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Home Basics

How well do you know your house?  Let's start from the bottom up with some home basics that will help you know your home.

Your Foundation
The foundation is the part of the house that interacts with Mother Earth.  There are several different types of foundations that can be constructed depending on the soils found on your homesite. If you got a good deal on plat of land be careful.  It may be cheap for a reason!  Swampland can be costly.

Most homes in the Chicago area are built on footings that bear directly on the soil.  The soil should be either undisturbed clay or can be properly compacted fill.  Ask your contractor if the fill has been compacted to 95% Proctor. 

Be careful if your home is adjacent to a wetland.  The house in the photograph to the right settled 6" due to a soft layer of soil deep underground.  The repair cost was $150,000!  Steel piles were installed 80 feet deep to good soil that allowed the house to be jacked level.

The footings and piers need to be installed below frost depth to prevent frost heave.  Frost heave occurs when water freezes under the footing and pushes the house upward.  When the ground thaws out in the spring, the house may settle back down but the damage to the structure will likely have occurred.  Frost depth in northern Illinois is 42" and may be deeper under paved surfaces (side driveways). 

If a house does not have a basement, an 8" wide frost wall needs to be built on top of the footing.  The frost wall needs to extend 6" above the finished grade elevation.  The wall can be made from treated wood, brick, concrete block, or preferably from poured concrete.  The foundation for a basement needs a footing drain and a sump pump to help keep the basement dry. 

Stormwater seepage is a major issue and causes headaches for many homeowners.  Stormwater seepage can be minimized by proper grading away from the foundation.  The minimum pitch should be 1/8" per foot toward a swayle or other drainage path.  Gutters and downspouts will also help direct stormwater from the roof away from the house.  Be sure to install downspout extensions so that the stormwater does not pool along the side of the house.  The homeowner forgot to reattach the downspout extension for the house shown to the right.  The hydrostatic water pressure pushed the concrete block foundation wall 6" off the footing.

Brick, concrete block, and poured concrete foundations crack for many reasons.  Structural Design Corporation (SDC) does not recommend hiring a contractor to determine the nature of a foundation crack.  The engineers at SDC have saved homeowners tens of thousands of dollars by determining the root cause of a crack and specifying the proper repair procedure.  Some cracks do not require repair and foundation repairs can be costly.

The Structural Frame
Most single family homes are constructed with a wood frame.  A wood sill plate is bolted to the top of the foundation and the 1st floor is built off the sill plate.  After the 1st floor deck has been completed, the exterior frame walls are built, tilted up, and nailed to the floor deck.

Your house has different types of walls.  Load Bearing Walls support the floor joists and transfer the floor loads down to the foundation.  Shear Walls transfer the wind or seismic loads to the foundation along the length of the wall.  All of your exterior walls are shear walls.  Shear walls need to have the proper sheathing (plywood) and nailing to transfer the shear loads (wind/earthquake) to the diaphragms (floors).  Some of the Interior Partition Walls that separate the rooms may also be load bearing or designed as shear walls.  Partition walls can not be removed or modified without determining if they are part of the structural frame.  

Openings in load bearing walls require headers or lintels to transfer the loads around the openings.  Double 2x8's or 2x10's are usually used to support the loads above doors and windows.  Your garage door may have a structural steel header since the opening is so large.  If your house has exterior bricks, the house may have two (2) headers at each opening.  One header is used to support the bricks while the header supports the roof and 2nd floor framing loads.

The floors in your house are designed for a live load of 40 pounds per square foot (40psf) along with their dead weight.  These are relatively light loads in the world of a structural engineer. Wood joists transfer the loads from the wood subfloor to the foundation walls or interior support beams.  The support beam can be constructed from either wood timbers or structural steel.  Most support beams have posts that carry the interior loads down to the spread footing foundations.  Some newer homes have floor trusses that have longer allowable spans than wood joists.  Trusses may eliminate the need for the interior support beams, columns, and foundations.    

Most structural problems with floors show up as excessive deflections or sags resulting in cracked floor tiles or drywall cracks.  Some floor trusses can be very soft or spongy due to their long spans.  Soft floors result in annoying vibrations and general discomfort. 

Many older homes are built with plaster walls which are very heavy.  The plaster walls are normally supported by a double floor joist.  It is likely that no structural engineering analysis was performed to properly size the beams/joists that support the plaster walls.  The undersized beams/joists result in large floor sags directly under the walls.

Floor deflections are normally limited to a value of L/240 where L is the length of the floor joist or floor beam.  In a home with plaster walls the floors are designed for a maximum deflection of L/360.  It is our experience that the new homes with ceramic floors need to be especially rigid.  We recommend that the floor deflections be limited to L/480 for expensive ceramic or marble floors.  We have found many cracks in ceramic floors directly above the main support beams.  This is due to counter flexure of the floor joists as they cross over the top of the beam.

Roof design is complex and can take many forms.  Your roof can be either stick built or constructed from trusses.  We recommend constructing your roof from trusses.  However, if you want a cathedral ceiling, ridge beams with rafters can be designed. 

Roof design has changed dramatically since Hurricane Andrew in 1992. FEMA has recommended a number of design changes that are now part of the International Building Code (IBC).  Most of the changes are not well understood by either design professionals, local building officials, or contractors.  If you are building that dream home ask your architect and contractor about your roof.  Is it designed by a Structural Engineer?  Does it have hurricane clips even if it is not near a coastline?  Is the ridge beam posted?

Here are some technical issues that need to be addressed when designing a roof:

  1. Is the roof designed for snow drift?
  2. Is the roof designed for unbalanced snow loads?
  3. Is the roof designed for uplift loads?
  4. Are the valley and hip rafters designed for the extra wind pressures required by the IBC?   

As a homeowner, remember that ceiling joists are part of the roof system and can not be cut for any reason without proper reinforcing.  The attic is not designed as a living space or for storage loads.




Related  Photographs














Workmen set up hydraulic jacks to level basement wall that settled 6 inches.










Concrete block foundation wall that buckled inward 6" due to hydrostatic pressure.  Note:  It is important to have a shear key or reinforcing bars to transfer lateral loads to the footing.