What Is The Best Foundation For A Modular Home?

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In general, the best foundations for a modular home are both a concrete slap foundation and a basement foundation since they are the most versatile types offering a good outcome in a wider range of situations. Still, the best choice of foundation will vary from one modular home to another depending on their situation. The purpose of the foundation is to offer support to the house and in some cases protect the building from weather events that are most common in the area. 

To determine the best type of foundation we should consider the following factors:

  • Very cold climates
  • Warm climates
  • Hurricane or flood-prone areas
  • Weak or damp soil at the surface
  • Rocky or hard land

Often a modular home can be built on different types of foundation and choosing one or another could be either a matter of visual preference, function or in most cases price. While the cost of building a foundation can fluctuate enormously depending on the land conditions, we can have an idea of a basic price range.

Types of modular home foundations

As permanent constructions, modular homes can only be attached to permanent foundations which are the same type of foundations used to build any traditional construction. Still, there is a variety of mainly five different types of foundations to choose from when building a modular home.

  • Concrete slab foundation: This is a very popular type of foundation because it is the most cost-effective. It consists of a horizontal block of concrete that is laid on the ground, setting a solid structural base for the new modular home to rest on. Generally, concrete slabs are about 4 to 6 inches (10 to 15 cm) [1] and are made of concrete reinforced with steel bars to offer a more resistant foundation. 
  • Basement Foundation: Basement foundations are very common because they represent an affordable way to add some extra square meters to the modular home. A basement foundation is a structure that is buried underground granting stability to the house while creating some additional space. This space can be utilized as storage or as an inhabitable area by turning it into a room.
  • Crawlspace or pier and beam foundation: This type of foundation elevates the whole modular home off the ground by about 18 inches (46 cm) [2]. This elevation is usually used to place the electrical and plumbing installations which in case of reparation can be easily accessed. Still, the only way to get into this area is by crawling, which makes this space uninhabitable. The elevation provides natural ventilation between the house and the ground, protecting the house from humidity when the ground is commonly wet[3]. 
  • Pile foundation: This type of foundation is considered a deep foundation because it consists of a long cylinder that is sunk deep into the ground to find support in areas where the surface isn’t solid enough. Given that it is buried deep down, this foundation is capable of reaching lower layers of solid ground, which is necessary where the land is too wet or weak on the surface [4]. It is not a common foundation, but in some situations where the land conditions require it, a pile foundation is the best type to choose from.
  • Elevated pile foundation or house on stilts: While this is a variation of the pile foundation, the main purpose of the elevated foundation isn’t to reach solid ground deep down, but to elevate the house as much as one whole floor above the ground. The main reason behind this structure is to protect the house from floods. In some cases, elevated piles are built as a combination of foundation types, often anchoring the elevated piles to a concrete pad [5]. 

The best type of foundation for every situation:

The best type of foundation for a modular home depends on a variety of factors. Below we have listed the most common situations in which a modular home will be built to identify which is the best type of foundation in every case. There is a best type of foundation for every situation, though the best choice isn’t necessarily the only one.

Very cold climates

The best type of foundation for colder climates is a basement. The most important thing to consider when building a foundation in a very cold climate is how deep will the land freeze because this is how deep foundations have to be sunk. In the Northern U.S. the frost line is typically 4 feet deep into the ground [6] which makes basements a really cost-effective solution. If necessary, it is possible to build other types of foundations such as a concrete slabs, though they will have to be carefully protected to prevent heat loss and avoid cracking due to the land freeze.

Hot-dry climates

The best type of foundation type for this kind of weather is either a concrete slab or a basement. They both offer a good outcome supporting the house and it is generally a personal decision to choose one over the other. A concrete slab is very popular in these areas because it is the most cost-effective method. However, a basement is a great way to save some energy once the house is functioning given that below ground the temperatures are cooler and more stable [7]. Building a basement to place rooms is a smart solution to keep the house from warming up without using cooling systems. 

Hurricane or flood-prone areas

The best type of foundation for a modular home based in an area where hurricanes and/or floods are common is an elevated pile foundation. In these circumstances, it is essential to elevate the house above the ground to enable the water to freely run underneath the house without causing any damage to the living space  [3]. Other foundations such as the crawl space could work very well when the main problem in the area is moist soil since the ventilated space avoids the filtering of water in the house. Though crawl spaces are usually not high enough to protect the house in front of floods. 

