function (basement or crawl-
Pad
drilled to bearing strata. This foundation type is
space) usually decide what
unsurpassed for lateral stability, whether it’s a
foundation type is used.
replacement foundation for old work or for new
construction. Also, concrete piers have a greater
cross section than driven steel piers and hence
pouring VerSuS PlacinG concreTe
greater skin friction against the soil, so they’re
much less likely to migrate. The stability of con-
Every trade has its jargon. Concrete snobs, for example, insist on using the phrase
crete piers can be further enhanced by concrete-
placing concrete, though concrete coming out of a 4-in. hose looks more like a pour,
grade beams resting on or slightly below grade,
albeit a sluggish one. Perhaps “placers” want to emphasize that concrete is so
which allows soil movement around the piers,
weighty that you should place it as close to its final location as possible. Point
without moving the piers.
taken. But to denote the general movement of concrete from truck to forms, pour us
The primary disadvantages of drilled concrete
a tall one.
piers are cost and access. In new construction, a
backhoe equipped with an auger on the power
Foundations and Concrete
233
takeoff requires 10 ft. or 12 ft. of vertical clear-
ance. Alternatively, there are remote-access por-
table rigs that can drill in tight quarters, even
inside existing houses, but they are labor inten-
sive to set up and move, increasing the cost.
Driven steel pilings are used to anchor founda-
tions on steep or unstable soils. Driven to bed-
rock and capped, steel pilings can support heavy
vertical loads. And, as retrofits, they can stabilize
a wide range of problem foundations. There are
various types of steel pilings, including helical
piers, which look like giant auger bits and are
screwed in with hydraulic motors, and push
piers, which are hollow and can be pushed in,
strengthened with reinforcing bar, and filled with
concrete or epoxy.
important elements
This pier and grade-beam foundation was built on a sloping site with expansive clay soil, so the
In many parts of North America, building codes
engineer specified parallel-grade beams and a more massive grade-beam perimeter. (The piers go
don’t require steel reinforcement in concrete
down 15 ft.) Integral concrete post piers atop grade beams support 4x6 joists spaced 3 ft. on center.
foundations, but steel is a cost-effective means of
avoiding cracks caused by lateral pressure on
foundation walls.
Steel reinforcement and fasteners. Steel rein-
forcing bar (rebar) basically carries and distrib-
utes loads within the foundation, transferring the
loads from high-pressure areas to lower-pressure
areas. It thereby lessens the likelihood of point
failure, either from point loading above or from
lateral soil and water pressures. Anchor bolts, or
threaded rods, tied to rebar, attach the overlying
structure to the foundation. Steel dowels are usu-
ally short pieces of rebar that pin foundation
walls to footings or connect new sections to exist-
ing foundations.
There also are a number of metal connec-
tors—such as Simpson Strong-Ties—that tie
joists to girders, keep support posts from drifting,
and hold down mudsills, sole plates, and such.
Several are shown in chapter 4.
The FounDaTion Within
Most perimeter foundations have a companion
foundation within, consisting of a system of
girders (beams), posts (columns), and pads that
pick up the loads of joists and interior walls and
thus reduce the total load on the perimeter
foundation. By adding posts, beams, and pads,
you can often stiffen floors, reduce squeaks,
avoid excessive point loading, support new par-
titions, and even avoid replacing a marginally
adequate perimeter foundation.
Before the pour, the rebar spine of this foundation wall is still visible. The green bolt holders will
position anchor bolts 3 in. from the outside face of the foundation; as the 2x6 mudsill lines up with
the outside of the foundation, the anchor bolts will be centered in the mudsill.
234 Chapter 10
GeTTinG The real DirT
zzzzzz Foundation drainage
Learn what you can about local soil conditions
before hiring a soils engineer. Start with local
Downspout
builders—especially those who’ve worked on
nearby properties. Next, consult with building
and land-use departments. Many have maps
Soil sloping at
1:20 ratio away
indicating watersheds, slide zones, contami-
from house
nated soil, and the like. Finally, the U.S.
Department of Agriculture (USDA) has extensive
soil maps. And the U.S. Geological Survey
(USGS) topographic (topo) maps show streams,
4-in. solid plastic drain
lakes, flood plains, and other natural features
2 ft. below grade
that could have an impact on your site.
Waterproofing
membrane on
foundation wall
Quality. Concrete quality is critically important,
both in its composition and in its placement.
