A mobile home spends a lot of time sitting in one location.
In theory, you can put it down anywhere, but a home (even a mobile home) needs a secure base if you want to avoid that sinking feeling or an unnatural tilt to your rooms.
1. Why Does the Foundation Matter?
Remember the parable about the wise man building his house upon the rock and the foolish one building on sand?
Although this story (and song) is a philosophical musing on good moral values, it also shows some practical wisdom.
A solid base is better for your manufactured home for long-term stability and fewer maintenance issues.
Foundations are either permanent (attached to the mobile home) or non-permanent (the mobile home leaves the foundation behind if you move).
Although a slab concrete foundation remains in place for decades, it classes as a non-permanent foundation because you can remove the mobile home and leave the foundation behind.
Putting a mobile home on a foundation makes sense, and most mobile homeowners opt for a slab foundation.
2. What is a Slab Foundation?
A slab foundation is a single slab of poured concrete.
The mobile home rests on piers on top of this concrete foundation in many cases.
Tethering bolts may secure the house in place to stop it from slipping or sliding off the foundation in severe weather.
The top benefits of a slab foundation are:
- Economy – it is the cheapest mobile home foundation to install.
- Ease – it is the most straightforward way to create a mobile home foundation.
- Speed – it is the fastest foundation to build.
- Bonus – you can create a crawlspace between the foundation and the mobile home, a valuable dry storage area.
There are some issues with slab foundations for mobile homes, and these include:
- Watching out for the frost line – you may need to insulate or add piers.
- The wrong type of foundation for a slope – you risk slipping downhill.
- Slab movement means you may damage any plumbing pipes embedded or passing through the slab.
3. How Do You Install a Slab Foundation?
The process is super simple:
- Start by leveling the ground.
- Build a wooden or polystyrene frame – pay attention to leveling and countersinking as this is the mold for your uniform slab.
- Lay 4-6 inches of gravel for additional support for the slab.
- Add piers if you live in an area where the ground freezes and thaws.
- Add connection points and sleeves for water, sewage, and other utilities.
- Add reinforcing bars or rods to prevent ground movement from cracking the slab.
- Pour the concrete to create a slab that is 4 to 6 inches deep.
- Leave it to cure.
- Remove the wooden supports.
Before you install the mobile home, you will add footers or piers to support the I-beams in the base of the manufactured house. Secure fastenings keep the mobile home in place.
To meet the HUD regulations, your concrete slab must be:
- Below the frost line.
- Certificated by an approved professional.
Plus, the infrastructure for your utilities must remain within the walls of your mobile home and your foundation.
For extra strength, use building blocks instead of the wooden frame, and these form part of the permanent structure.
Compacting the gravel sub-base before pouring the concrete reduces the potential impact of ground heave. Ground heave is when the soil expands and contracts due to moisture or freezing conditions.
4. What About Weight?
The foundation supports the weight of your mobile home.
You calculate the weight on the force of pounds per square foot.
Older homes are lighter than newer homes; as an approximate guide, older homes (pre-1980) are 40LBS/sqft, and newer ones are 50 LBS/sqft. A mobile home with higher ceilings and drywall can be 70LBS/sqft.
To calculate the weight of your mobile home, take the width and the length and multiply them to find the area in square feet. Then multiply the area by 40, 50, or 70 (depending on the house type) to estimate the weight.
If in doubt use 70, because it is better to over-engineer your concrete slab than underestimate the load.
The strength of the concrete slab depends on:
- Inherent compressive strength – ranges from 1,800 to 4000 psi depending on the formula.
- Thickness of the concrete slab – thicker is not necessarily stronger; you need the most appropriate depth.
- Load-bearing capacity of the underlying soil.
- Load distribution – even or concentrated at the edges.
- Reinforcement bars.
The total load on a slab is a simple sum of the following appropriate loads:
- Deadweight – the weight of the mobile home.
- Live load – the added weight of moveable furniture and people.
- Wind load – varies according to location.
- Self-weight – the weight of the concrete slab.
- Snow load – in some areas, winter snow adds seasonal weight variations.
- Seismic load – the movement of the ground transferred to the slab produces additional load impacts.
Although the calculation seems complex, you need to consult the HUD codes for your area when you want to build a slab foundation.
An adequately supported 6-inch concrete slab foundation with reinforcement and an appropriate sub-base will support your mobile home in most areas.
