to eliminate the
chance that any of these weak spots might telegraph through to
the new finish floors.
Figure 2. Extensive saw cuts indicate the positions of
new footings within the original slab (top); the trenches
continue into the surrounding soil. The semicircular cut
(bottom) is intended for a stem wall to support two curved
Otherwise, the old slab was in good condition and easily met
the 4-inch minimum thickness required by the engineered plan.
The sawing operations took about two days to complete.
New Footings and Stem Walls
With the help of a backhoe operator, we removed the cut-out
sections of concrete and trenched for the new footings. As is
typical for California, all the footings — as well as the
slab — required engineering, and the plans included a
rebar schedule and specs for seismic hold-downs.
Since the site was known to have expansive soil, all the new
footings had to be 2 feet deep. Many of the existing footings
bottomed out at 18 inches, so in a couple of places we had to
hand-excavate underneath them to reach the required depth
(Figure 3). We used #5 rebar and 3,000-psi
concrete for the new grade beams. The column pads were
typically 12 inches thick — ranging in size from 1 square
foot to 16 square feet, depending on the load — and
reinforced with #4 rebar.
Figure 3. Where existing footings intersect the new
trenches (left), deeper footings underpin the old work, and
continuous rebar passes right through the concrete to unify the
structure. To carry exterior wall loads, perimeter grade beams
require #5 rebar cages and a 3,000-psi concrete mix (right).
Quadruple verification. To make sure all the anchor
bolts and seismic hardware are properly installed in the right
locations on our projects, our crew foreman carefully checks
all the formwork before any concrete is ordered (Figure
4). As job superintendent, I double-check the layout.
Then the project engineer performs a site inspection and signs
a Structural Observation report, confirming that all is
according to plan. Last, the city inspector checks everything
on site and collects the engineer’s report.
Figure 4. Formwork for the perimeter stem wall
includes Simpson Strong-Wall shear-wall hold-down hardware,
shown here mounted in reusable templates (top). Bottom, the
shear panels in place.
Once the inspector signs off, we pour. The pads (poured first
for steel moment-frame installations) get a city inspection
without the engineer.
Fill and Compacting
Because the lot on this job sloped, the stem walls at the rear
of the slab were higher than in the front, requiring a fair
amount of fill. Soil fill has to be compacted in 6-inch lifts,
which made it an impractical and expensive option given the
size and complexity of the slab layout. Plus we would have had
to pay a soil technician to supervise the entire backfilling
Gravel, on the other hand, doesn’t require compacting and
can be placed in one shot. We used 375 tons of 3/4-inch stone.
Though not required, we opted to compact it — by
vibration — as common-sense insurance against minor
settlement that could crack the slab (Figure
Figure 5. Though it was not required by code, the
author chose to compact the gravel fill to eliminate any risk
of settlement cracks forming in the slab.
On top of the stone, we spread 2 inches of clean sand,
installed a 10-mil vapor barrier, and covered that with another
2 inches of sand to protect it from punctures.
Doweling the Slab Edges
To create a stable cold joint, we doweled the new slab into the
old one. We first drilled 1/2-inch-diameter holes 8 inches deep
every 16 inches along the existing slab edge. After blowing the
holes clean, we injected SET 22 High-Strength Epoxy-Tie
and inserted #4 rebar (Figure 6). Once the
epoxy had set, we wire-tied these dowels into the #4 rebar grid
in the new slab areas.
Figure 6. Rebar dowels tie new slab areas to old
(left). To tie the slab to the stem walls, the crew grouted in
30-inch-long dowels on 16-inch centers. These are bent over and
wire-tied to the grid (right).
To ensure compliance with the design specifications, the
municipality requires that a deputy inspector be on site during
doweling operations. He observes the process and signs off on a
form for compliance, later collected by the city
Slab: Infill and New
Because of its size, the slab took three days to pour
(Figure 7). We used a 2,500-psi mix. Concrete
reaches its design strength at 28 days, but construction
operations can resume after 48 hours. In our typical hot
weather, we usually mist the slab for at least the first few
hours to control curing.
Figure 7. Once all the rebar and subslab mechanicals
were in place (left), it took three days to pour the entire
slab (bottom). After framing is complete, a lightweight
concrete topping will eliminate surface irregularities in
preparation for finish flooring.
Later in this project — a couple of weeks away as of this
writing — we’ll pour 3/8 inch of lightweight
self-leveling concrete over the entire slab (as well as a 1
1/2-inch-thick layer over the subfloor upstairs). This will add
$10,000 to the cost but will eliminate minor irregularities and
provide an optimal substrate for the finish floors.
All told, reworking the slab took about seven weeks, or two
weeks longer than if we’d started from scratch with an
all-new foundation. Weighed against a lengthy and costly
permitting process, I’d say we made the right
decision.Alon Toker is president of Mega Builders in Chatsworth,