not structural repairs, polyurethane foam is probably a better choice.
side by side, with a 2-ft.-long 4x6 placed perpen-
Polyurethane foam is applied in many ways, including the surface-port injec-
dicular atop the 4x12s. Footing blocks of that size
tion just described for epoxies. polyurethane sets up in minutes, so it’s unlikely to
are wide enough to accommodate a jacking post
and a shoring post side by side while you make
sag or run out the back of the crack. it’s largely unaffected by water, so you can
the transition from jacking to shoring.
inject it into a damp crack. unlike epoxy or cement-based fillers, polyurethane is
For two-story houses or heavy single-story
elastomeric (meaning it stays flexible), so it’s great for filling foundation cracks
houses, most professionals specify W6x18 or
that expand and contract seasonally. one disadvantage is that it has little compres-
W8x18 steel I-beams to support the loads. (A
sive strength and hence does not create a structural repair.
heavy single-story house might have a stucco
exterior, a plastered interior, and a tile roof.)
Foundations and Concrete
237
Because steel beams can span greater distances
Screw jacks vary from 12 in. to 20 in. (closed
P R O T I P
than wood, they require fewer posts underneath,
height), and extend another 9 in. to 15 in. Never
which frees space under the house and improves
raise the threaded shaft more than three-quarters
shoring timbers are heavy, so
access for workers. However, if you use steel
its total length because it would be unstable
get help. it takes three strong
beams, stick with wood posts: Fir 6x6s are less
beyond that. Screw jacks are extremely stable: Of
workers to raise a 16-ft. 4x8. a
likely to migrate than steel columns.
all types, they are the least likely to fail or lower
4x12 that same length requires
four people.
unexpectedly under load. But they require a lot
cribbing. Cribbing refers to a framework of usu-
of muscle and at least 2 ft. of space around the
ally squared timber (often 6x6s) stacked in alter-
jack for operation.
nate layers to create a stable platform for jacking
Hydraulic jacks are the workhorses of founda-
or shoring house loads. In earthquake country,
tion repair and are rated according to the loads
foundation contractors “shear-wall” cribbing
they can bear, such as 12 tons. In general,
higher than 8 ft.—that is, they temporarily nail
1
hydraulic jacks are easier to operate than screw
⁄2-in. plywood to the cribbing using duplex nails. jacks, and they fit into tighter spaces. They are
The precaution is worth the trouble: In 1989 a
lowered by turning a release valve, so they can’t
California house resting on 13-ft.-high shear-
be lowered incrementally. Because hydraulics
walled cribbing remained standing through a
release all at once, many house movers use
7.1 quake.
hydraulics to raise a house and screw jacks to
Braces and connectors. To keep posts plumb
lower it gradually.
and prevent structural elements from shifting,
Because the head of a hydraulic jack is rela-
builders use a variety of braces and connectors
tively small, you need to place a 4-in. by 4-in. by
such as these:
1⁄4-in. steel plate between it and the wood it sup-
Diagonal 2x4 braces 3 ft. or 4 ft. long are
ports so the head doesn’t sink in during jacking.
usually nailed up with double-headed nails for
Safety note: As the jack is lowered, have a helper
easy removal.
keep a hand on the steel plate so it doesn’t fall
Plywood gussets are acceptable if space
and injure someone. Alternatively, have the plate
is limited.
predrilled so you can screw or nail it to the
Metal connectors such as Simpson
underside of the beam.
hurricane ties, post caps, and post-to-beam
unsuitable for raising a house. Post jacks
connectors are widely used because they are
employ a screw mechanism but are of flimsier
strong and quick to install.
construction than the compact screw jack
described in the previous section. They tend to be
Jacks. House-raising screw jacks and hydraulic
fashioned from lightweight steel, with slender
jacks are by far the most common types. For
screws that could easily distort when loaded
safety, all jacks must be placed on a stable jack-
beyond their capacity. So use post jacks only for
ing platform and plumbed.
low-load, temporary situations.
Jacks. Left: a hydraulic jack, the workhorse of house-
raising. Right: a screw jack. Textured heads on both reduce
chances of post slippage.
Unsafe! Although post jacks such as this are widely used
as temporary shoring, they are not strong enough for
house loads. Here, footing blocks are undersize, and the
post is badly out of plumb.
