Slab-on-grade prefab form kits

Formed EPS foam kits for frost-protected shallow foundations can reduce building costs by eliminating the need to build footings.

Prefab frost-protected shallow foundation form kit
Prefab frost-protected shallow foundation form kit © Iso-slab

Prefab form kits for frost-protected shallow foundations can in many cases save time and money. 

A slab-on-grade (also known as a frost-protected shallow foundation) is a more durable and usually much more affordable alternative to building a basement on most building lots, and there are a few different ways you can go about installing one.

If you're not familiar with the concept, check out our building guide page for a better understanding. We also have an instructional video series documenting all the construction steps of our recently completed LEED Platinum slab-on-grade Edelweiss House. But those pages cover a different installation technique than the time-saving option of buying a prefab kit.

We chose to build our demo house slab with Roxul stone wool instead of EPS foam for a couple of reasons - stone wool is made with over 50% recycled content, it contains no fire-retardants and there is evidence to suggest that termites and carpenter ants are less likely to be a problem with stone wool than EPS foam.

Legalett corner edge element
Corner edge element © Ken Williams

Termites are not a great concern in all of Canada, but they have already moved up into southern Ontario and other warmer regions, and due to rising global temperatures they will only keep coming. EPS can be treated to repel insects, and while commonly used compounds are said to be safe for humans, how well and for how long they will continue to work for is uncertain.

Given that there are no companies currently offering a stone wool prefab form kit, we had to build forms the traditional way, but if you are planning to use EPS foam, prefab kits will save you a lot of time and money .

metal fasteners prefab slab on grade kit
Top rail metal fasteners © Ken Williams

Rather than building wooden forms and taking them apart afterwards, a prefab slab-on-grade form kit uses shaped foam as the form itself. Once the top rail metal fasteners are in place, no further bracing is required.

This makes the installation much quicker by eliminating the steps of building forms, driving stakes in the ground and bracing it, then taking all that apart afterwards. While the initial cost of a prefab kit will be higher than the price of simply purchasing EPS foam, remember that the speed at which this can be accomplished will most likely mean substantially lower labour costs. 

The fact that site-built forms are secured to the ground makes it more difficult to do fine adjustments to keep forms true and square before pouring concrete. In contrast, floating forms can be very easily adjusted prior to pouring concrete to ensure they are straight and level.

Slab on grade wooden forms
Wooden forms for site built slab-on-grade construction. The natural flexibility of wood as the form means forms need to be secured by driving stakes into the ground. This makes fine adjustments very difficult to do. Alternatively you can backfill it with gravel, but the gravel will then need to be removed to remove the forms. Most foundation contractors will however © Ecohome

Securing the insulation to the exterior surface of concrete is a potential headache with some designs; that is avoided with a kit as the solid exterior surface of foam is secured simply by the pouring of concrete. All that is left to do afterwards is to parge the exterior surface, which  can be applied directly to the EPS foam.

There are two types of forms that are commercially available for this, both technically known as a 'frost-protected shallow foundations' but more specifically within that category are 'insulated raft-slab foundations'. Both installation methods begin the same way:

Cutting around plumbing drains
Cutting around plumbing drains © Iso-Slab

1)  Create a level building surface with at least 6-8 inches of crushed stone for drainage.

2)  Install the formed perimeter edge pieces to give a reference point for plumbing installation.

3)  After plumbing is installed, rigid board insulation is laid over the interior surface of the floor.

The difference between an insulated raft-slab and a standard frost-protected shallow foundation:

  • Raft-slabs fall under the general category of frost-protected shallow foundations, but raft-slab foundations have a uniform thickness over the entire surface rather than a thickened edge or footing.
  • With a standard frost-protected shallow foundation the load of exterior walls is carried by the footing alone, with a raft-slab the weight is distributed evenly over the entire surface of the floor.
  • Raft-slabs have a more robust reinforcement mesh and a thicker layer of concrete, typically between 5 and 8 inches for residential projects compared to the standard 3.5 - 4 inches with other slabs where the load is not spread across the entire surface. With more concrete comes more thermal mass for retaining heat and balancing temperatures, though the value of thermal mass much beyond 4 inches starts to fall off due to the limits of heat dephasing on a 24-hour cycle. The extra concrete also results in more embodied carbon due the greehouse gas emissions from cement plants.

Either design should come out cheaper than a traditional slab-on-grade made with hand-built forms, though in some circumstances with poor soil conditions the raft-slab is likely the more financially viable option due to the uniform distribution of weight.

Building sites with poor soil conditions (disturbed soil, poor bearing capacity, high water tables, etc.) may incur significant additional costs in drainage, soil replacement and compaction before an engineer will approve a project. Even with a modest-sized house, those costs can easily climb into the 10s of thousands and stop a building project in its tracks. By distributing the weight evenly, a raft-slab acts a bit like a snow shoe and can effectively 'float' on terrain that would otherwise be unsuitable for building.

Note: As specified in the National Building Code, the minimum bearing capacity of soil (also called soil loading) is 1500lbs per sq. ft. for traditional footings, though foundations designed by an engineer can be used with much lower bearing pressures, such as with raft slabs.

Finding slab-on-grade form kit suppliers:

There are a few more suppliers in the US, but in Canada we came up with two relevant manufacturers. 

Prefab slab on grade form kit
Top layer of foam designed for hydronic tube installation  © Iso-slab

ISO-SLAB is a Quebec-based company that distributes prefab slab kits across Canada. They provide EPS insulation forms, metal locking rail fasteners for the exterior edge, EPS board insulation for under the slab and exterior skirt insulation, all customized to each building design.

Clients need to provide a soil sample and the company can provide stamped engineering documents if required. They have a PDF installation guide as well as an installation video. For clients who wish to include radiant floor heating, ISO-SLAB includes an optional top layer of EPS that is designed for a quick install of hydronic tubing.

© Ken Williams

Legalett is a company based in Cornwall, Ontario, and distributes EPS raft-slab kits across Canada and the U.S. Stamped engineering documents and detailed construction and shop drawings are provided. Due to the even distribution of weight and the ability to be built on much softer ground, soil samples are not generally required except in extreme cases.

Legalett slabs can be left unheated, but the company offers an optional air-heating system and their kits are perfectly suitable for hydronic or electric-wire heating systems. 

EPS foam footing forms
EPS foam footing forms © Isomax / Polyform

Polyform is another Quebec manufacturer with a product called Isomax, an EPS footing form only. Isomax is a 4” thick moulded form with grooves for the inclusion of skirt insulation. 

Soil samples and engineering documents must be provided by clients, as well as all additional materials for the interior of the slab. 

While Polyform  is not offering a complete slab-on-grade package, we thought we would include it as it is something of a hybrid between a full kit and a traditionally built slab.

Watch for more players getting into the game in the future as this type of construction gains in popularity, but be careful of going with knock-offs. There have been cases of builders trying to go it alone on this and coming up with poorly-engineered designs or the wrong density of foam that have led to building failures. Ask about warranties before signing any contracts.