zzzzzz Joists perpendicular to a Foundation Wall
2 ft. or less
Joists
Pony wall
Support beam
Jack
Mudsill
Cribbing
Foundation
Supporting the structure: When joists
run perpendicular to the foundation
wall you’re working on, support their
load with a beam parallel to the wall.
To determine the height of the shoring posts, jack the beam
Al owing yourself room to work, put this
to the desired height, level the footing block, and measure
beam as close to the wall as possible—
between the two. Nail the post cap on before inserting the
in most cases, 2 ft. is optimal.
post. Because this needle beam is simply holding a wall in
place—not raising it—it was jacked just snug to a wall plate.
3. Positioning a jacking beam requires prep
mine the height(s) of the shoring posts. (If you
work. Ideally, the crew should raise the beam
jack up an additional 1⁄8 in., you’ll find it easier to
into place and then immediately plumb and set
slide shoring posts in.)
the jacks. But they may need to catch their
5. After you’re done jacking, install the shoring
breath or gather equipment before setting the
posts, which are more stable than jacks on posts.
jacking posts. In that event, cut two 2x4s approxi- To keep the shoring posts in place, nail steel caps
P R O T I P
mately 1⁄2 in. longer than the distance from the
to their tops before installing them. Once you’ve
underside of the beam to the top of the cribbing
placed the posts under the footing beams, attach
if a new foundation section
plates, and use a sledgehammer to wedge a
the post caps to the beams, and add cross brac-
isn’t terribly long or deep, use an
2x4 under each end of the beam. Caution: This
ing or plywood gussets to keep the beam from
electric demolition hammer with
setup is inherently unstable, so workers should
a shovel bit to excavate the
rotating. Once you’ve plumbed the shoring posts
monitor both 2x4s continually to make sure they
trench. this tool is particularly
and braced the beams, lower the jacks slowly
don’t kick out.
helpful when there’s no room
until they no longer bear weight, and then remove
to swing a pick. shovel bits
4. As soon as the beam is in place, cut posts
them. With shoring supporting all necessary
cut a clean edge and dig them-
10 in. shorter than the distance from the under-
bearing members, you’re ready to begin repairs.
selves in, even in heavy clay.
side of the beam to the top of the footing blocks.
Note: Some foundation contractors install two
then you simply shovel out the
This 10 in. is roughly the (closed) height of a
shoring posts—one on either side of the jacking
loosened soil.
hydraulic jack plus a little room to move. With a
post—for greater stability. Once the jack is
helper, place jacks and posts under both ends of
removed, nail two 3-ft.-long 2x4 diagonal braces
the beam, plumb the posts, and start jacking.
between the two shoring posts; for this, use a
Center each jack on its footing blocks so there’s
pneumatic nailer. Hammer blows could dislodge
plenty of room for the shoring post(s) that will
the posts.
follow shortly. (If you use hydraulic jacks, you
6. When your repairs are finished, begin to
can position the shoring posts 3 in. to 4 in. from
remove the shoring by reinserting the jacks and
the jacking posts.) As you jack, try to raise both
then simultaneously raising all the jacks slowly
ends of the beam evenly, using a 4-ft. or 6-ft. spirit and evenly to take weight off the shoring. Leave
level to check for level.
the cross bracing in place until those loads are
When the beam is at the desired height, mea-
removed. Then, keeping the jacking posts plumb,
sure down to the tops of footing blocks to deter-
carefully remove the post-and-beam bracing and
242 Chapter 10
lower and remove those elements. Gradually
and jack just enough to take the load off the post—
lower the building onto its new pads, posts, and
plus 1⁄8 in. Remove the rotted post, measure from
foundation, then remove the jacks.
the underside of the girder to the pad (remem-
bering to subtract the 1⁄8 in.), and cut a new
post—preferably from pressure-treated lumber.
