Design Course
for
The Permanent Wood Foundation System

Each building, including its wood foundation, is unique. The foundation will perform as expected when thoughfully designed and then strictly using this design to build it during construction, It is important to follow the design without any deviations.

Foundations are the most important part of any structure because foundations are the support for the building above. It is the foundation that transfers the weight of the building to the bearing soil. If the foundation is flawed, its problems will affect the rest of the building. However, there are many variations that influence a correct design of a wood foundation. To recongnize these variations and identify them in a plan is important. This is written to help you become more familiar with all the variations, how to recongnize them, and cover them in a plan. No reference should be made to published tables. They may be correct or partly correct for a particular building, but they are more often incorrect. This is true because there are so many variations to consider. A book of tables an inch thick would be hard pressed to keep up with all the variations. A small variation such as column height will change everything in a design.

Applicable Referenced for Permanent Wood Foundation Design

1. Technical Report Number 7 - AF&PA
2. National Design Specification for Wood Constuction - AF&PA
3. Minimum Design Loads for Building and Structures - ASCE
4. Michigan Residential Code 2000

In Section R401.1 Application.......Wood Foundations shall be designed and installed in accordance with AF&PA Technical Report No. 7.

Exceptions: 1. Wood Foundations to be permitted only: (1.1) In building that have no more than two floors and a roof; and (1.2) When no dimension in a basement room or crawl space exceeds the smaller of the building width or length.

Wood foundations in Seismic Design Categories D1 and D2 shall be designed in accordance with accepted engineering practice.

In secrion 404.2.3 Height of backfill.........The height of backfill against a foundation wall shall not exceed 4 feet.

The above is interpreted as meaning, according to the position of AF&PA: "The International Residential Code (IRC) or Michigan Residential Code 2000 (MRC) provides a simple, perscriptive solution for the PWF. Wood foundations may comply with this prescriptive solution in lieu of design. Wood foundations must strictly comply with the prescriptive requirements in addition to the provisions of TR7. The prescriptive requirements limit the backfill to 4' in height, there must be equal height of fill aroung all sides of the foundation, and the distance between points of lateral support (the point at which an interior wall is perpendicular to and abuts the exterior foundation wall) cannot exceed the width of the building (cannot exceed the smaller of either the building length or width). Minimum dimensions and strength characteristics are specifird in the IRC (MRC) for wall studs, along with minimum thickness for plywood sheathing. When these limitations cannot be met, the perscriptive provisions of the code does not apply. Therefore, any foundation structure or portion thereof outside the prescriptive requirements must be completely designed."

In addition, the prescriptive requirements apply only to 8' high walls and the use of a 30 lbs./cu. ft. equivalent fluid weight soil pressure. A 30 lbs./cu. ft. soil pressure applies to ordinary non-expansive clay soil with a small overdig. If the soil type is anything other than this, it does not meet the prescriptive requirements, and must be completely designed. Also, if the wall height is greater than 8', it does not meet the presciptive requirements, and must be completely designed.

TR7 does not provide for prescriptive solutions, but is the building code recognized reference for the design and construction requirements of all PWF's.

The loads induced on foundation walls are unique and may result in a required analysis of the resisting system. Foundations must be designed for each unique building configuration and soil condition. The only exception is the prescriptive requirements described above.

The Permanent Wood Foundation System, Design, Fabrication, and Installation Manual (DFI) is no longer published by the American Forest & Paper Association. The DFI has never been referenced in the model building codes, but has been a guidance document for wood foundation designers. Permanent Wood Foundation Design and Construction Guide is now published by the Southern Forest Products Association on behalf of the Southern Pine Council. The Guide is not referenced in the building codes. Its purpose is to use it as a guide to provide a homeowner, builders, designers, or building officials with the best practices reference materials. It must not be considered an alternative to the requirements for structural design.

Basic Requirement for Design Before the Design Process Begins

Note: Panel foundations can be site built or made in a shop. When made so it cover up items that need inspection by the Building Inspector. Third Party Inspection may be required. (inspect for grade stamps, treatment stamps, stud spacing, insulation, nailing, dip of saw cuts, plywood requirments are some of the item that a third party inspection will note.

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For more information:

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Note: Panel foundations can be site built or made in a shop. When made so it cover up items that need inspection by the Building Inspector. Third Party Inspection may be required. (inspect for grade stamps, treatment stamps, stud spacing, insulation, nailing, dip of saw cuts, plywood requirments are some of the item that a third party inspection will note.

All Panel Wood Foundations must be designed and installed in accordance with:
All current Building Codes Standards

While wood foundations are easy to build, this is only true if one is building from an accurate well designed plan. When such a plan is incorrect, or if something is left out of the plan or absent, or if a design is made using a guide manual (these are not design manuals); major mistakes can be made during the construction process. These errors cause problems for the owner, builder, and the building department.

PermanentWoodFoundation.com a service of Permanent Wood Foundation System (PWF), supported by Southern Pine lumber users affiliated with the training and installation of wood foundations.
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