Slab-on-grade technical guide

A home built on a slab-on-grade can be more affordable, more comfortable and more durable. Here is a technical guide to building a home on a slab instead of a basement foundation.

Slab-on-grade construction technical guide
Slab-on-grade construction technical guide © Yanni Milon,Ecohome

TECHNICAL GUIDE : Slab-On-Grade Construction 

Text adapted into English by Natalie Pavey

slab on grade construction
 

Starting your home with a slab-on-grade instead of a conventional basement foundation can greatly reduce your environmental impact during construction, and has the potential to offer significant financial savings as well.

By replacing concrete with lower-impact and better performing building materials, you can eliminate many tons of greenhouse gas emissions from your building materials as well as during the future operation of the home.

Slab-on-grade construction is a somewhat uncommon method of home building that replaces the conventional foundation wall and basement with a concrete slab that rests directly on grade. The technique lends itself to most types of terrain, with the exception of heavily sloped sites.

Why build a slab-on-grade instead of a basement?

To save money and carbon emissions, and have a healthier and more durable house.

Typically, the construction of a single family home in Canada starts with a poured concrete footing followed by an 8-inch thick foundation wall, generally 8 to 10 feet in height.

A basement foundation counts on being buried at depths of between four and five feet to protect the footing and basement floor from freezing temperatures. Since this creates additional floor area underground, it is a natural desire to claim that space, and so born is the basement rec room.

Whether you start construction with a slab-on-grade or a foundation, in the absence of bedrock both will be resting on dirt. So one is not more 'stable' than the other, or more suceptible to frost heave than the other, if it is properly insulated around the perimeter.

In the case of a 'walk-out basement' you effectively have a slab, only with a very expensive and poorly insulated wall. Envision a slab-on-grade as a four-sided walk-out basement with much cheaper and better performing walls.

In cold climates, a slab-on-grade can easily avoid frost heave with the simple addition of skirt insulation, which protects the perimeter of the footing so that it will never be subjected to the freeze/thaw cycle, and instead remain close to the relatively constant 8 -10 degrees Celsius temperatures of the earth.

Skirt insulation around slab-on-grade construction prevents frost heave
Skirt insulation around slabs and foundations prevents concrete from freezing © CMHC

Skirt insulation is a very simple and cost-effective solution that could (and should) be applied to basement foundations as well, as it would help reduce heat loss by keeping basement walls at a much higher temperature.

In addition to significant cost savings during construction, a slab-on-grade will reduce the risk of problems caused by humidity and water infiltration, which are typical challenges faced by basements.

What's wrong with basements?

Cost - ecological and financial:

Concrete is very expensive to purchase and extremely energy intensive during manufacturing. The process of building a basement requires 3 costly visits from a team of concrete trucks; once to pour a footing, again to pour walls, and a third time to pour a basement floor. For every ton of concrete produced, one ton of greenhouse gases is released into the atmosphere.

Once a basement foundation is completed, a subfloor must be built on top. This is another cost that will be incurred in order to create a surface on which living space is built, compared to a slab which is ready for construction.

* A home built on a slab-on-grade has a lower risk of flood damage, which is favourably looked upon by insurance companies and can be reflected in your premiums.

Health, durability and comfort:

Despite how common finished basements are, there is a general lack of understanding of the additional challenges posed by building underground. This can mean that in a lot of cases, the proper building techniques are not being followed, according to the principles of building science.

Mold in basement
Mold in basement © Thomas Anderson

Case in point: walls need to be able to dry in at least one direction. In a cold climate, the walls above ground should be designed to dry predominantly to the outside. But, since soil is for all intents and purposes 'water', basement walls need to dry to the inside.

Common building practice does not account for this, and a disturbingly high number of homes have moisture damage and mold, which is in part blamed for an increase in respiratory illnesses.

This happens for a number of reasons, the first of which is that we finish basements too soon. Concrete is largely made up of water, and with soil on the exterior a foundation requires a minimum of 2 years to fully dry to the inside.

Secondly, as the National Building Code now requires basements to be insulated and new home buyers usually want the space finished, the cheapest way for developers to do this is to treat them as they do above ground walls. So basements are generally insulated on the inside, long before unwanted moisture has left, and in the same manner that we build above ground where walls can dry to the exterior.

By sealing moisture sensitive materials (wood and fibreglass insulation) between a wet concrete wall and a polyethylene vapour barrier, we are inviting mold. A slab-on-grade avoids this all together.

Why we like slab-on-grade construction

Quality of life:

Polished concrete slab on grade floor
Polished concrete slab-on-grade floor © Bala Structures

In comparison to a basement, building above ground has the advantage of providing more natural light. It also helps maintain clean interior air quality as it reduces the possibility of mold.

