Foundation Capping and House Bolting
Foundation capping of an existing un-reinforced masonry foundation refers to the addition of a new concrete foundation on the top of an existing foundation. It is sometimes referred to as a foundation retrofit. When concrete is placed on all 3 sides of the old foundation, it is called a cap. When caps are installed the original mudsill becomes embedded in the foundation. This makes seismic retrofitting difficult because the plywood cannot be nailed to the mudsill which is necessary to create a good retrofit shear wall. It is hard to justify a foundation cap if the house has not had the 6 inches of code required clearance between the house and the dirt for 50 or more years with limited or no ill effects to the building,
The retrofitted foundation should have some reinforcing steel connecting the cap or saddke to the existing foundation but there is really no tell to without a metal detector. The concern is that the cap will slide on the old foundation.
Retrofit foundation capping is recommended with some regularity by San Francisco Bay Area pest inspectors for wood framed buildings in order to increase the clearance between soil and wood wall framing. This has nothing to do with the installation of an earthquake retrofit. This is almost always driven by a desire to make existing buildings comply with CBC Section 2306.8, which for new construction requires a minimum clearance of six inches from earth to wood that is neither treated nor of natural resistance to decay. This was not a requirement when older houses were built and this is recommended to bring the older house “up to current code” without considering if it is really necessary and how it will impact future earthquake retrofit attempts. Foundation caps cause the mudsill to become deeply embedded into the concrete, making a seismic retrofit very difficult.
When retrofitting the foundation with a foundation cap, the high cost and the impact it has on future seismic retrofit plans would put this at the bottom of the priority list.
Uses, Limitations and Risks of Foundation Capping
Retrofit foundation capping of a concrete foundation is a complex and expensive operation that provides a number of opportunities for damage to the building. A contractor shores up the building just inside the foundation line in the crawlspace with “cribbing” (large beams) to hold the house in place. The bottoms of the cripple wall studs are then cut off at the new height and they are nailed into the same mudsill of the original cripple wall. After that the concrete is poured so that the mudsill is now embedded into the concrete which, as stated before, makes the cripple wall to shear wall conversion very difficult.
Concrete Saddling of Brick Foundations
If a brick foundation is capped you often see a saddle. This means the brick is covered on the top, inside, and outside edges. To find out if it is a saddled brick foundation dig below the concrete and see if you find brick. It does make sense to treat a brick foundation this way as it encapsulates the bricks and keeps then together in an earthquake.
Better ways to Address Earth to Wood Contact.
The soil to wood problem can be addressed using pressure treated wood and plastic spacers as shown in the photos below.
Wood to soil clearance is likely to be only one of many aspects of an existing building that may not conform to the current code for new construction. There is seldom any code-based requirement for an existing building to meet current requirements for new construction (change of use of the building might be one reason for such requirements). The decision to modify the existing building should be based on the ease with which modifications can be made and the likely hazard if modifications are not made. As one example, if a building has existed for 80 years without decay or pest damage, it is hard to justify expensive measures to avoid possible future decay or pest damage; periodic monitoring might make more sense in this case. Where possible, lowering of the soil is the quickest, easiest, and cheapest solution to increasing soil to wood clearance.
Where lowering the soil is not possible, it may be reasonable to do nothing or possibly to add decay-resistant framing.
Capping both sides and the top of a foundation (sometimes called a saddle) provides a good approach to re-supporting studs and posts at a local area of foundation fracture or deterioration.
Where foundation fracture or deterioration occurs in more than a couple of local areas, where it is determined desirable to increase the strength of the foundation, or where hardware anchorage from retrofit of woodframe portion of the building is needed (other than anchor bolts), the other two methods proposed for foundation retrofit should be pursued: replacement or parallel systems. A foundation poured parallel to the old foundation would be the cheapest of the two alternatives because the old foundation would not have to be removed.
Design Guidance for Capping
In all cases where damage to existing foundations appears to be caused be by soil conditions such as settlement or expansion, it is desirable to consult with a geo-technical engineer prior or drainage specialist. Sources of water causing deterioration should also be eliminated where possible.
Foundation capping is an expensive retrofit, and the relative benefit of foundation is probably minimal. Where capping is proposed, it is appropriate to ask what the benefit to cost ratio might be, and whether the expense of capping is justified. Foundation capping has been recommended in buildings that have unbraced cripple walls and soft first-stories (due to parking); retrofit of the cripple walls or soft story would generally produce a much greater benefit to cost ratio and possibly also reduce risk to life.