Slab on grade, or foundation and basement

The first step to home design deciding what sort of base you will rest it on. The following pages outline the benefits and drawbacks of slab-on-grade construction and basement foundations.

Slab on grade or a foundation and basement?
Choosing between a slab-on-grade and basement © Ecohome

Traditionally, home construction starts with a concrete basement foundation. Depending on the situation there are other options that can be more durable and affordable that will better protect your home air quality.

The first factors to consider when designing the foundation of a home are lot size and soil conditions. When space is limited (with total footprint and height restrictions) a basement foundation may be the best option, but if space allows there is a strong case for avoiding basements altogether with a slab-on-grade.

Related pages:

Concrete foundations:

For a modest sized home, a concrete foundation will cost you easily between $20,000 and $30,000. Factor in a subfloor and finished flooring, you’ll be lucky to stay under $40,000. Building basements ‘just because’ invites unnecessary costs, potential humidity problems, and greater environmental consequences.

The production of one ton of Portland cement releases one ton of greenhouse gases, and the average home foundation wall uses between 75 and 100 tons. It will also account for between 10 to 15 percent of your total building costs, which does not include finishing the inside basement walls, or building a floor on top of it.

And if you remember the 'One-Tonne Challenge' that asked Canadians to reduce thier carbon emissions by one tonne in a year, well this one move could offer you a century of success in that. It can also be much cheaper, more ecologically sensible, and offer a better quality of life to build 'up' instead of 'down'.

Slab-on-Grade Construction:

Slab on grade polished concrete floor
Slab on grade polished concrete floor © Bala Structures

A slab-on-grade means no basement, no basement walls, just one slab of concrete on which you build your house. They aren’t suitable for all building sites, which we will get into later, but for now let’s assume you can build on one.

There is a lot of prep work to do first, you need well-packed soil, proper drainage, insulation, vapour barrier, and a lot of mechanical infrastructure before you pour. None of that should discourage you, that will all be necessary under your basement floor anyway.

A slab floor is a fantastic way to start a passive solar home, as your entire floor surface consists of several inches of thermal mass to absorb heat. A slab is a great place to bury electrical work, plumbing, central vacuum tubes, phone and internet lines, speaker wires, and most importantly, tubing for radiant floor heat.

You can put flooring material on top of a slab, but simple finishes include acid staining, adding colour, cutting tile patterns, or the simplest and cheapest is just polish it and call it done.

All the electrical, plumbing and other mechanical infrastructure you will have to do anyway throughout your house, so don’t think of it as added cost. In fact with a slab that can all be done cheaper. Laying out plumbing and wiring on a flat surface is a lot easier than drilling a thousand holes and fishing wires through studs and joists.

At a cost of about $8 a sq. ft. to pour a concrete floor plus a couple of thousand to polish it, a finished slab over basement construction can potentially save you tens of thousands of dollars during construction and leave you with a final product that is extremely durable, energy efficient and does not pollute your indoor air as so many finshed flooring products do, not to mention moldy basements.

So those are the “pros”, and I would be remiss if I didn’t share the “cons”.

  • Concrete is hard, and consequently not as comfortable for standing. It’s a good idea to have cushioned mats where you stand frequently, like in the kitchen.
  • If your kids wipeout, the stakes are a bit higher than on a shag carpet or pine floor, and if you drop a glass, you definitely won’t be drinking from it again.
  • Without a basement you will need to account for a main floor laundry and utility room, so factor that into your plans. A utility room can be noisy, so try to situate it away the general living areas and be sure to soundproof the walls.
  • Slab-on-grade Technical guide

Foundations

Foundation forms
© Bala structures

As we mentioned, not all sites are suitable for a slab. Municipal restrictions for footprint and height may leave you with no option but to build down in order to have sufficient space.

If your building site is on a slope and will require terracing, a foundation can often be the only solution.

In such cases, be sure it is well-insulated and well protected outside from moisture infiltration and it’s always best to insulate the outside of your foundation rather than the inside.