What's more, slab-on-grade construction can make your living space more comfortable. Thermal mass within the conditioned living space has the ability to absorb and store heat, greatly helping to regulate interior temperatures.  Homes with significant thermal mass inside the building envelope are also easier to keep cool in summer.

Affordability:

Bringing a building project to the point where it is ready for main floor framing can be done much cheaper with a slab-on-grade than a basement. With a slab, the same milestone is reached without having to build an 8-foot concrete wall, nor do you have to build a wooden subfloor on top.

Comfort and efficiency:

In the absence of 5 feet of dirt, a slab-on-grade requires additional measures to avoid frost heave, so it includes levels of insulation that otherwise seem to be omitted from basement construction. That insulation can be paid for with the thousands of dollars that would have gone into purchasing concrete for a foundation wall.

Slab floors also easily accommodate radiant floor heating, which offers a very balanced and comfortable environment, transforming the concrete mass into one big radiator. 

One of the great advantages of radiant floor heat is that the further you get from anything radiating heat (imagine a woodstove), the cooler it gets. So heat is concentrated at ground level where we are rather than collecting in the highest points of or house, where we aren't. This facilitates an overall lower temperature, without sacrificing comfort. Warm feet are happy feet!

Reducing your environmental impact:

The slab-on-grade building method reduces your impact on the environment in two different ways: by greatly reducing the amount of CO2 produced in the manufacturing and transportation of materials, and by providing - dollar for dollar - a wall with much better insulation.

By building a slab-on-grade home, you are choosing to substitute foundation walls with above-ground walls. In other words, you are replacing concrete walls with a much more affordable and energy efficient wall assembly.

cold climate slab on grade technical guide
© Yanni Milon for Ecohome

Sacrifices, challenges and solutions:

In order to be able to make an informed decision about this type of foundation, there are several precautionary steps that must be taken and challenges to be addressed.

Your municipality may require plans that have been approved by an engineer, and some may not be familiar with slab-on-grade homes. Be sure to check with your municipality before beginning construction, and even before getting too far into your building plans.

While we are firmly in favour of slab-on-grade construction as a concept, we recommend carefully considering your options before moving ahead with any plans. There are many legitimate reasons to begin construction with a basement foundation:

  • A slab-on-grade will require more above ground space, so in order to have the same size of home you will need to build either out or up. You may run into height restrictions where you choose to build, meaning you may have to build out instead of up. This is not always possible, certainly if your site is an urban infill lot, leaving you two options: a smaller house or a basement. 
  • Despite their disadvantages, basements are often very practical since they provide a significant amount of storage space. Without a basement, everything that would have been down there must fit into the rest of the house or in a shed.
  • Basements usually house mechanical rooms. Keep this in mind during the design phase should you choose a slab, as mechanical systems will now need to be housed on the main floor. And don't be stingy with that space - think about everything that may potentially need to go there: a furnace, boiler, water heater, air exchanger, water softener, septic pump, sulphur tank, central vac, etc.

Slab-on-grade design:

Given that you are committing a certain area of the main floor to mechanical systems, this is as good a time as any to plan some storage, and maximize the efficiency of that room. Along with some space for storage, you could consider including laundry facilities, or even a pantry in that space.

With the amount of action going on in a mechanical room, it will be a bit noisy. In order to mitigate that, those walls should include sound reducing measures.

Plumbing systems are normally accessible from the basement or a crawl space, but not so with a slab. The nature of a slab-on-grade means plumbing systems will be permanently fixed in the concrete and not easily modified.

Installing a second toilet on an existing drain pipe is virtually impossible in this situation, so plan ahead. Worth investigating is the concept of 'flexible housing' where future changes are anticipated so that the necessary infrastructure can be put in place at the time of initial construction.

Floating slab / Monolithic slab:

Monolithic slab-on-grade form
Monolithic slab form @ Ecohome

The term floating slab refers to a two-stage slab construction, where footings are individually poured, and the centre floor of the slab is poured after footings have cured. The forms of a monolithic slab are designed so that both footing and slab floor are poured at the same time.

We have found no great advantage to support either method - the main reason for a pouring a monolithic slab would be to reduce the visits by the concrete trucks to one only.

