Before adding new living space above an existing garage, make
sure the footings can handle the load
As part of a recent renovation project, we added a second floor
to an existing one-story attached garage. The plan was for a
new bedroom suite in this added space to tie into a two-story
addition that we would build in conjunction with the garage
Simply demolishing the garage and starting from scratch would
have made more sense but wasn't an option: Since the garage
sidewall was in violation of the local zoning ordinance's
side-yard setback requirement, taking it down would mean losing
5 feet of variance. So, to conform to the ordinance, the new
second-floor sidewall needed to be built 5 feet further in from
the property line than the existing garage sidewall.
To build the second floor and prepare the garage for the
tie-in, we would have to demo the roof, demo the sidewall
gable, jackhammer the slab, and lengthen the back of the garage
by 2 feet.
We knew that we'd have to pour a footing for the new wall at
the back of the garage. What we didn't know was that we'd also
have to underpin the front-wall footing to make it strong
enough to support the loads from the new second floor.
Check Existing Footings
Over the years, we've learned to carefully check porch and
garage footings; they're often substandard. We don't want to
get a phone call from a client's attorney a year after project
completion because we hadn't bothered to check a footing before
building a second-floor addition. All of our contracts plainly
state that footings are assumed to be in accordance with code,
making it clear that any work on existing footings is
Hence, early in this project, we asked the excavation
contractor to dig test holes next to the garage foundation when
he was digging the main hole for the new addition.
The test holes revealed that while the footing under the
sidewall was adequate, the front-wall footing was only 18
inches below grade (see Figure 1). Because the plans called for
the new second-floor loads to be carried primarily by the front
and back walls of the garage, it was clear to me that we would
have to do something about the existing front-wall footing when
we poured the footing for the new back wall.
Figure 1. A test pit revealed that this
shallow foundation footing, though adequate for a garage, could
not support the loads created by the addition of second-floor
living space without reinforcement.
We didn't spend too much time speculating about how to fix the
problem: This situation called for a site visit from the
project architect. Whatever the solution, we knew that the town
would want an architect's or engineer's seal on it.
The architect turned the problem over to an engineer, who
provided us with a design for underpinning the front wall (see
illustration). Then we filed for a permit update and got
started with the garage demolition.
Demolition Lightens the Load
Equipped with a five-pack of circular saw blades, we started
cutting through the shingles and roof decking, carving the old
roof into 2-foot-by-5-foot sections that we could leverage with
a digging bar and throw in the dumpster section by
Once the decking was removed, we cut the rafters in half with a
reciprocating saw and pulled them apart. Then we took the gable
wall down in sections, with the studs and sheathing together.
All we left standing was the garage's side and front
Next we busted up the garage slab. Since our electric rotary
hammer wasn't up to the challenge, we rented a
compressor-driven jackhammer to quickly finish the job (Figure
Figure 2. Before framing could begin, most
of the existing garage had to be demolished, except for the
sidewall — which was needed to retain the structure's
5-foot zoning variance — and the front wall, which needed
to be reinforced with a deeper footing.
Demolition of the garage plaster, ceiling joists, roof rafters,
and gable wall removed a considerable amount of weight from the
front wall. Just the same, we decided to underpin the existing
footing in two steps, doing half of the wall at a time.
Building the Form
With our rented backhoe, we dug a trench next to the garage
foundation for access, then used hand tools to excavate under
the footing itself.
To loosen the soil under the footing, we used a rotary hammer
equipped with a chisel, then removed the dirt with a pick and
shovel (Figure 3).
Figure 3.After digging a trench
alongside the existing front-wall footing with an excavator,
the author's crew used hand tools and a rotary hammer fitted
with a chisel to remove the compacted soil under the footing
(top). To prevent damage to the foundation, half the wall at a
time was excavated and temporarily shored up
Because soil would form the back wall of the underpinning, we
were careful to square up the excavation under the existing
footing as much as we could.
