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Mobile Home Foundation Major Design Factors

Related Links ->: Major Design Factors | Design Criteria | Best Design Practices | Foundation Types

1. Initial Cost

Among the many factors that shape the selection of the foundation system, cost is one of - if not the most-important, especially for the sellers and buyers of modestly priced housing. Generally, discussions of cost focus on the initial home price. While the purchase price is only part of the total cost equation-maintenance and upkeep being the other major considerations-it is the part that can be quantified pre-purchase.

Price is in many cases the determining factor in choosing a foundation system, and this guide compares foundation systems in terms of the relative first cost.

Among the foundation systems reviewed in this guide there is considerable variation in cost, with pier and anchor systems among the least costly and basements the most expensive. Price is conditioned on local site conditions and design parameters, such as the case in seismic and flood hazard areas where design constraints generally increase the cost of all foundation systems. Other factors, such as amenity, aesthetics and desire to qualify for real property financing, are variables that may compete with or override the desire to select a low-cost foundation.

2. Real Property Classification

Be sure that the foundation system selected is one that is familiar and acceptable to construction and mortgage lenders. In advance of settling on a system that is intended to have a real property classification, it is always prudent to confer with area lenders as to the locally acceptable foundation systems. In California these are specifically classified as Permanent Foundation Systems and require the filing of a 433a form; other classifications are also in place in other states.

3. Installation Time

To speed up construction time for foundation systems, it's smart to be aware of local design and construction practices. Using prefabricated components, such as precast concrete grade beams, manufactured structural panels, prefabricated steel stanchions, pony walls framed for the non-proprietary systems, or one of the proprietary systems described in Chapter 4, can significantly reduce installation time.

Simple installations can be done in one day, but the time required for more complex jobs can extend to a week or more. When planning the time needed for an installation, it may help to set up a schedule of tasks that are to take place both before and after the home's arrival from the factory.

Compared with site building where construction schedules are often subject to lengthy delays, the manufactured home delivery dates are usually accurate to within a day or so. Use this to work backwards and schedule preliminary site work and foundation construction. If possible, try to have all pre-delivery work done a day or so before the home arrives. Doing pre-delivery work any earlier could affect the cost of borrowed construction funds; any later could mean the house is in the way while the work is being finished. It is prudent to budget extra time for site work the first few times a new system is specified. The home manufacturer can be a valuable resource in the foundation planning process. Developers and retailers who consistently get homes done on time do so because they plan their projects much like the home manufacturer plans for factory production. Each person working on a foundation or home installation should know the role they are expected to perform, the time in which they are to do it, and the standard to which they are to perform.

In general, the proprietary systems will require less time, because the components are prefabricated/preassembled in advance and generally require less work at the site. In budgeting foundation construction time, it is generally true that pier and anchor systems are the speediest, followed by crawl space systems, slab-on-grade, and basements. Foundation systems that require a crane for moving the home on to the foundation, such as a basement, typically take longer and usually cost more to install.

4. Matching Up the Chassis System

Some of the foundation alternatives we engineer require modifications to the floor and chassis system. While many manufacturers offer these variations in their regular option lists, some do not. It's wise to contact the manufacturer to make sure their floor and chassis can be adapted for use with the selected foundation.

For example, some basement designs work best with floors that are capable of clear spanning from the outside wall to the centerline (mating wall). To allow for basements uncluttered by a forest of posts that may be required to support the traditional chassis rails, many manufacturers offer, (usually at a cost premium), one of several types of integrated floor/chassis systems. These systems place the chassis in line with the floor system and move the structural support to the exterior wall where it bears directly on the foundation wall.

Other designs make use of direct fastening connections between the perimeter stemwall and the rim joists of the home's floor. This system calls for recessing the steel chassis parts away from the edge of the floor joists. Additionally, all utility dropouts need to be clear of this contact zone between the foundation sill and the floor joists.

For all of the foundation systems we design, there are numerous ways the homes can actually be attached to their foundations, and ultimately to the ground. Direct bolting and nailing are very common. If a steel-to-steel connection is involved, welding is optimal. We coordinate with you to optimize the foundation for your specific location, budget, and construction techniques available (and contractor preferences) within your area.


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In North Carolina, all projects are performed by Harrison Engineering Services PLLC our affiliated company.
In Montana, all projects are performed by Harrison Engineering Services our affiliated company.


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