Minor Repairs and Upgrades
To keep this new post from rotting, cut a
The category minor repairs includes anything
22-ga. sheet aluminum plate and place it under
short of replacing a failed foundation, which is
the bottom of the post. A dab of silicone caulking
covered in the next section. Repairing surface
will hold it in place while you plumb and posi-
cracks is explained on p. 237.
tion the post, then lower the jack so the new post
bears the load. Or replace the wooden post with a
replaCing posts and pads
preprimed metal column. However, if basement
If floors slope down to a single point, there’s a
floors are wet periodically—suggested by sedi-
good chance that a post or pad has failed. If a
ment lines along the base of the walls—build up
floor slopes down to an imaginary line running
or replace the existing pad with a taller one to
down the middle of the house, there’s probably a
elevate the base of the post. Add a sump pump,
girder sagging because of multiple post or pad
too, as explained on p. 258.
failures. Fortunately, the cures for both condi-
replacing pads. Replace concrete pads that
tions are relatively straightforward.
are tilting or sinking because they are too small
Post repairs. The most common cause of wood-
for the loads they bear. Likewise, you’ll need to
en post failure is moisture wicking up through a
pour a new pad if there was no pad originally
concrete pad, rotting the bottom of the post. To
and an overloaded post punched through the
replace a damaged post, use the techniques just
concrete floor. Pads for load-bearing columns
described in “Jacking and Shoring.” Place footing should always be separated from floors by
blocks as close as possible to the existing pad,
isolation joints.
Cutting into a Concrete Floor
to enlarge an existing load-bearing pad or create a new one, you
thick and reinforced with rebar, you can spend a day accomplishing
may need to cut through a concrete floor. depending on the condition
very little. Well . . . you get the picture.
and thickness of that floor, the job will range from nasty to horrible.
Fortunately, for a few hundred bucks you can hire a concr
ete-cutting
Cutting concrete is noisy, dirty, and dangerous, and the tools are
subcontractor to cut out a pad opening in about an hour. (don’t forget
heavy and unpredictable. Wear safety glasses, gloves, hearing pro-
to allow for the thickness of the formboards when sizing the opening.)
tectors, and a respirator mask. adequate ventilation and lighting
the subcontractor also can bore holes needed for drainpipes and such.
are a must.
if the floor was poured before the 1950s, you’ll likely find that it is
only 3 in. to 4 in. thick and is without steel reinforce-
ment. the floor also may be badly cracked. in this
case, you can probably break through it with a pickax,
but to minimize floor patching later, rent an electric
concrete-cutting saw with a diamond blade to score
around the opening. then finish the cut (the saw-
blade rarely cuts all the way through) with a hand
sledge and a chisel.
Be advised, however, that a concrete-cutting saw
cuts dry and thus throws up an extraordinary amount
of dust. therefore, you may need to seal off the base-
ment with plastic barriers and then spend an hour
vacuuming afterward. alternatively, you can rent a
gasoline-powered wet-cut saw, which keeps down the
A gasoline-powered, wet-cut saw has a diamond blade and is connected to a water
dust but fills the basement with exhaust fumes. and,
supply (via the yellow fitting), which reduces airborne dust when cutting concrete.
if the concrete floor is a modern slab that is 5 in.
Foundations and Concrete
243
If your floors are springy and joists exceed the
joiSt SizE
tyPiCAL SPAN (ft.)
following rule-of-thumb lengths, consider adding
26
8
a girder.
An engineer can size the girder for you. “Beam
28
10
Span Comparison” below shows maximum spans
210
12
for built-up girders in two-story houses.
Ideally, the new girder should run beneath the
212
14
midpoint of the joist span, but if existing ducts or
drainpipes obstruct that route, shift the girder
location a foot or two. Once you locate the girder,
snap a chalkline to mark its center, and plumb
Load-bearing pads should be 24 in. by 24 in.
down from that to mark positions for pads and
by 12 in. deep, reinforced with a single layer of
posts. Place posts at each end of the girder and
No. 4 (1⁄2-in.) rebar arranged in a grid. Pads sup-
approximately every 6 ft. along its length. If you
porting a greater load (such as a two-story house) create a girder by laminating several 2xs, keep at
should be 30 in. by 30 in. by 18 in. deep, with
least one member of the “beam sandwich” con-
two layers of No. 4 rebar; in each layer, run three
tinuous over each post.
P R O T I P
pieces of rebar perpendicular to three other
Size and reinforce pads as described in the
pieces. In either configuration, keep the rebar
preceding section. After the concrete pad has
Before adding a girder to cor-
back 3 in. from the edges of the pad.
cured for a week, bring in the girder or laminate
rect springy floors, go under the
Line the forms with sheet plastic so that the
it on site from 2-in. stock. Prescribed widths
house and see if there’s solid
water in the concrete won’t drain into the soil
for built-up girders are usually three 2x boards
blocking or cross bridging
and weaken the pad. (Plastic also prevents soil
(41⁄2 in. thick when nailed together). For built-up
between joists. if not, add it,
moisture from later migrating through the pad
girders and beams, the Uniform Building Code
and that may be all you’ll have to
and rotting the post.) If you carefully level the
do to stiffen the floors. if floors
recommends the following nailing schedule:
tops of the formboards and screed off the con-
sag between joists, the subfloor-
20d nails at 32 in. on center at the top and bottom
ing may be too thin.
