Site Construction
There are different types of foundations for prefab homes. Here is a list of some of them along with a brief description of what they are. Be sure to check with your township’s building inspector to see what your area requires!
SINGLEWIDES
Singlewides are typically set on piers, slab, or slab with piers. These prefab homes can go in mobile home parks and sometimes placed on land depending on local building codes.
DOUBLEWIDES
Doublewides are typically on a slab with piers, crawlspace, or basement. Prefab homes like these can go into mobile home parks and generally on private land, be sure to check with the local building department!
MODULAR (BOCA) HOMES
Modular (BOCA) homes can ONLY go on top of a crawlspace or basement. These homes do not require steel I-beams and do not have frames under them. Modular Homes are built to local building codes and are considered “stick-built” homes. For financing and appraisals, they are also viewed as regular stick-built construction.
Foundation Types
Cookies
Round cement discs. Normally 24 inches diameter and 4 inches thick. Very few townships allow this type of foundation for prefab homes. If you have an older home that has been on the property for a long time, you may find these under your home. (This will be a Flat Set.)
Piers
Piers are cement cylinders that go under the frame of the home at every blocking station (typically about every 8 ft.). Piers are normally 18 inches in diameter and go 42 inches deep. This goes down below the frost line. That way, when the ground freeze and rises, the prefab homes stay anchored in place. Sometimes, in a location with very sandy soil, piers can be shallower than the 42 inches. (This will be a Flat Set.)
Slab With Piers
This is a slab with piers underneath it. This is a very common foundation. (See above for slab and pier definitions. This will be a Flat Set.)
Basement
This is basically the same thing as a crawlspace, just deeper (read above description). Modulars do not have a frame or I-beams and only need an 8-foot wall. In order to have bedrooms in the basement, egress/daylight windows are required in each bedroom. This allows for a safe and accessible escape in case of fire. (This will be a Roll-On Set and might require a crane.)
For a HUD (doublewide home) there will need to be steel I-beams going across the top of the basement to support the home. Because the home will have the frame and I-beams, the walls should be at least 9 feet tall to have 8-foot ceilings. Doublewides are not normally placed on basement anymore. Most factories no longer offer the option to build them basement ready.
Ribbons/Runners
These are long rectangles that run the entire length under the steel frame of the home. For a singlewide, there are 2, for doublewides, there are 4. Not too many townships allow this type of foundation. (This will be a Flat Set.)
Slab
A solid rectangle of cement the same size as the home. We recommend pours an extra 2 inches on each side to make it easier to install skirting or lay perimeter blocks. Slabs are typically 4 inches thick. We recommend pouring a 12 inch footing/rat wall along the end that the home will be pulled/backed on to prevent any cracking. A slab without piers is called a “floating slab.” This is because without the piers to anchor it, when the ground freezes and expands, the slab may slightly rise. (This will be a Flat Set.)
Crawlspace
This is a 48 inch deep rectangle that is excavated out to match the size of the home. Typically, a crawlspace has poured walls and a poured floor. Sometimes, a blocked wall is done and the floor is sometimes pea-stone gravel on top of a vapor barrier. On top of the wall (blocked or poured) a sill plate (2×6 treated lumber) is installed. This is what the perimeter of the home sits on. With doublewides, steel beams will be required to go across it (normally about 4, depending on the home size, layout, and structure) which require beam pockets in the wall. Under each beam will be a jack post that will be on top of a footing. Proper venting and an access door will be required to meet building codes. (This will be a Roll-On Set and might require a crane.)