Renovation 4th Edition

by Michael Litchfield

  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