Lets talk a little bit about foundations.
(NOTE: IANAGOSE – I am not a structural or geotechnical engineer — you should refer to one if you need project-specific advice).
When an engineer is not involved in a project, the house needs to be built to the prescriptive codes that your local authority having jurisdiction (AHJ / city / township) has enacted into law. For us, that is the International Residential Code 2006 Edition (IRC2006).
The IRC Chapter 4 has what are called prescriptive guides for designing foundations for common building styles and soil conditions. Ultimately, the soil beneath your house supports your footer, which supports your walls, which supports your roof. The bearing capacity of the soil determines how wide of a footer you will need to support the house.
The ASCE-10 determines how you add up the loads on your footer. For most structures, you will be summing the live, dead, and weather loads multiplied by applicable factors. Dead loads are things that are always there, and never move (e.g. concrete, wood studs, dry wall). Live loads are things that may be added or removed at any time (e.g. furniture, people). Weather loads are things like snow, rain, wind, or earthquakes (in seismic zones).
Lets run through an example for our house:
STEP 0: What is the soil?
If you want to be certain about the soil bearing capacity of your property, you will need to hire a geotechnical engineer to come out and do a soils report. Typically this involves drilling a number of 20′ deep test bore holes, and pulling out cores of sample material at various depths. From these cores, an engineer will be able to give you an estimate of the soils bearing capacity, and recommend foundation options.
A good first step that doesn’t cost any money, is to take advantage of the USGS soil survey. Its a rough classification of soils covering every square inch of the United States previously performed by the government. The resolution is not down to the lot-level, but it can give you an idea of what to expect beneath your feet. It also provides the USGS soil classification that will be useful when reading off the tables in the IRC.
STEP 1: What is the load on the roof?
For us, the biggest roof load is going to be snow. When doing foundation calculations, we examine a 1′ slice of the building. While the main roof spans 40′, that load is split between two exterior walls. This is a concept called tributary area.
We can’t forget the weight of sheathing, roofing, and drywall either! IRC Table R301.4 is a convenient list of assumed dead loads for common building materials:
Wood Sheathing (per 1″) = 3 PSF x (5/8″) = 2 PSF
Underlayment = 0.7 PSF
Asphalt Shingles = 2 PSF
Fiberglass Insulation (per 1″) = 0.7 PSF x 12″ = 8.4 PSF
Gypsum Drywall (per 1/8″) = 0.55 PSF x 4 = 2.2 PSF
Total: 15.3 PSF
Our heaviest truss is a main girder weighing 669 lb. Trusses are 24″ O.C., and again, that weight is split between two walls. Adding the contribution, we get:
STEP 2: What about the weight of the walls?
Our walls are made out of concrete (a dead load), and it weighs a lot (150 PSF), so we’ll want to account for it.
STEP 3: Add it up.
The total load on the footer is:
This is where the bearing capacity of the soil is assumed. Since we have terrible soil, and did not have a geotechnical engineer do a soils report, we need to assume worst-case performance. We can find the prescribed bearing capacity for clay soils in IRC Table R401.4.1 (1,500 PSF). We can use this bearing capacity to determine our required footer width.
We also could have used IRC Table R403.1 Minimum Width of Concrete Footings to size our footer. It takes wall construction (e.g. conventional light-frame, brick, or fully-grouted masonry), elevation, and soil bearing capacity as inputs and gives you a required minimum footer thickness. Around our area, a 16 x 8″ footer is typical which exceeds the minimum for most soil conditions.
| CLASS OF MATERIAL | LOAD-BEARING PRESSURE |
| Bedrock | 12,000 PSF |
| Sedimentary Rock | 4,000 PSF |
| Sand Gravel or Gravel (GW and GP) | 3,000 PSF |
| Sand, Silty Sand, Clayey Sand, Silty Gravel, and Clayey Gravel (SW, SP, SM, SC, GM and GC) | 2,000 PSF |
| Clay, Sandy Clay, Silty Clay, Clayey Silt, Silt and Sandy Silt (CL, ML, MH, and CH) | 1,500 PSF |
| 8″ Fully Grouted Masonry | 1,500 PSF | 2,000 PSF |
| 1-Story | 16″ | 12″ |
| 2-Story | 29″ | 21″ |
| Conventional Light Frame | ||
| 1-Story | 12″ | 12″ |
| 2-Story | 15″ | 12″ |
STEP 4: How tall is the footer?
IRC R403.1.1 specifies that the footer must be at least 6″ thick, and that the footer shouldn’t project further from the foundation wall it is thick. If we have a 6″ wall siting on a 16″ footer, then the footer only projects 6″ on either side.
This can become an issue as you got with progressively wider footers.
STEP 5: How deep should the footer be buried?
According to IRC R403.1.4.1, the bottom of the footer must extend below the frost line for your area. Per IRC Table R301.2(8) Frost Depth, that puts us at 40″ below finish grade as a minimum.
What did we ultimately choose?
For me, concrete is cheap, so I went conservative with 20 x 8″ footer. We are forming the footer up with a product called form-a-drain that combines stay-in-place plastic forms and footer drain tile. More on this later!
UNITS:
psf = Pounds Per Square Foot
plf = Pounds Per Linear Foot
kip = 1,000 Pounds
A Valiant Effort 