Modular Home Foundation Options

A modular home can sit on a full basement, a crawlspace, screw piles or concrete piles, and occasionally a slab. Which one is right comes down to five things: the soil, the depth of frost, whether the space underneath will be heated, how the lot is serviced, and what you want to use that space for. This guide walks through each option, what the National Building Code asks of it, and how the foundation fits around a home that is built somewhere else.

One point first, because it shapes everything else. The home is manufactured indoors, but the foundation is not. CSA A277, the standard for factory-built buildings, covers the work done in the plant. British Columbia's building branch puts it plainly in its bulletin on A277 buildings: the standard does not cover the parts of a building that are not completed in the factory, and the siting, foundation and service connections have to meet the building code like any other house. The foundation is designed for your lot and inspected by your local building authority.

Why the foundation matters more on a modular build

On a site-built house, a late foundation delays framing. On a modular build, a late foundation leaves finished modules with nowhere to go. The home is built in parallel with the site work, which is how most Herauf homes are manufactured and delivered in roughly 16 weeks, up to 50% faster than conventional construction. That only works if the foundation is ready before the delivery week.

CMHC's review of modular housing projects across Canada found that moisture during transportation and storage can be a serious problem. A foundation that is finished on time means the modules are craned straight onto it and closed up, rather than waiting on blocking. The foundation type is settled in the quote, so the site schedule and the factory schedule are planned together from the start.

Full basement

A full-depth concrete foundation that adds a level under the home. It gives room for mechanical equipment, storage, or finished living space, and on the prairies, where a foundation already has to go deep for frost on many soils, the extra depth to a full basement is a smaller step than it would be in a mild climate.

A basement needs a site where excavation is practical: workable soil, a water table that stays below the floor, and room for the excavator. It is the longest on-site job of the options here, so it is the one to start first. Because the basement is heated space, the code tables treat it differently from an unheated foundation (more on that below).

Crawlspace

A short perimeter foundation wall that lifts the home above grade and leaves an accessible space underneath. A crawlspace keeps the floor off damp ground, gives plumbers and electricians a way to reach services later, and copes with a gently sloping lot better than a slab. It is less excavation than a basement and is a common middle ground for modular homes.

Whether the crawlspace is heated matters. A heated, insulated crawlspace is part of the conditioned building; an unheated one follows the frost rules for a foundation with no heated space.

Screw piles and concrete piles

Piles carry the home on points rather than a continuous wall. Steel screw piles are turned into the ground by machine; concrete piles are drilled and poured. A beam system or grade beam on top of the piles carries the modules. Herauf's site-readiness checklist notes that screw piles install quickly in most prairie soils and avoid large excavations, which is why they suit acreages, lots with poor access for a concrete truck, and sites where digging a basement would hit water.

Piles leave an unheated space under the floor, so they have to reach stable ground below the frost line or be designed by an engineer for the conditions. On permafrost, very soft soils or steep ground, piles are often the practical answer, but they are an engineering decision for that site, not a default.

Slab-on-grade

A concrete slab poured on prepared ground is common under site-built homes and garages. Under a modular home it is less common, because each module arrives with its own insulated floor, so the slab ends up supporting a floor rather than being one. It can still be the right answer on some sites, with a perimeter wall or grade beam to set the modules on, and with plumbing and electrical stub-ups placed exactly where the modules need them. If a slab is on the table, settle the layout with the plan before any concrete is poured.

What the building code asks of a foundation

Most houses fall under Part 9 of the National Building Code, which covers buildings of three storeys or less with a building area of 600 square metres or less. Part 9 sets a table of minimum foundation depths by soil type:

• Rock, and coarse-grained soils with good drainage: no minimum depth for a foundation that contains a heated basement or crawlspace.
• Silt, or coarse soils with poor drainage: a foundation with no heated space has to go below the depth of frost penetration.
• Clay, or soils that are not clearly defined: 1.2 m, and for a foundation with no heated space, not less than the depth of frost penetration.

Two details catch people out. First, if the footings are insulated to keep heat from reaching the soil beneath them, the code treats the foundation as having no heated space, because the ground underneath can freeze. Second, the code allows shallower foundations where local experience with the soil shows they work, or where an engineer designs for them.

The code also exempts some homes from the depth table: those whose structure passes the deformation-resistance test in CSA Z240.2.1, the standard for manufactured homes, which is what lets many of them sit on surface blocking. A modular home built to CSA A277 is built to the building code itself, so plan on its foundation following the ordinary depth rules or an engineered design. Your local building official has the final word on which applies.

Radon and the space under the floor

Radon is a radioactive gas from the ground, and the foundation is where it gets in. The National Building Code requires an air barrier wherever a heated space is separated from the ground. For homes, it also requires a rough-in for a radon extraction system, unless the space between the air barrier and the ground is left accessible so a subfloor depressurization system can be added later. A basement slab usually gets the rough-in; an accessible crawlspace or pile foundation may meet the rule by staying accessible.

Health Canada's guideline is 200 becquerels per cubic metre, and it is clear that a rough-in or a passive system is not a substitute for testing: homes with these systems should still be tested. Plan to test once the home is lived in, whatever it sits on.

Financing and the foundation

For a home buyer, the foundation can decide how the home is financed. CMHC's Prefab Plus mortgage insurance covers prefabricated homes, including modular homes, that are permanently affixed to the land on a permanent foundation, with chattel financing as a separate route for homes that can be moved. Ask your lender which applies before choosing a pile or skid design for a house. The lender side is covered in financing a modular home.

How to choose

• Start with the ground. A geotechnical report or local knowledge of the soil tells you which row of the code table you are in, and whether digging is practical.
• Decide whether the space below is heated. A heated basement or crawlspace and an unheated pile foundation follow different depth rules.
• Think about the services. Where the water, sewer or septic, and power come into the home has to line up with the foundation plan. Rural lots usually need a septic system designed and permitted first.
• Think about access. The crane or picker truck needs room beside the foundation, and the approach has to carry a heavy load. Frozen ground carries loads that spring thaw cannot.
• Decide what the space is for. Storage or extra living space points to a basement. A dry, serviceable platform points to a crawlspace or piles.

Commercial modular buildings add one more option, the engineered steel skid, for buildings that may move. That decision is covered separately in steel skid or foundation for a modular building.

Planning the foundation with Herauf

The foundation type is decided early because the quote is built around it, and Herauf matches the foundation to your site conditions as part of quoting. The site-readiness checklist covers access, services, permits and timing in the order they usually come up. Every Herauf home meets or exceeds CSA A277 and the National Building Code for climate zones 6 to 8, and the winter builds page explains how a foundation poured before freeze-up pairs with a home built over the winter. For acreage and farm sites, see building on rural properties.

To have your lot assessed and a foundation matched to it, request a free quote or call 1.639.382.4106. A written quote usually comes back within one business day.