How to build a slab-on-grade on standard soil

There are many ways to build a slab-on-grade (or frost-protected shallow foundation), the following is a step-by-step walk though of a thickened-edge slab floor we built. XPS or EPS rigid foam is typically used under concrete slab floors, in this case we used mineral fiber insulation.

Note: our mineral wool was provided by Rockwool (formerly Roxul) and their engineers at could not confirm with us that their product had the compressive strength to carry the load under the footing, so where the weight of the walls would bear, we installed EPS foam instead. 

Slab floors are often built in a way that requires footings and sometimes frost walls to be poured first, with the interior slab floor being poured later once the footing has set. Insulation underneath the footing is often omitted with this technique, which leads to thermal bridging and heat loss. building a slab with a thickened edge footing also requires a second trip with a concrete pump truck, which depending on your region can run anywhere from $700 to $1000. Instead, what we have done here is known as a 'monopour', where the footing and slab are poured at the same time.  

Here are the basic steps for slab-on-grade construction on soil that does not have a high water table and has sufficient compressive strength to carry the load of the building. For problem soils see our pages on raft slabs and how to build on problem soils. 

1. Creating a level building surface

After the land was cleared, compactable fill was brought in to bring the site to a workable level. The house sits at about grade at the north side, but needed about 3 feet of fill at the south edge to level the building site.

A plate packer was used to tamp the ground in stages to ensure a solid base, as seen above. A retaining wall was built using rocks from the site to hold the compacted gravel and future slab in place. 

2: Building forms

A common technique for build forms is to drive stakes into the ground then attach boards to them to act as concrete forms, but we decided to pre-build forms then raise and square them the way you would with walls. This was intended to make disassembly easier and allow us to reuse the wood. Hard to say if it ended up being easier or not in the end, but it went fine. 

Forms need to be well-secured to avoid a blow out, including lots of braces to hold the weight of the concrete so forms don't bend. Three runs of 2x6's easily held concrete and insulation, and after forms were removed, that wood was used for framing. 

3. Plumbing and mechanical infrastructure

All plumbing infrastructure (water pipes, drains, radon stack, central vac, power conduits, etc.) was put in place next, below the coming insulation and concrete. 

You need to be very sure about the placement of plumbing at this point as concrete is not so forgiving when it comes to making changes afterwards. You will need to pour concrete right up to drains in the floor, but for sink and bathtub drains that you will not see, you can make your life a lot easier if you don't encase them in the concrete. Either build a wooden form around the drain pipes or even just cut a hole in a bucket and stick it over top. This way the drain will have a little play, making it much easier to hook up bathtub and sink drains. 

4. Installing insulation

After all the plumbing was in place, 8 inches of rigid insulation was installed - Rockwool underneath the slab and on the vertical exterior surface, and  8 inches of  EPS under the footings. 

Comfortboard is great to work with, it cuts very easily and stays securely in place without slipping, allowing for tight clean joints. This is a nice added benefit when you are doing multi-layers as we did here. The joints of each layer were offset as well to further reduce heat loss.

We placed 8 inches of rigid mineral wool vertically to protect the exterior of the slab, and we included a cement board outside the insulation boardsbut inside the form. We later attached the cement board with plastic tie straps that passed through the insulation and vapor barrier into where the future footing will be; a 1.5" screw was put into the end of the tie strap to act as an anchor inside the concrete after it sets.

The reasoning behind this step was to have a well-insulated slab with no thermal bridging, and a cement exterior ready for parging when the forms were removed. After disassembling the forms we were happy to see it worked exactly as planned. See a short video of how we did this. 

Plastic ties were chosen over long nails or screws to prevent the heat loss that would come through metal, which our engineer assured us would be significant, far more than we would have expected before seeing software energy simulations. Metal fasteners would have effectively reduced the total R value by almost half, testament to the conductive powers of metal and why as a material it should be very selectively used in wall assemblies.

6. Radon gas barrier / vapor barrier

For added durability over the commonly used 6-mil polyethylene vapour barrier, we used a 10-mil barrier instead. The added thickness gives us more protection against accidental holes made during the construction phase, giving us greater confidence in it for radon gas protection.  

To support the concrete mesh as it extends over the footing, we cut sections of mesh that sit on small foam feet. This also supports 4 rebars that will be imbedded in the concrete footing at exactly the position we want them. 

Despite that it may look somewhat labour intensive, this little detail took only a few hours to do and it ensured that our mesh was level, our rebars were well-positioned and our vapour mat was not punctured. 

Radiant floor hydronic tubing

Limiting the length of tube installed per zone is very important at this stage; with too long a run the water will have cooled too much towards the end of its return trip to effectively supply heat.  We had our zone system designed by our sponsor Uponor which ensured that the heat would be balanced as best as possible. 

9: The final floor will be a polished concrete, which is completely non-toxic and very durable. The production of concrete is a heavy polluter, releasing one ton of GHGs (greenhouse gases) per ton of concrete. So we chose a concrete mix that included 50% recycled material, which significantly lowers the total emissions of our build, and brings in some LEED points as well.

There is an often overlooked advantage when you choose a slab over a basement - concrete is very expensive, so the reduced amount of concrete used in a slab along with not needing to install a subfloor and finished flooring product translates into a savings of many thousands of dollars.