crete to them, the top of the pad will be level as
and two 20d nails staggered at the ends and at
well. Allow the concrete to cure before putting
each splice.
weight on it: three days minimum but seven days
Whether solid or laminated, if the girder has a
recommended.
crown, install it so it faces up. Installing a new
girder is essentially the same as positioning a
adding a girder
temporary shoring beam, except that the girder
will stay in place. Have helpers to raise the girder
Adding a girder under a run of joists shortens the and support it until permanent support posts are
distance they span, stiffens a springy floor, and
in place. Properly sized, the pad will have more
reduces some loading on perimeter foundations.
zzzzzz Beam span Comparison
Beam span Comparison
Typical
joiSt SPAN (x/2 + y/2)
joist
Header or
other support
8 Ft.
10 Ft.
12 Ft. 14 Ft.
Beam
BEAM tyPE
BEAM SPAN (ft)
(2) 2x8 built-up beam
6.8
6.1
5.3
4.7
4x8 timber
7.7
6.9
6.0
5.3
y/2
y/2
Joist
31⁄8-in. x 71⁄2-in.
Header
x/2
span y
glue-laminated beam
9.7
9.0
8.3
7.7
x/2
Use this drawing and table for
Beam
estimating beam sizes and
supports
31⁄2-in. x 71⁄2-in. PSL beam
9.7
9.0
8.5
8.0
1
comparing beam types for
/2 of each
joist span, or
(2) 13⁄4-in. x 71⁄2-in.
uniform floor loads of a 40-psf
Joist
x/2 + y/2. See
span x
LVL (unusual depth)
10.0
9.3
8.8
8.3
(pounds per square foot) live
table at right.
load and a 15-psf dead load.
Header
4x8 steel beam
Have a structural engineer
supports
(W8 x 13 A36)
17.4
16.2
15.2
14.1
calculate your actual loads.
1/2 of single
joist span.
244 Cha
pter 10
replacing a Wooden girder with an i-Beam
as level building lots continue to disappear, we’ll need to make the
most of sites once thought unbuildable, such as the remodel in the
photos here, which takes place on a steep, south-facing slope above
san Francisco. the lot’s high side bordered a city street, so a garage
right on the street became the portal to descending stairs, an elevat-
ed walkway, and a dramatic, modern house set downhill about 50 ft.
from the road.
in time, the homeowner realized that the cavernous space under
the garage could accommodate an in-law suite, so he hired stephen
shoup, principal of Building lab in emeryville, Calif., to design and
build it. the great challenge of the project involved transferring the
loads borne by the main girder—which supported a garage floor
above—to a steel i-beam set flush to the finished ceiling that would
span the 22-ft. width of the suite without intervening support posts.
the weights were considerable. the garage floor system consisted
of 2x12 douglas fir joists spaced 12 in. on center, a 3⁄4-in. plywood
subfloor, and a 4-in.-thick reinforced concrete slab. the clearances
Unfinished space under the garage. The 6x12
were also quite tight when it came time to cut through the garage
Douglas fir girder, at top, spanned 22 ft. with
the aid of 2x6 beam pockets at both ends and a
floor joists: the slot cut was roughly 3⁄16 in. wider than the width of
6x6 post at midspan. The white sticks about
the i-beam—allowing 1⁄16 in. on each side of the i-beam and 1⁄16 in.
halfway up the wall indicate the height of the
of “wiggle room.” the fit had to be that exacting so joist ends could
subfloor to come.
be hung off the 2x12s bolted to the sides of the i-beam.
you can see the finished suite on pp. 38–39.
An I-beam on the way up.
The subfloor in place, with an I-beam
At left, shoring; upper
waiting to be hoisted. Note that a slot
right, a rectangular hole
has been cut into the 2x12 ceiling-
into which the beam end
joist array above, into which the
will fit once it has been
W12x26 I-beam will fit. At this
fully raised. At either end,
transitional point, the girder and post
posts will be cut to exact
seen in the photo above right are now
length, set into the wall,
carrying roughly half the load—the
and bolted to the legs of
joists at left—and shoring, at right,
the I-beam cap, which is
supports the cut-through joists. Once
an integrally welded
the I-beam is in place, the severed
Renovation 4th Edition Page 57
