DESCRIPTION
The crawl space foundation system described here has two main distinguishing characteristics: it incorporates full perimeter wall support together with internal, independent support points; and the space itself is not habitable. Within that very broad definition, there are many styles, designs, and ways to build pit-set crawl space foundation systems. This section describes one design that has worked effectively with manufactured homes that we design for. Crawl space foundation systems are intended for use where a traditional site-built foundation look and feel is preferred. This foundation is less well suited for instances where economy, speed, or installation flexibility is paramount.
In some of our designs, the perimeter foundation wall rests on an excavated footer. The wall itself may be constructed of one or more conventional building materials (such as poured concrete, concrete block, or treated wood). The entire perimeter of the manufactured home floor bears directly upon this wall. The chassis is also fully supported, but with relatively economical piers. The manufacturer-designated ridge beam support points are carried by economical piers or posts. The home’s resistance to horizontal or uplift forces is achieved through attachment of the floor joists to the exterior foundation wall. No additional anchoring devices may be necessary – our engineers will determine this.
This foundation can be used on sloping lots and for recessed “low profile” installations. In the latter case, the structural walls form a barrier to the entry of water underneath the home and act as a short retaining wall. The low profile design, providing a site built “look,” is much more difficult to achieve with a traditional anchor set or slab foundation.
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Access to the crawl space for utility hookups and repairs must be considered by our engineers. This is potentially problematic in a low-profile installation. One solution is for the contractor to install an access panel in an appropriate location, such as a closet floor.
The low profile crawl space foundation system provides more of a site built appearance as shown below.
CONSTRUCTION
Crawl space foundation systems require more care and precision than conventional anchor systems. The exterior wall of the foundation should not exceed the dimensions of the manufactured home’s perimeter floor joists (not including the thickness of any exterior siding or sheathing). After staking the site, excavation to the depth of the footing is done with a backhoe. Interior footings may be individually dug with a power soil auger or poured as a grade beam. Some variations of this system allow placing interior piers on crushed rock.
As forms are constructed, they are double checked to make sure they are level, dimensionally accurate, and square. Reinforcing steel specified by our engineers is set as required. Concrete is poured, tamped, and dressed (e.g., anchor bolts are carefully placed for the sill).
Forms are stripped and the structure is again measured. The walls may also be constructed of mortared and grouted hollow core blocks, again with bolted sills. A third option is a concrete or block stemwall in combination with a wood-framed ponywall.
Pony walls are useful when a low-profile installation is either not possible or not desirable. They also afford the installer with a bit more dimensional tolerance when placing the home directly on the stem wall.
A well thought out plan for setting anchor bolts will prevent trouble later when installing the home. Bolts should be carefully placed so that they will not coincide with the floor joists. This is a matter of good planning and careful workmanship. Cutting off bolts or coring out the sill to allow for the bolt’s washer and hex nut is not recommended. Interior support footers, both along the chassis and at the ridge beam columns, may be poured concrete or crushed rock (if locally approved).
VENTILATION
Crawl space ventilation is provided through perimeter wall vent openings. Planning for vents varies, depending primarily on whether or not the home is being installed in a low-profile configuration.
INSTALLATION
Normally, the foundation, especially the low-profile version, is completed before the home arrives. The axles, tires, and hitches are removed, and the home is installed on the foundation by craning or rolling. Once placed on the foundation sill or pony wall, the floor is brought to a level position. Interior piers are placed along the chassis beams and positioned at the designated ridge beam columns.
Where an open endwall permits the truck to back the home inside the foundation, a home can be moved into position by the toter. Building the missing endwall then finishes the foundation.
There are a number of options for constructing the interior piers that carry the chassis and ridge beam loads to the ground. The most economical is approved dimensional lumber, such as redwood or treated fir. Also popular are non-grouted hollow core concrete blocks and manufactured steel piers. Since these structural components only provide vertical support, they may be selected for economy and ease of installation.
METHODS OF ATTACHMENT
The crawl space design allows attachment of the entire perimeter of the floor joist system to the foundation sill, the preferred and most economical approach. The connection is secured with engineer-approved nailing strips (or approved steel nailing plates), fastened according to our engineer’s nailing schedule. The strips may then be painted and left as a finished surface. If vinyl siding is to be applied over the sheathing, it is installed last.
