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Renovation 4th Edition

Page 56

by Michael Litchfield


  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

 

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