Restoration Guide: Wood and Prefabricated Foundations

Roger Diez

Editor's Note: This is article 3 of 9 in the Foundations Chapter of the Old House Web Restoration Guide. This guide was developed and edited for old homes from original materials in the U.S Housing and Urban Development (HUD) Rehab Guide.


Section 1--Permanent Wood Foundation Systems

Permanent wood foundation systems are an alternative to concrete or masonry foundations. When replacing an existing concrete foundation wall that is damaged or deteriorated, a permanent wood foundation may be a solution. If an old house has a damaged permanent wood foundation, the same material should be used for repairs.

Permanent wood foundation systems consist of a footing plate on a base of crushed stone that is then topped with a stud wall sheathed in plywood. A number of factors, including stud spacing and number of stories, determine whether this stud wall should be constructed of 2x4, 2x6, or 2x8 lumber.

For preservation purposes, the wood in a permanent wood foundation system must be pressure-treated using either Ammoniacal Copper Arsenate (ACA), Ammoniacal Copper Zinc Arsenate (ACZA), or Chromated Copper Arsenate (CCA). Fasteners used in the system should be stainless or galvanized, and the exterior plywood sheathing should be covered with polyethylene sheeting.

1.1 Replacing Your Foundation with a Permanent Wood Foundation System

When replacing a damaged portion of your existing foundation, follow these steps to make your home renovation project easier:

  1. Correct the source of the damage
  2. Remove the damaged portion of the old foundation wall
  3. Excavate down to the existing footings
  4. Replace existing footings if necessary
  5. Connect the new permanent wood foundation section to an existing concrete or masonry foundation with half-inch diameter galvanized lag screws and lead expansion shields
  6. Use lag screws to connect to an existing permanent wood foundation; additional studs may be needed in the older section

The advantage of using a permanent wood foundation for repair is that it can be installed in cold weather that would prevent the use of concrete or masonry. It can also be installed more easily in areas with limited access.

Section 2--Prefabricated Foundations

Prefabricated foundation systems are of two types:

  • Structural insulated panels (SIPs)
  • Precast concrete panels

Although both systems were originally developed for new construction, they are also useful in home restoration projects for replacing damaged sections of existing foundation walls. The benefit of the SIP system is its light weight, making it easy to install without heavy equipment, while precast panels must be set in place using a crane. Because both systems are prefabricated off the job site, they can be installed in cold weather which would preclude using concrete or masonry.

2.1 Replacing Sections of Damaged Foundation with a SIP Foundation System

The construction of an SIP foundation system is similar to that of a permanent wood foundation system. It differs in its materials, which consist of a foam core panel of expanded polystyrene (EPS) and sheathing of pressure-treated plywood on the exterior and either plywood or oriented strand board (OSB) on the interior. The interior sheathing must also include a fire barrier that provides a 15 minute thermal barrier.

Panels come in standard 4x8 or 4x9 foot dimensions. Installation of SIP panels is similar to that of permanent wood foundation systems, using a footing plate on a base of crushed stone, gravel, or concrete. The SIP panels are attached to the existing foundation using a 2x splice fastened with lag screws.

Like the permanent wood foundation systems, SIPs can be installed without the use of heavy equipment and in areas where access is limited. Their one drawback is that the standard size may not match up with the portion of the existing foundation to be replaced.

2.2 Replacing Section of Damaged Foundation with Precast Concrete Panel System

Precast concrete foundations can be made to order to fit the section you want to replace, but you'll have to use a crane to put the panels into place.

Sections of precast concrete can be bolted together, and some are made with pre-drilled holes for plumbing and electrical conduit. Most precast concrete sections only need waterproofing at the joints and allow for varying amounts of polystyrene insulation to be added. Precast panels can be attached to your existing foundation walls.




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