Guide to foundation for modular home

All modular houses are required to be attached to a permanent foundation and despite being built off-site, it isn’t until the prefabricated modules are attached to the foundations that the modular house is considered complete. The foundation’s main goal is to grant stability to the modular house by creating a solid base. To do so, it is essential to choose the right type of foundation by taking in considering factors such as the soil characteristics or the structural design of the house.

Main things you should know about modular home foundations

1.It is recommended to get a soil survey

In the UK and most European countries, a soil survey isn’t required, though it is highly recommended. A geotechnical test offers a detailed composition of the deeper layers of soil. Having this information beforehand helps identify the best type of foundation and saves the risk of encountering unexpected elements during excavation.

2.The modular home manufacturer is often in charge of building the foundations

Some modular home manufacturers take care of the whole building process, including installing the foundations on-site. The manufacturers who don’t offer this service will require you to hire a contractor to build the foundations, though they will still offer guidance in the foundation selection and building process.

3.The modules are directly attached to the foundation

The modules conforming to the modular home are manufactured in a factory-like environment and delivered to the building site once completed. There, they are lifted with a crane and generally attached to the foundations with connectors or bolts. The connector or bolt is usually welded to both the module and the foundation[1].

What type of foundation is required for a modular home

There are five main types of permanent foundations commonly used for modular construction which are no different from the foundations utilised to raise a traditional on-site-built house.

Raft foundation

A raft foundation is a type of shallow foundation consisting of a slab of concrete that lays horizontally on the ground covering the entire footprint of the house. Generally, to improve the resistance and durability of the foundation, the concrete slab is reinforced with steel bars and thickened at the edges.

The excavation work needed to install this type of foundation is minimal since rafts are basically sitting on a levelled bed of soil. As a result, this type of foundation is usually faster and cheaper to build.

Individual footing or single pad foundation

Individual footings are a type of shallow foundations that consist of a set of square, rectangular or round pads that are usually placed below ground. These pads are most often made of reinforced concrete and separated from one another evenly. Individual footings are very common within traditional construction, though not the most frequent within modular houses.

Modular homes are most often built with structures composed of load-bearing walls and single pads are designed to support structural columns. Still, in some situations such as sloped lands, individual footings are a good solution to avoid severe excavation work, though to connect the house to the foundations it is necessary to attach an additional horizontal structure below the house’s walls.

Strip or wall footing foundation

Strip or wall footings are a very common type of shallow foundation in modular construction, they are similar to an individual footing, though in this case, instead of being composed of multiple pads, it is built as one continuous slab strip. This footing is built right below the house’s bearing walls with a width that generally is 2 to 3 times the walls’ width.

Deep foundations

This type of foundation is only used when the soil is weak at the surface. For foundations to perform properly and provide the necessary support to the modular home, it is required that they rest on solid land. When the first layers of soil are weak and lack the strength to support the foundations, it will be required to dig deeper into the ground until better soil is found.

There are different types of deep foundations, though in residential buildings pile or screw pile foundations are the most common. Piles are long cylinders that can be built on-site or prefabricated and are usually made of different materials such as concrete, steel, or wood.

Basement foundations

It is definitely possible to build a modular house on a basement foundation. This type of foundation requires a significant amount of excavation work, which will consequently increase the construction cost. However, it is usually still cost-effective since it offers extra square metres that can be utilised as either living space or storage.

The basement is composed of two different structural elements: a raft foundation and the walls which are, in most cases, all built of reinforced concrete. The modules conforming to the modular home are placed on top of the basement walls.

What is the best foundation for a modular house?

Each one of the foundation types described above can offer good support to a house built with modular construction, though the best foundation type will vary from one modular house to another. Here are some of the most important aspects to consider when choosing the best type of foundation for a modular home:

  • The soil composition: The composition of the soil is one of the most important aspects to consider since the foundations need to rest on solid ground. Identifying any weak layers can be very helpful to determine the need for a deep foundation. Though, identifying elements such as rocks can be beneficial as well since it can save the difficulties we would encounter if we would decide to excavate.
  • The structure of the modular house: Knowing if the modular house is built with a load-bearing wall structure or a column structure will help choose a foundation type that satisfies them without adding an extra cost to the construction.
  • The topography of the land: Taking the topography into consideration can help us choose the most economic and even environmentally friendly foundation type. Flat lands usually offer us an easier starting point, though that doesn’t mean we should always excavate to flatten the terrain. Playing around with the topography and the different types of foundations can offer really cost-effective solutions.
  • The manufacturer’s expertise: To ensure the foundation is solid and safe, its selection should be done by the architects and engineers in charge. The modular home manufacturer usually takes care of these services. In those cases where the foundations are built by an external worker, the manufacturer will still provide the necessary guidance to ensure the type selected is the most suitable.

[1]Pujadas Gispert, E. Prefabricated foundations for housing applied to room modules Universitat Politècnica de Catalunya
Featured image: Modern Exclusive V1 by Mevena

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