238 Chapter 10
JaCking saFely
For safe jacking, you need to proceed slowly and
observe the following precautions.
zzzzzz Jacking Components
Beam
Preparatory steps
Survey the building, noting structural
failings and their probable causes as well as
which walls are load-bearing. Also determine
Steel plate
whether joists or beams are deflecting because
Jack
Cribbing
of heavy furniture, such as a piano; which
pipes, ducts, or wires might complicate your
repairs; where the gas pipe shutoff is; and so on.
Have a plan. If excavation is necessary,
who’s going to do it? And where will you put
the displaced dirt? (Disturbed dirt has roughly
twice the volume of compacted soil.) Will you
need to rent equipment, such as a compressor,
a jackhammer, or jacks? Where will you store
materials? How will rain affect the materials
and the work itself? Can a concrete-mixer
Jacks must be supported on a level, stable platform. Here, cribbing beneath
truck reach your forms or will you need a
the jacks and steel plates atop them disperse loads to larger surface areas.
separate concrete pumper, an auxiliary pump
Without the steel plates to spread the load, jack heads can sink into wood
on wheels that pumps (pushes) the concrete
beams.
from the mixer truck to the pour?
Assemble safety equipment. This is
DON’T DO THIS!
FOOTING PIT
mostly hard-hat work. You’ll also need safety
glasses that don’t fog up, sturdy knee pads,
and heavy gloves. For some power equipment,
you’ll need hearing protectors. Update your
tetanus shot. Set up adequate lighting that
Uns
afe!
keeps cords out of your way—and, on a post
near a suitable light, mount a first-aid kit.
Even though a cell phone is handy if trouble
strikes, never work alone. Workers should stay
within shouting distance.
Have all necessary shoring materials on
hand before you start jacking. Remember,
jacks are for lifting, not supporting. Within
Level pit
reason, level the ground where you’ll place
footing blocks or shoring plates. As soon as a
section of the house is raised to the proper
Where you must jack on a slight incline,
level, be sure to set, plumb, and brace the
When the jack is loaded, bearing blocks
dig down into the soil to create a level
shoring. Hydraulic jacks left to support the
placed unsafely close to the edge of an
pit. The pit will also surround the bearing
structure too long may slowly “leak” and settle
excavation can cause it to collapse.
blocks, giving them no place to go when
or—worse—kick out if bumped or jostled.
loaded.
Jacking basics
Support jacks adequately. The footing
blocks or cribbing beneath the jacks must be
thick enough to support concentrated weights
without deflection and wide enough to
distribute those loads. It’s difficult to
generalize how big such a support must be; a
4x12 footing block 3 ft. long or two layers of
4x4 cribbing should adequately support a jack
beneath the girder of a single-story house. In
this case, the soil must also be stable, dry, and
level. If the soil isn’t level, dig a level pit for the
Foundations and Concrete
239
bottom right drawing on p. 239, so footings or
cribbing can’t migrate under pressure.
lookinG For trouble
(Typically, the foundation contractor digs the
Certain conditions make raising houses diffi-
pits and prepares the site before the house
mover arrives to install the cribbing.)
cult. When foundation contractors see the fol-
Surrounded by the walls of the pit, the bearing
lowing conditions, they get a second or third
blocks have no place to go.
opinion from engineers and house movers before
Keep checking for level and plumb as you
bidding on a job:
jack. If supports sink into the soil, posts tilt, or
Sloping sites, unstable soil, site erosion, or
the jack starts “walking” under pressure,
excessive ground water.
lower the jack, reset the supports, and begin
A masonry building.
anew. First thing each day, check jack supports
Multiple-story house or single-story dwell-
and shoring for plumb and level. Diagonal
ing with heavy materials, such as stucco exterior
bracing, plywood gussets, and metal
walls; plaster interior walls; and tile, slate, or
connectors will each help posts stay plumb.
three-layer roofs.
When cross-bracing temporary posts and
Quirky framing visible in the basement,
beams, use screw guns or pneumatic nailers to
such as undersize or cut-in girders, joists run-
attach braces. Hand nailing braces could
ning in several different directions, and multiple
knock posts out of plumb or cause beams to
additions to the original structure.
rotate or jack heads to migrate.
Floors that pitch in different directions or
Raise jacks in small increments—say,
1
are badly out of level.