Comments

We have a slab-on-grade house in North Vancouver. The water pipes used to run under the slab but I guess that caused problems whenever there was a leak - you'd jackhammer up the floor. When we bought the house the pipes were re-routed through the attic. Trouble is, attic is not heated. The pipes have a little bit of insulation on top of them, but when it gets below -10 outside, the pipes freeze. Therefore we leave the water running at night - not very eco!!! What is the alternative? More insulation on the pipes? Anything else?

yes, absolutely more insulation. If you used one single foam pipe wrap it likely wouldn't be enough, I'd triple it and see how it goes. Another idea may be running them through the conditioned space, perhaps building a bulkhead to hide them. Good luck, and let us know how it works out.

yes, absolutely more insulation. If you used one single foam pipe wrap it likely wouldn't be enough, I'd triple it and see how it goes. Another idea may be running them through the conditioned space, perhaps building a bulkhead to hide them. Good luck, and let us know how it works out.

yes, absolutely more insulation. If you used one single foam pipe wrap it likely wouldn't be enough, I'd triple it and see how it goes. Another idea may be running them through the conditioned space, perhaps building a bulkhead to hide them. Good luck, and let us know how it works out.

yes, absolutely more insulation. If you used one single foam pipe wrap it likely wouldn't be enough, I'd triple it and see how it goes. Another idea may be running them through the conditioned space, perhaps building a bulkhead to hide them. Good luck, and let us know how it works out.

Has any one added a slab on grade to an exsisting home with a basement to produce an extension to the home. If yes please advise if it was the same construction with the shared wall of the exsisting home. Is there a reference I can review

I am in SK where winter temperatures reach -30 frequently and-40 occasionally. Would slab work for here? Also how do you deal with septic/waste water plumbing on an acreage?

We would like to build slab on grade with south facing windows for solar gain. Can you recommend a place to find ready made plans for a home that is under 2000 square feet (including mechanical room etc.)
Thanks

"Freegreen" has a nice passive house plan under its free category. :)

I am building a slab on grade workshop with lots of insulation in the floor (I asked a related question in another article). In this article you suggest the use of an insulation skirt but you are providing no information on how we size it. What thickness or R value should we use. Is there a minimum depth below ground level in order to make this worthwhile? Is the span out from the slab decided by the depth or ? Is it laid horizontally or on an angle or does it matter? Does it matter at what point it comes off the slab...right at the bottom, halfway down...?
Thanks

The above post is mine...this is an update. I ended up contracting a geotech engineer while digging the hole for the slab because I thought I had run into a clay like substance. While it wasn't clay it did result in the need for more drainage and protection from frost. A skirt was recommended. It covers 4ft out from the bottom of the footing away from the building for the entire perimeter. It is set at a slight angle down away from the building to shed water. I have two layers of 4" styrofoam under the slab and the skirt starts from the edge of that insulation. A vertical section of styrofoam sits on top of it and against the footing to a point six inches above the slab floor and will be capped with flashing to protect it from the sun once the wall is in place this spring. (right now it and the slab are covered in straw and protected with tarps). Under the skirt, against the bottom piece of styrofoam, is socked big-O drainage tile that drains into a massive French drain. The purpose of the skirt is to stop frost from getting under the slab due to moisture that is trapped in my soil type freezing in the winter. If the soil was of a type that would drain better then there would be no need for it at all. As it was the geotech engineer said that putting in the drainage tile was the most important addition and would probably be sufficient...that kind of vagueness on the future health of a slab I had invested almost $30K on was not going to cut it so I did both.

Typically I like to use the same as what went under the slab, extending out as far as you like. I went two feet out, horizontally with an angle so as to shed the water. My pump house stayed warm all winter without a hiccup.(4 months of at least -20) Colby.

I want to build a home on a slab on grade in a cold climate.
Frost line is quite deep. 4-6ft
My question is about plumbing. Line from well and to septic system should be run below frost line? And rest of driains ie shower, sinks, washing machine. Run under slab? Or below frost line?

The pipe from the well must be below the frost line. My frost line is seven feet. However it can go deeper where vehicle or foot traffic push it down so ensure styrofoam or some sort of cover in a vehicular area. Even a sheet of poly plastic under the driveway/above the water pipe will slow the frost. As for your under slab plumbing. It does not have to be deep. Frost travels down. It hardly moves horizontally. Dig trenches and do your plumbing then fill with sand. then seal a layer of poly and styrofoam. also a styrofoam skirting extending out from your slab will ensure everything under your slab will not freeze(obviously if you leave for the winter and turn the heat off it will). Always put styrofoam down between the plumbing/dirt and the slab in a cold climate. This is my personal opinion based on my area and experience.

can i build with slabs ontop of slabs

What is the best way to run ventalation to all the rooms if I'm putting in slab heat? I was going to get an air to air exchanger put in and run a main line with trunk lines to all rooms should it be done in the attic or put pipes in the cement? Thanks JV

I would not recommend putting the ventilation in the slab. You will run into condensation issues and potential air quality issues down the road with that approach. Either on the warm side in the attic or more ideally in bulk-heads on the warm side of the air barrier inside.

What is the best way to run ventalation to all the rooms if I'm putting in slab heat? I was going to get an air to air exchanger put in and run a main line with trunk lines to all rooms should it be done in the attic or put pipes in the cement? Thanks JV

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