Foam board or asphalt
Water, sand, and aggregate must be clean and
protector board
well mixed with the cement. Concrete with com-
pressive strength of 2,500 pounds per square inch
Porous backfill
(psi) to 3,500 psi is common in residential foun-
dations, yet there are many ways to achieve that
Footing
strength, including chemical admixtures. Discuss
your needs with a concrete supplier who’s famil-
iar with soil conditions in your area, and read
“Ordering Concrete: Be Specific” on p. 255.
Drainage. The drainage system is not technically
a part of the foundation, but the flow of water
alongside and under a foundation is an impor-
tant consideration. In some soils, it’s essential to
mediate water flow. At the very least, water seep-
ing through foundations can cause damp base-
Filter fabric
4-in. perforated
ments and encourage mold. Worse, excessive
3/4-in. gravel
plastic perimeter drain
water can rot framing, undermine footings, and
cause unreinforced foundations to crack, bulge
inward, or fail altogether. Moisture problems
often can be mitigated simply by keeping gutters
Holes at
and downspouts clear, grading the soil away
4 o’clock and
from the foundation, and improving drainage
8 o’clock
around basement window wells. Beyond that,
4-in. solid plastic
cures such as excavating along the outside of the
&
nbsp; drain (1:20 slope)
foundation to add gravel and perimeter drainpipe
and to apply waterproofing treatments are
DETAIL OF
increasingly expensive.
PERFORATED PIPE
indoor symptoms oF
If you’re willing to excavate, you can retrofit a first-rate
Foundation Failings
drainage system such as this.
Most foundations that fail were poorly designed,
poorly constructed, or subjected to changes
(especially hydrostatic pressure or soil move-
ment) that exceeded their load-bearing capaci-
ties. Exact causes are often elusive.
Foundations and Concrete
235
Large, 1⁄2-in. or wider vertical cracks
through the foundation that are wider at the
top usually mean that one end of the
foundation is sinking—typically at a corner
with poor drainage or a missing downspout.
Large vertical cracks through the
foundation that are wider at the bottom are
usually caused by footings that are too small
for the load. You may need to replace or
reinforce sections that have failed.
Horizontal cracks through a concrete
foundation midway up the wall, with the wall
bowing in, are most often caused by lateral
pressure from water-soaked soil. This condi-
tion is common to uphill walls on sloping lots.
Concrete-block walls with horizontal
cracks that bulge inward are particularly at
risk because block walls are rarely reinforced
with steel. If walls bulge more than 1 in. from
vertical and there’s a chronic water problem,
foundation failure may be imminent.
This dramatic crack through the
In cold climates, horizontal cracks
corner of this building was caused by
a downspout and drainage system
localized springiness or low spots in flooring
through the foundation, just below ground
that was clogged for decades. Instead are probably caused by an undersize pad or by a
level, are usually caused by adfreezing, in
of runoff being directed away from
deteriorated or absent post beneath a girder. If
which damp soil freezes to the top of the
the house, water collected at the base you find wet rot or insect damage at the base
foundation and lifts it. If these cracks are
of the foundation, undermined the
of the post, correct that situation before doing
accompanied by buckled basement floors, the
footings, and caused a corner to sink.
anything else.
foundation’s footings may not be below the
frost line.
Widespread springiness in floors and joists sag-
ging in midspan are caused by joists that are too
Gaps between the chimney and the house are
small for a span or by a failed or absent girder. If
usually caused by an undersize chimney pad. If
an existing girder seems sound, adding posts or
the mortar joints are eroded, too, tear the chim-
new pads may fix the problem. Otherwise, add a
ney down and replace it.
girder to reduce the distance joists span.
Failure of all or part of a perimeter founda-
P R O T I P
tion often explains flooring that crowns above a
girder, sloping downward toward the outside
if you’re not sure that a foun-
walls; doors and windows that are difficult to
dation crack is active (moving),
epoxy a small piece of glass to
open; and cracking at the corners of openings.
both sides of the crack. if the
Foundation cracks often signal foundation fail-
glass breaks over two months’
ure. Cracks may range from short surface cracks
time, the crack is active. a glass
to through-the-wall cracks that should be exam-
microscope slide is perfect for
ined by a structural engineer. Here are some
this test. or substitute a scrap of
common symptoms and remedies:
window glass.
Narrow vertical or diagonal surface
cracks that are roughly parallel are likely
caused by foundation settlement or soil move-
ment but are probably not serious. If water
runs from cracks after a storm, fill them with
an epoxy cement, and then apply a sealant.
