The foundation of a tiny home, like that on any house or building, serves to transmit the loads of the superstructure and substructure safely to the ground. The majority of tiny homes are built on mobile foundations; for example, a flatbed trailer in which the foundation is comprised of axles and leveling jacks and the substructure is the trailer frame, which is also the floor system. Tiny homes can also be built on permanent foundations; for example, a shallow foundation with spread footings and the floor system framed directly on the foundation wall.
Foundations systems are designed to consider both the form and layout of the superstructure and varying ground conditions. In the case of the flatbed trailer, the surface material the trailer will be parked on and the specific contact points for the axles and the leveling jacks should be considered in detail. And in the case of the shallow foundation with spread footings, the condition and composition of soil, rock and water should be known and evaluated for soundness.
The loads that the foundation system must consider are a combination of dead and live loads. Dead loads include the weight of the superstructure and all immovable objects, e.g. appliances and fixed furniture and MEP systems. Live loads include the number of occupants, snow, wind-induced sliding, overturning, uplift, and earthquakes.
Disclaimer: You should always consult architects, engineers, or foundation specialists for specific instruction on special conditions. The documentation below is for typical conditions and does not accommodate, for example, earthquake- or hurricane-prone areas.
Mobile Foundation System
The choosing of a mobile foundation system will likely mean the choosing of a trailer to serve as the substructure and foundation. The trailer is an extremely important part of the design process and careful selection or design must be considered. Most standard flatbed trailers are not designed to accommodate the necessary loads of a tiny house. As a result, many trailer manufactures have begun to produce trailers specific to tiny homes, and tiny home building companies have been creating and providing their own lines of tiny home trailers.
The trailer will serve as the substructure and foundation of the tiny home and become the system that transfers all loads to the ground. The following factors should be considered when buying a trailer or designing and building one independently:
First, and most importantly, are the leveling jacks (A1) and axles (A2), which are the foundation. Leveling jacks are either attached or unattached; the two most common attached jacks are swivel or scissor jacks, and typically, unattached jacks are stackable jacks. The most cost-effective and durable jacks are stackable jacks; however, they require manual removal and storage space. Swivel jacks cost more than scissor jacks, but are much more durable as they have fewer moving parts. For a detailed analysis of trailer jacks, please visit etrailer.
The second consideration should be the type of steel section used for the frame and crossmembers (A3) and the trailer perimeter (A5). Trailers that utilize a C-section steel construction for frame and crossmembers are the most cost-effective, but are not suitable for wind shear, torsion, and earthquake forces that are typical of tiny homes. Therefore, utilizing a rectangular (tubular) steel section are the most suitable. The trailer perimeter should be critically considered, as the wall system will be mounted to it and will transfer loads to the frame and crossmembers.
The final consideration for a trailer should be the features that are optional, but drastically affect the cost of the trailer. These features include fender type (A4) and frame flashing. Each fender type correlates to wall construction type: round fenders to curved wood construction, angled fenders to angled wood construction, and squared fenders to perpendicular wood construction. And underside flashing will protect the trailer from weather, external debris, and rodents and/or insects. Frame flashing will drastically reduce long-term mainatinece costs and preserve the integrity of the floor system.
TRAILER COST ANALYSIS
For our Case Study 1 tiny home, three tiny home trailer manufactures were surveyed. This generated a comparitive analysis based on cost, frame and crossmember material, weight, and miscellaneous features, such as leveling jacks, frame flashing, and brake type. These variables yielded a cost gradient of $3,600-$5,600 USD, excluding taxes and the cost of shipping to a designated site. The $2,000 cost differential is the result of variables such as brakes (electric vs hydraulic), frame flashing (cost range of $400-$600), type of leveling jacks (cost range of $40-$300), axle type (high capacity axles vs drop axles), type of steel section construction (c-section vs tubular), and type of fender.
If looking into pre-designed and manufactured trailers, we recommend the tiny home trailer by Trailer Made Trailers. Their design process first involved the engineering and construction of a foundation for a tiny home and then figured out how to turn it into a trailer. And while Trailer Made Trailers is the most expensive comparatively, their trailers are designed to withstand tiny home forces and come equipped with all of the necessary capacities to begin home construction right away.
Permanent Foundation System
Foundation walls and spread footings are the most common permanent foundation systems for tiny homes, and are constructed of cast-in-place concrete (A3). The height of the foundation wall can vary according to design preferences, such as the accommodation of a basement or crawl space. The floor system is anchored directly to the foundation wall with a pressure-treated (water proof) sill plate and anchor bolts (A1). The floor system is framed on the sill plate utilizing wood headers and joists (A2).