5. Why Use a Contractor?
Although you may feel that you will save money by doing it yourself, using a contractor to build your foundation may be the most effective and economical option.
A local contractor working to HUD codes will have all the equipment and labor available to:
- Excavate the correct depth of soil.
- Choose and compact the most appropriate sub-base.
- Calculate depth and reinforcement necessary for the climate and soil conditions on-site.
- Layout utilities and drainage for easy access.
- Pouring and leveling.
- Providing weather protection while the concrete cures.
A contractor has experience which assists in surveying the site, choosing the right concrete formula, site preparation, local conditions, and potential hazards.
You expect your foundation to last for the life of your mobile house (and beyond), so it makes sense to invest in a professional to give you a solid foundation.
Finally, a contractor can arrange official certification that your foundation meets the FHA standard for permanent housing.
This certificate is an essential step for loan finance and maintaining your resale value.
6. How Much Will It Cost?
The cost of your concrete slab foundation depends on the area you need. For example, a single-wide mobile home needs a smaller slab than a double or triple-wide home.
When planning your foundation, consider whether you want more hard standing than the current mobile home for a vehicle or utility area.
The mobile home park may have regulations covering the quality of slab foundation, and you need to consider the HUD code.
The costs of the concrete vary from $2,500 (single-wide) to around $8,000 (double-wide). But the costs of installing a mobile home foundation include labor and a sub-base, and for a complete job, expect to pay $6,000 to $15,000.
However, the floating slab foundation can class as a permanent foundation, and the cost included as part of your mortgage finance.
7. What About Resale?
Regular annual maintenance of your foundation (and your manufactured home) keeps the potential resale value high, so you either recoup your investment or make a small profit.
A poorly-maintained home only has scrap value when it is time to move on.
The most vulnerable time for a mobile home concrete slab foundation is the first two to three years – it may crack or sink as it settles into position.
A contractor or an engineer has the necessary expertise to design the concrete slab adequately for the soil condition and your mobile home’s weight.
Checking that your mobile home is level (and adjusting the level if necessary) means your windows and doors open and close properly, and you avoid warping and damage to the structure.
8. What Are the Alternatives to Slab Foundations?
A floating concrete slab foundation is the popular choice for speed, economy, and practicality, but there are other legal options for suitable foundations for your mobile home:
- Basement foundations – structural support and extra living space.
- Pit foundations – less expensive than a basement, but no extra living space.
- Runner foundations – support the mobile home where structurally necessary seldom in use for newer homes.
- Pier foundations – adjust well to slopes.
The basement foundation is a permanent foundation that is the most expensive option but has some significant advantages:
- Extra living or storage space in a purpose-built basement.
- Attractive to lenders as a financing option.
- Excellent resistance to earthquakes.
- Can build into a slope.
The downside of a basement foundation is that they are more complicated (must meet building codes), expensive, and can flood.
A pit foundation is less expensive than a basement and does not give you additional living space. It does provide easy access to your utilities with a roomy crawlspace, but it shares all the disadvantages of a basement foundation.
Pier foundations are less expensive than pouring a whole slab, and they provide excellent frost resistance. The massive downside of a pier foundation is that it can move independently and cause severe leveling issues with your mobile home.
Often a pier foundation combines with a slab foundation to provide double the benefits.
Runner foundations are the cheapest to install and don’t interfere with access to your utilities, but your crawl space includes the base soil leaving you at risk from vermin and rising dampness.
Runner foundations are easy to damage, and although the cheapest, they are the least effective for long-term use. You may find them on older properties.
9. What is a Permanent vs. Non-Permanent Foundation?
The general definition of a permanent foundation is one that remains and lasts.
Still, a permanent foundation is an integral part of the mobile home in mobile home terms – a basement, pit, or floating slab foundation can all class as “permanent” in this context.
A non-permanent foundation like a pier, single slab, or runner foundation is a temporary structure that disperses the weight of a mobile home across the underlying ground.
If you want to finance your mobile home, you need a certified, approved permanent foundation.
10. Can You Put a Mobile Home onto the Ground?
In theory, you can put a mobile home directly onto the land, but that won’t meet the HUD code for a permanent home.
Unprepared ground is subject to “heave” – local movement up and down resulting from different moisture levels, drying out, and freezing.