The engineer's design called for the underpin footing to be a
minimum of 36 inches below grade and 24 inches wide, with #4
rebar running continuously throughout, set on chairs. We built
the form with 2x4s and 3/4-inch plywood, securing the bottom
with 30-inch form pins and bracing the top with 2x4
Since we planned to do the underpinning in two pours, we
extended the rebar 12 inches beyond our first form so that we
could tie in later with the second pour (Figure 4).
Figure 4. Built with 2x4s and 3/4-inch
plywood, the underpinning form was braced with stakes and form
pins (left). The side of the excavation underneath the existing
footing provided the back wall for the poured concrete
(center). Extending the rebar past the first form (right)
helped tie the two halves of the new 24-inch-wide-by
36-inch-deep footing together when the second form was
Pouring the Concrete
When we pour concrete, we always try to schedule the first
available morning pour. Afternoon pours tend to run past
quitting time and into the evening.
For this job, we ordered 7 cubic yards of concrete; I figured
we'd use 1.5 cubic yards for the first half of the front-wall
underpinning and 4.75 cubic yards for the back wall
Even though I expected an easy pour, I followed my usual
practice of handing the concrete truck driver a $20 tip when he
arrived. It doesn't take long to use up the allotted six
minutes per yard, and I've found that it never hurts to have
the driver on your side.
As it turned out, we needed all the help we could get, since
the back wall form suffered a blowout during the pour (see
To extend the garage, plans called for a new
foundation underneath the relocated back wall (which
would contain the new garage door). Rather than pour a
footing below grade and then build up the foundation
out of block, we formed a 24-inch-wide monolithic block
from footing depth to grade, which we felt would make a
more attractive transition apron between the asphalt
driveway and the garage slab.
As we were pouring the last of the 4-plus yards of
concrete needed to fill the form (A), though, the form
wall on the driveway side began to bulge, then
collapsed (B). Since the inner wall remained intact,
project manager Danny DoCouto decided to pull the
collapsed wall out entirely and spread the concrete
into the outer portions of the trench, allowing it to
establish a solid, level surface 24 inches below grade
that we could build off later (C).
To get the form out of the trench — with
concrete on one side and form pins on the other —
we had to shovel concrete away from the wall, then lift
the form by leveraging it with concrete blocks, 2x8s,
and our digging bar. Even after the form was removed,
we had to work our shovels into the rebar grid to
expose more rebar and level out the concrete, a process
that took more than an hour.
The following day, Danny cut and removed the old rebar
so we could rebuild the form. Since the new outside
form wall was now sitting on concrete, we wedged it
into place with 2x4 stakes along the bottom plate. And
this time, we used plenty of stakes to brace it along
the top plate (D).
After laying in new rebar on chairs and straightening
out the bowed inner form, we were ready to pour once
We had to spend five man-hours cleaning up the mess,
four man-hours rebuilding our form, and two man-hours
on the second pour.
We also had to spend about $200 for additional
With half our four-man crew working on fixing the blowout,
we knew we still had to get the first section of the
underpinning poured. To play it safe, we quickly beefed up the
underpinning form with additional stakes and 2x4s — we
didn't want a repeat performance. But the front underpinning
pour went smoothly, and 15 minutes later the whole crew was
working on removing the collapsed form.
The next day, we prepared for the second underpinning pour by
stripping off the first form, then excavating under the
remaining footing as we had before (Figure 5).
Figure 5. After stripping the form from
the first half of the front-wall underpinning, the author's
crew finished excavating underneath the remaining wall (top).
Form pins helped hold the bottom of the form in place (middle
left), while 2-by braces secured the top plate (middle right).
The completed underpinning (bottom).
After building the form and installing rebar, we secured it
with plenty of form pins and stakes, making sure that the
second underpinning pour would be as uneventful as the
Once again, when the concrete truck arrived to pour the
remainder of the underpinning and the new back-wall foundation,
I greeted the driver with a $20 tip for added insurance. This
time, the pour went without a hitch, and soon we were able to
start framing up the new garage addition.Rob Corbo is a building contractor in