Weak or damp soil at the surface

The best type of foundation for a modular home being built in an area where the soil is weak or damp at the surface is a pile foundation. It is very common to encounter weak or damp soil areas near the beach or in locations where there is a lot of groundwater or swamps. Since a pile foundation is expensive to build, in some situations it can be avoided. The most important aspect to consider is to ensure the foundation rests on a piece of land that is solid enough to support the whole modular home. 

Rocky or hard land

The best type of foundation for a modular home being built in an area where the soil is rocky or very hard is a concrete slab foundation. When the soil is rocky or very hard, any excavation works will be difficult and very expensive. Therefore, in these types of situations choosing a shallow type of foundation such as a concrete slab will generally be more suitable. 

How much will every type of foundation cost?

When we are building a modular home that could work well on different types of foundations, it might be helpful to consider the cost of the different types of foundations to help us decide which option works best for us. The prices listed below are just a reference though because the cost of building the same type of foundation can be completely different depending on the type of land.

  • Concrete slab foundation: The concrete slab foundation is the most cost-effective type of foundation since in most cases it doesn’t need excavation works. The average cost of building a concrete slab is from $4 to $8 per square foot [8]. 
  • Basement Foundation: Building a basement foundation isn’t the cheapest option, though it could be seen as an investment since once finished it offers us some extra livable space for the house. The average cost of building a basement foundation under a modular home is about $33 per square foot [9] considering the construction of the foundation only.
  • Crawlspace or pier and beam foundation: This type of foundation is not as popular nowadays as it used to be some years ago. Though in some situations it might offer the most benefits. The average cost of building a crawl space is from $10 to $15 per square foot [10].
  • Pile foundation: Building a pile foundation is usually very expensive since it is an engineering work that requires multiple deep but narrow excavations. The average cost of building a concrete pile foundation is from $23.45 to $69.80 per square foot [11].  
  • Elevated pile foundation or house on stilts: While building a house on stilts isn’t the cheapest foundation solution, it is cheaper to invest in these foundations than to repair the overall house in case of a flood. The average cost of building an elevated pile foundation is from $20 to $60 per square foot [12]. 

References:

  1. Vanderwerf, P. (2007, July 23) Concrete floor slabs Concrete Construction https://www.concreteconstruction.net/how-to/construction/concrete-floor-slabs_o
  2. Crawl Space – Everything you need to know about Crawl Spaces Groundworks https://www.groundworks.com/glossary/crawl-space/
  3. Patil, R. Pier and Beam Foundation Constructionor https://constructionor.com/pier-and-beam-foundation/
  4. What is Pile Foundation? Types of pile foundation Civil Engineering https://civiltoday.com/geotechnical-engineering/foundation-engineering/deep-foundation/176-pile-foundation-definition-types
  5. Homeowner’s guide to retrofitting – Chapter 5: Elevating your House Federal Emergency Management Agency (FEMA) https://www.fema.gov/sites/default/files/documents/fema_elevating-your-house-chapter-5.pdf
  6. Hall, D. How to defend your home in colder climates LiteForm https://liteform.com/defend-home-colder-climates/
  7. Hassan, H.,  Arima, T., Ahmed, A., Sumiyoshi, D & Akashi, Y. (2014). Testing the Basements Thermal Performance as an Approach to the Earth-Sheltered Buildings Application at Hot Climates, Case Study (Egypt). 10.13140/RG.2.1.4789.4564. 
  8. How much does it cost to pour a concrete slab? Home Guidehttps://homeguide.com/costs/concrete-slab-cost
  9. How much does it cost to build a basement or foundation? Home Guide https://homeguide.com/costs/foundation-cost
  10. Pow, A. Pier and beam, foundation cost The Pricer https://www.thepricer.org/pier-and-beam-foundation-cost/
  11. Foundations – Section UIP 1 State of Michigan https://www.michigan.gov/-/media/Project/Websites/treasury/VOL/Vol228UIP1FoundationsFrames.pdf?rev=af0c0f1cf71c4ed1acf4defc41acc8e5
  12. (2022, January 21) How much does it cost to build a beach house on stilts? Home Advisor https://www.homeadvisor.com/cost/architects-and-engineers/build-beach-house/