Chassis piers may be placed and tightened to the chassis beam through compression, wedges nailed in place, spot welds, or a number of proprietary attachment devices.
CONSTRUCTION CHALLENGES
Typically, a crawl space foundation is fully constructed before a home is installed. This presents a few challenges to the foundation contractor and home installer:
- The foundation must be precisely measured and constructed. Installers/builders are advised to consult with the home manufacturer to obtain the exact floor dimensions. There is less tolerance for error than if the foundation were intended for a site-built home.
- The manufacturer must provide a “foundation ready” floor-chassis system. This involves recessing all steel chassis components 8�10 in. from the edge of the floor joists.
- The home is normally moved onto the foundation with rollers. If the site is not accessible from the street, a crane is used. The use of a crane is often the method of choice for multiple installations.
- Often, the working conditions under the home are cramped and dark, due to the enclosed perimeter.
- When placing a home in a “low-profile” installation, planning for adequate slope in the home’s drain line is important. If the home site and proximity is high enough above the street sewer, the drain line can be routed under the perimeter foundation footer; otherwise, a sleeve must be placed in the foundation wall to allow the drain to exit. Depending on the floor plan, the drain may need to be routed a substantial distance before it exits the foundation, creating potential problems with inadequate slope. Some manufacturers can supply “drops” through the floor and ship sufficient loose material to hang the drain line with proper slope.
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COST
Crawl space foundation systems generally are more expensive than slabs and anchors but less than basement foundation systems. If carefully planned, savings gained in some parts of the system will pay for costs generated in other areas. For example, many times the cost of tie-downs (ground anchors) is eliminated in crawl space foundations and piers used for the support of the chassis may be scaled back significantly. These savings partly offset the additional costs associated with building a perimeter wall that both supports and finishes the home.
REAL PROPERTY CLASSIFICATION
A crawl space foundation system utilizing the perimeter load-bearing enclosure walls could be expected to meet the conditions for real property financing. With the structural perimeter wall, the home can be securely attached to the foundation to resist wind, gravity, and seismic forces.
INSTALLATION TIME
Installing a manufactured home on a perimeter wall crawl space foundation system is typically a two-step process, similar to an installation over a basement. First, the home is delivered alongside or near the foundation and uncoupled from the transporting truck. Next, the house is raised, the axles, wheels, and hitches are removed, and then it is rolled or craned onto the foundation. This is inherently slower than driving the home into its final setup position, as would be the case with an anchor and pier or slab system. A crew of three typically can construct the crawl space foundation system in three days before delivery and install and finish the home in five days after delivery (not including a garage or other ancillary structures).
WIND LOAD RESISTANCE
Although the areas where this system has been popular are not prone to hurricanes, pit-set crawl space systems offer effective resistance to the buffeting forces of high winds. The continuous fastening of the home’s floor joists to a sill ( i.e., anchored to a structural concrete wall and footing) provides good resistance to horizontal forces. Further, homes set in “low profile” offer less wall exposure to high winds thus reducing the loads required to be resisted by the connections.
GRAVITY LOAD RESISTANCE
The perimeter load-bearing enclosure wall support system provides excellent gravity load resistance. The perimeter wall carries much of the roof load directly to the earth. The chassis main beams and piers carry only the interior floor loads. The perimeter enclosure wall supports the full perimeter of the home.
SEISMIC LOAD RESISTANCE
The perimeter load-bearing enclosure wall support system combined with a deep stemwall provides adequate seismic load resistance. The structural connection of the home to the perimeter foundation wall can be designed by our engineers to effectively transfer the forces to the earth around the foundation. This system has been used effectively in the San Francisco Bay Area, one of the highest seismic risk zones in the nation.
FLOOD RESISTANCE
The low-profile version of the crawl space foundation system described here is not intended for use in floodplain areas. However, when FEMA-recommended designs are specified, crawl space system foundation systems are suitable for use in floodplains. Among the FEMA requirements are the following: the foundation system resists flotation, the floor level is set above the base flood elevation, and the home site is not in the floodway.
FROST HEAVE
As long as the footers reach to below the maximum frost depth, crawl space foundation systems can be and have been used in any climate.