Wide cracks in foundations less than 2 ft.
tall indicate little or no steel reinforcement, a
common failing of older homes in temperate
climates.
If your foundation upgrade will require big steel I-beams,
hire house movers. They’ll have the cranes, beam rollers,
cribbing, and expertise to handle I-beams safely.
236 Chapter 10
Jacking and Shoring
patching Foundation Cracks
Jacking refers to raising or lowering a building so
you can repair or replace defective framing or
determine the cause of the crack and fix that first; otherwise, the crack may
failed foundations or to level a house that has
recur. shallow foundation cracks less than 1⁄
settled excessively. Shoring refers to a temporary
8 in. wide are usually caused by normal
system of posts, beams, and other structural ele-
shrinkage and needn’t be patched, unless their appearance disturbs you or they let
ments that support building loads. Temporary is
water in. However, you should repair any cracks that go all the way through the
the crucial word: Shoring supports structural ele-
foundation: probe with a thin wire to see if they do.
ments between jackings. Once repairs are com-
of the many crack-repair materials, there are three main types: cement based,
plete, you need to lower the house and remove
epoxy, and polyurethane foams. When working with any of these materials, wear dis-
the shoring as soon as possible. If repairs are
posable rubber gloves, eye protection, and a respirator mask with changeable filters.
extensive—say, replacing foundation sections—
Cement-based materials such as hydraulic cement are mixed with water and
have a structural engineer design the new sec-
troweled into cracks. to ensure a good connection, first use a masonry chisel and
tions, specify jack size, and specify the posts,
hand sledge to enlarge the crack; angle the chisel to undercut the crack, making it
beams, and bracing needed to safely jack and
wider at the back. then wire-brush the crack to remove debris. next, dampen the
shore the building.
surfaces, fill the crack with hydraulic cement, and feather out the edges so the
Jacking a house is nerve-wracking. It requires
repaired area is flat. Work fast because most hydraulic cement sets in 10 to 15 min-
a deep understanding of house framing and how
utes and expands so quickly that it can stop the flowing water of an active leak.
structures transfer loads. It also requires superb
Epoxies range from troweled-on pastes to injection systems that pump epoxy
organizational skills and a lot of specialized
deep into crack
s. application details vary, but many injection systems feature sur-
equipment. For that reason, foundation contrac-
tors routinely subcontract house-raising to house
face ports, which are plastic nozzles inserted into the crack along its length. you
movers with crews who know what they’re doing
should space ports 8 in. apart before temporarily capping them. then seal the wall
plus have on hand the heavy cribbing blocks,
surface with epoxy gel or hydraulic cement, which acts as a dam for the epoxy liquid
hydraulic jacks, and cranes to lift steel I-beams
you’ll inject deep into the wall through the ports. Working from the bottom, uncap
for bigger jobs. Structural engineers usually will
each surface port, insert the nozzle
know qualified house movers. (By the way, these
of the applicator, and inject epoxy
specialists are still called house movers even
until it’s visible in the port above.
when the house stays on the site.)
Cap the port just filled, and then
move up the wall, port by port.
materials and tools
epoxy is famously strong. the
Basically, timbers used for jacking and shoring
manufacturer of simpson Crack-
stages are the same size. For example, once you
pac® claims that its injected epoxy
have jacked the building high enough with 4x4
achieves 11,000 psi compressive
jacking posts, you can plumb and insert 4x4 shor-
strength when cured for seven
ing posts next to the jacking posts. Then, with
days. (Foundation concrete aver-
shoring posts solidly in place, you can slowly
ages 3,000 psi to 4,500 psi.)
lower the jacks of the jacking posts and remove
Consequently, injected epoxy,
each jacking component.
which bonds to both sides of the
Posts and beams. To support a single-story
crack, is a true structural repair,
house, 4x4 posts with 4x8 or 4x10 beams should
not just a crack filler. there are a
be adequate. Because harder woods compress
couple of disadvantages: cost and
less when loaded, shoring should be Douglas fir,
curing time. epoxy takes hours to
oak, or a wood with similar compressive
harden, so it can ooze out the back of the crack if there’s a void between the soil
strength. You’ll also need footing blocks under
and the foundation wall—as there often is. if your main concern is water leaks and
each post: typically, two 2-ft.-long 4x12s placed
Renovation 4th Edition Page 55