The soil conditions of some building sites are not suitable for a slab-on-grade foundation with a thickened-edge footing without first beginning with extensive and costly soil remediation. Opting for a raft slab in such conditions can be a much cheaper option. 

A raft slab is a frost-protected shallow foundation, or slab-on-grade, that does not include the standard thickened-edge footing on which the entire load of the house bears. It is engineered to distribute the load evenly over the entire building surface. 

Building sites with poor soil conditions (disturbed soil, expansive soils, low-bearing capacity, high water tables, etc.) may require significant investment in drainage, soil replacement and compaction before an engineer will approve a project. 

The photos below are images of drainage measures that were required to support a thickened-edge footing slab-on-grade floor on a site with a high water table. Images courtesy of Yanni Milon.

Drain placement for slab on grade with high water table
Drain placing with gravel and geotextile under a slab on grade foundation
Placing gravel for draining slab on grade with high water table

A raft slab acts a bit like a snowshoe in the way it distributes weight evenly over a larger surface. For this reason, they can often be built on soil that could not support other types of structures. 

The typical soil bearing requirements for a thickened edge footing are 150 kpa (3,000psf), where a raft slab may be able to sit on soil with one third of that bearing capacity, or even less with additional engineering measures. This will often be the most affordable (and possibly only) option for building on sites with particularly unsuitable soil conditions. Even with an averaged-sized house, such costs can sometimes climb into the tens of thousands of dollars and possibly stop a building project in its tracks.

The risk of encountering poor soil quality at the depth of traditional foundations is also avoided, so a raft slab on any site can avoid possible costly surprises once excavation has begun. Given its more robust design, it is also less prone to movement and cracking than buildings that bear on a footing. 
You may or may not have success in finding an engineer experienced in raft slab design, alternatively you can look into companies that specialize in custom designed prefabricated ICF slab on grade form kits.  

Excavation and slab placement

  • Remove the approximately 6 inches of organic material, two feet beyond where the building footprint will be.
  • Build a retaining wall if necessary to create a level building surface.
  • If there are large holes where tree roots have been removed, they can be filled with aggregate and compacted. 
  • If the site is sloped at all, bring it up to level with 0-2.5 inch compactable fill, being sure to compact it with a plate packer at the required intervals.
  • Lay down 6 inches of levelled clean stone, two feet out from the building perimeter.
  • Install pins where the corners of the building will be.
  • Install all sub-slab plumbing, electrical conduits and radon gas evacuation tubes.

Note: We would strongly recommend seeking a plumber with experience in slab-on-grade construction. As all plumbing work will be embedded in concrete, the accuracy of location, drain height and proper sloping of drains is essential. 

  • Install raft slab insulation forms, interior floor insulation and reinforcement mesh as per engineering specifications. 
  • If you are installing in-floor radiant heating, be sure to have it designed by engineer to properly locate the heating delivery system so as not to affect the structural integrity of the slab. 

Raft slab Installation

First the formed edge pieces are laid in place, the corners need to be squared and secured in their location.  following that is the installation of interior board insulation, a radon gas/vapour membrane, reinforcement mesh and any heating systems, all done according to the engineered building plans and directions. All slab images courtesy of Legalett.

Compacting gravel for under a raft foundation
Insulated slab forming systems
Corner detail for insulated slab forming system
Skirt detail for frost protected shallow foundation slab forms 
Rebar detailing for slab foundations

Slab-on-grade floors are often heated, which provides a very comfortable and even distribution of heat throughout a home. A large volume of heated concrete inside a building envelope will act as a thermal battery by storing and releasing heat, which helps balance temperatures in both summer and winter.

Such quantities of heated thermal mass inside the building envelope also provides heat security in the event of a power outage, by slowly releasing its heat over the course of days. Raft slabs can be heated with hydronic systems (liquid) or air-heated tubes as seen below

A better basement design:

Traditional basements begin with a poured footing, then a foundation wall, and finally the slab floor. Rarely are footings insulated, and depending on how the walls are insulated, the result can be a thermal bridge between the footing and walls or floor. This brings unwanted heat loss, as well as a greater risk of condensation forming on those colder portions of concrete. 

Alternatively, a basement can be constructed by beginning with an insulated raft slab, followed by an ICF foundation wall. This provides a continuous layer of insulation separating the concrete from the ground. The result is a very comfortable and energy efficient basement, with no thermal bridging and reduced risk of mold. 

Basement mockup with raft slab and ICF wall courtesy of  Treehugger 
Mockup of  a raft slab with radon/vapour barrier protected between layers of EPS foam © Legalett 


Read more about Insulated Slab-on-Grade Foundation Form Systems here, from the EcoHome Green Building Guides