⁄4 in. per day—to minimize damage to finish
Catastrophic foundation failures, such as
surfaces inside the house. When you’re jacking
foundation rotation and sinking corners.
a structure to be repaired, as when replacing a
mudsill, jack just enough to lift the weight off
the sill to be removed. If many jacks are
involved, raise them simultaneously if
possible, so excessive stress (and damage)
doesn’t build up above any one jack.
cribbing, as shown on p. 239. (Avoid precast
concrete piers as jacking blocks because their
Steps in jacking and installing shoring.
footprints are too small and the concrete could
Setting jacking equipment varies according to the
shatter when loaded.)
type of jack, the structural elements to be raised,
and site conditions, such as ceiling height,
Don’t place jacking or shoring platforms
P R O T I P
access, and soil stability. That noted, the follow-
too close to the edge of an excavation.
ing observations hold true in most cases.
Otherwise, the soil could cave in when the
it’s generally safer to place
timber is loaded. The rule of thumb is to move
1. Position the jacks and jacking beams as
jacks atop posts rather than
back 1 ft. for each 1 ft. you dig down. Also,
close as possible to the joists, girders, or stud
under them. as jacking pressures
don’t put jacks or shoring where they could be
walls you’re jacking. If you’re adding posts under
increase, there’s increasing dan-
undermined later. For example, if you need a
a sagging girder, support may be directly under
ger that posts sitting on jacks
needle beam to support joists parallel to the
the girder, but more often, it will need to be offset
will kick out. However, if the load
foundation, excavate on either side of what
slightly—say, within 1 ft. to 2 ft. of joist ends—to
is light, either way is ok. For
will be your new foundation, and place jacking
give you working room. In other words, place
example, in the photo on p. 242,
platforms in those holes. In that manner, you
them close enough to joist ends so they won’t
the jack is merely holding a nee-
can remove foundation sections without
dle beam in place while workers
deflect, yet far back enough to let you work.
undercutting the jacking platforms.
measure and set a shoring post.
Again, don’t put jacks or shoring where they
Level support beams, and plumb all
could be undermined by unstable soil later.
posts. The logic of this should be evident:
2. Level and set the footing blocks or cribbing
When loads are transmitted straight down,
on compacted soil. Each jack base should be
there is less danger that jacks or posts will kick
about 2 ft. by 2 ft. Or, if you’re using a single tim-
out, injuring someone and leaving shoring
ber block, use a 4x12 at least 3 ft. long or, if the
unsupported. Accordingly, cut the ends of the
soil is crumbly, at least 4 ft. long. If you spend a
posts perfectly square, plumb the posts when
little extra time leveling the footings, the posts
you set them,
and check them for plumb
will be more likely to stay plumb. To support a
periodically as the job progresses.
single-story house, set posts every 5 ft. or so
Where the ground slopes, dig a level
beneath an adequately sized beam—typically, a
footing pit into the soil, as shown in the
4x8 or 4x10 set on edge.
240 Chapter 10
needle Beams
When replacing a mudsill or sections of a foun-
dation whose joists run perpendicular to the adja-
cent foundation wall (see p. 242), place a 4x8 or
4x10 carrying beam on edge under the joists within
zzzzzz Joists parallel with a Foundation Wall
2 ft. of the foundation. a jack every 6 ft. under the
Pony wall
beam should suffice.
top plate
When joists run parallel to the foundation wall
being replaced, you’ll need to run needle beams
through exterior walls and support each beam with
Needle beam
Joists
one post underneath the house and a second post
outside, roughly 2 ft. beyond the foundation wall.
Doubled
For this, you’ll need to remove sections of siding so
rim joists
you can insert a beam every 6 ft. to 8 ft. if the sid-
ing is stucco, you’ll need to punch large holes
through it. to keep the rim (outer) joist from
deflecting under the load, nail a second rim joist to
it, doubling it before jacking. also, add solid block-
ing from those doubled rim joists to the first adja-
cent joist. use metal connectors to affix the block-
Mudsill
Note: Some studs,
ing and 10d nails to face-nail the rim joists.
sheathing omitted
this big needle beam needed to fit under the
for clarity.
top plate of the pony wall (short wall) on the right.
as a result, the top of the beam needed to be built
Where joists run parallel to the foundation wall, remove a section of
up with a 4x4, which now supports the floor joists.
siding and run a temporary needle beam through the wall as shown. To
the metal post cap keeps the post from migrating.
prevent its deflection under pressure, double the rim (outer) joist and
run solid blocking to the next joist inward.
This big needle beam needed to fit under the top plate of the pony wall
(short wall) on the right. As a result, the top of the beam needed to be
built up with a 4x4, which now supports the floor joists. The metal
connector atop the post keeps it from drifting out of plumb.
Foundations and Concrete
241
Renovation 4th Edition Page 56