The ground can be soft, and your mobile home will sink in some places resulting in sloping floors, warping of the structure, and ill-fitting doors and windows.
Plus, it may damage your utility connections and cause your drainage to leak.
Bare earth is a poor foundation for your mobile home, and contact with the ground exposes your mobile home to dampness and vermin.
11. Why Do You Need a Skirting Board?
You might think leaving the space between your mobile home and the foundation open will improve airflow and keep your mobile home drier.
The skirting board is necessary to meet HUD regulations and because:
- It hides the understructure giving a better appearance like a stick-built house.
- Reduces heating and cooling bills.
- Creates dry storage space for seasonal items.
- Prevents vermin from taking up residence under your home.
Some airflow is necessary, and your skirting board requires appropriate ventilation to reduce heat and humidity.
12. What is the Best Mobile Home Foundation?
The best mobile home foundation depends on:
- Budget available.
- Site conditions.
- Time available.
- Legal requirements.
- Your needs.
Suppose you have the budget, time and need extra space. In that case, a well-constructed basement foundation is ideal because it gives you an excellent foundation equivalent to any standard housing with an additional living room.
The significant barrier to using a basement foundation for all mobile homes is the expense and complexity of creating a structure meeting the building code.
If your site has a significant slope, your foundation options are limited unless you want to excavate a level platform and use a retaining wall.
A floating slab foundation is the most practical and economical approach to providing a permanent foundation for your mobile home on most sites.
13. Is a Gravel Layer Essential?
You can pour a concrete slab onto level ground without the gravel layer – a straight slab foundation.
Adding gravel or other substrate layer and then pouring the concrete slab creates a “floating” slab.
The gravel layer evens out ground imperfections, adds drainage, and cushions the concrete slab (and your mobile home) from the effect of ground movement.
Some community mobile home sites may legislate that you must have a floating slab foundation.
14. Is a Crawlspace Essential?
The HUD code insists on clearance underneath the mobile home of at least one foot below the chassis beam.
This space allows for inspection of the underneath of the mobile home and access to utilities and drainage.
The minimum clearance (1 foot) provides little practical access. Some mobile homeowners opt to design a more spacious crawlspace to give generous storage space and better access for maintenance and inspection.
15. What is a Vapor Barrier?
A moisture barrier (sometimes called a belly wrap) under your mobile home protects the metal stands and wooden beams from water damage.
The vapor barrier prevents problems with humidity in your crawl space and protects your insulation from pests.
Most people fix the vapor barrier underneath the mobile home and extend it around 6 inches up the walls – like a damp-proof course in a standard home.
If you live in a humid climate (like Florida or Texas), a vapor barrier is essential to avoid rotten wood and mold problems. Further, an annual inspection is necessary to check for damage and tears as the belly wrap must be intact to work correctly.
The HUD code refers to the vapor barrier as a “bottom board.”
If you get a leak inside your mobile home, the water will pool at the lowest point of the belly wrap. You will need to cut the belly wrap to fix the plumbing issue, but you then need to repair that section to preserve the protection benefits.
Concrete absorbs moisture from the ground and evaporates it into the space below your mobile home.
As an added protection against excess water in your mobile home, you can insert a vapor barrier on a gravel bed underneath the concrete slab.
The advantages of adding a vapor barrier to your concrete slab foundation are that it keeps soil gases like radon out of your house and prevents moisture transfer from the soil through the concrete.
A concrete slab foundation is a popular choice for most mobile homes because it provides the most benefits for the least expense.
However, it is unsuitable for a slope without extensive ground preparation, and in the wrong conditions, it can crack.
But generally, slab foundations are stable and last as long as your mobile home.
Although a slab foundation is straightforward to install, it is worth taking time to prepare the ground and use a compacted gravel layer. The gravel layer adds extra material and labor costs, but it confers benefits in maintaining ground stability.
Your mobile home is as stable as its foundation.
Installing a vapor barrier beneath the concrete slab (even in desert areas) is an effective way of adding moisture protection to your living space.
Less moisture in your mobile home is healthier for the structure and your family. Plus, it makes the storage space beneath your mobile home drier and more useful.
Finally, an approved contractor will certify your foundations as being code compliant and consider local conditions such as climate hazards and soil type.
You rely on your foundation to support your home, and it is worth doing it right the first time.