Allowable Variations in Foundation Elevations
I’m frequently asked what the acceptable variance in foundation elevations taken on a home inspection. Here’s some general guidelines about foundation elevation standard and residential foundation movement standards.
1. International Residential Code (IRC):
According to the 2018 International Residential Code (IRC), on graded sites, the top of any exterior foundation should extend above the elevation of the street gutter at the point of discharge or the inlet of an approved drainage device. This extension should be **not less than 12 inches (305 mm) plus 2 percent. The IRC also covers other aspects related to foundations, including materials, footings, foundation drainage, waterproofing, and more.
2. HUD Guidelines:
For manufactured homes, the ground level must be at least “18 inches below the bottom of wood floor joists” and “12 inches below the bottom of the chassis beam”. If access is needed for maintenance and repair of mechanical equipment located in the under-floor space, the ground level in the affected area should not be less than “2 feet below wood floor joists”.
3. FEMA Recommendations:
Regardless of the elevation technique used, the foundation of an elevated house must withstand expected loads from hydrostatic pressure, hydrodynamic pressure, debris impact, and erosion. It should also resist undermining due to expected erosion and scour .
4. Texas Residential Foundations Guidelines:
The Texas Section of the American Society of Civil Engineers (ASCE) provides guidelines for residential foundations. These include:
– Recommended Practice for the Design of Residential Foundations.
– Guidelines for the Evaluation and Repair of Residential Foundations.
These guidelines are not intended to be strict standards but serve as valuable guidance for engineers evaluating complaints related to residential foundation engineering. They reflect the collective engineering opinions and practices of committee members.
5. Acceptable Foundation Movement:
As of 2021, widely used standards include “1% tilt” (where foundation slopes exceed a rise or fall of one inch in a span of 100 inches) and “L/360 for deflection” (where deflection should not exceed the span length divided by 360). For example, 100 inches corresponds to eight feet four inches (8’4″)³.Generally, acceptable foundation movement is defined in terms of fractions of an inch or millimeters.
– For vertical movement, a guideline suggests that “1/2 inch to 1 inch” over a **20-foot span** can be considered acceptable.
– Lateral movement (side to side) should ideally not exceed “1/4 inch”.
6. In Conclusion
Remember that these guidelines provide valuable insights, but good engineering judgment based on education, experience, wisdom, and ethics remains essential in any specific engineering application. The standards are crucial for ensuring the stability and safety of structures. If you need more detailed information, I recommend referring to the official codes and guidelines provided by relevant authorities.
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