By doing so, you greatly reduce your risk of interior condensation and you keep the concrete walls as thermal mass to help balance temperatures.

Be sure to use a foundation wrap or membrane extended over the footing, and proper drainage.

Regardless of the base you build on, insulate the surface extending out 4 feet from your home to keep either your foundation or slab from freezing. This is done by grading away from your foundation, but stopping a foot or so below your final desired height. Lay down 3 inches of extruded foam, then the final 8 inches or so of soil.

If you have to build a basement, it makes sense to use the space. But living underground is not the most ideal situation at the best of times and especially if your basement is experiencing moisture issues, as most do eventually. A common mistake in basement finishing is doing it too soon, before concrete walls are dry. Allow 2 years for concrete to shed excess moisture.

All walls need to breath in one direction and since the exterior foundation walls are buried in wet ground, they must breathe to the inside. A vapour barrier prevents that from happening and you will inevitably trap moisture inside your walls.

Insulating under concrete floors

Many building codes have only recently insisted on the inclusion of insulation below concrete basement floors, and as always, it is important to realize that the requirements of building codes are the bare minimum only. Increasing sub-slab insulation to R15 or R20 (or more) will make floors much more comfortable and reduce heating costs. The added cost for additional floor insulation can be paid back in as little as a couple of years though energy savings.

Comments

I have been reading everything I can find on insulated slab on grade and I am having difficulty finding information to deal with vapor barriers. Is there a reason to use one for a slab? What is the objective of using a vapor barrier in this application? I have consistently read that no matter that you do if water can't infiltrate now it will at some point...and if it can't escape it becomes a problem. A search of vapor barriers suggests that we need 15mil thickness. A search for providers in Ontario generates one supplier. This suggests to me that a vapor barrier under concrete floors in Ontario is not common. Can you talk to this issue please.

We are located in Alberta and looking into ordering an RTM as the cost of houses around our area is extremely expensive. We are considering not having a basement but putting a rec room above the garage for extra space. Is there a huge cost savings when putting in just a slab rather than a full basement?

This guy is a real COWBOY BUILDER, you listen to him, your house will fall on top of you. Adding foam insulation from outside the foundation walls and you know what will happen, nothing solid like earth to hold it in place, your house will be like boat swaying in water, the earth is constanltly moving and the house will have crack walls, standing structural supports will warp and the next thing is the house will drop on your head. The soil breaths so don't worry, the concrete walls will not have to breath from inside your house. You don't have to wait for concrete to dry in two years, your house has heat and constant air flows in and out. Which island does this guy come from? he should take some basic highschool science lessons.

I came across a wonderful product called FLOODSILL for finishing basements. It elevates partitions 2" above the basement floor, protecting them from both internal and external flooding, allowing for water to flow under the wall partition and directly to the floor drain (instead of being trapped behind the wall) when there's a flood (as we all know this eventually will happen to all basements at one point or another - it's inevitable!).
I highly recommend it, it's very inexpensive, easy to isntall and worth the peace of mind. Thought I'd share my experience here!

I am working on a house plan with a three car garage, with kitchen,dining area, laundry and washroom directly behind the garage area. would there be noticable cost savings, by having the slab from the garage, extended into this living area, with a standard poured concrete basement area under the remainder of the house. Gross area of house approximately 2100 square feet, with basement area representing approximately 850-900 square feet. Located in Ontario.

I am building a house slab on grade on southern Vancouver Island. Due to our mild climate I wonder how important underslab insulation is. What would you recommend as a minimum? I am also concerned with drainage as the building site is wet with surface water when we have heavy rain. The lot is very flat until the front property line when it drops 5 feet in the last 30 ft and will connect to municipal storm drain. But this is a long 150 feet from the building site. Will this allow enough slope for the drainage?

I am currently looking into buying a house built on a slab on grade foundation. Have you ever heard of issues with re-sale value of these homes?

I am looking to build a slab on grade foundation for our new home. Are there particular exterior sidings that I should be looking to use or this type of design? I'm concerned that with no basement wall, the soil for lawns and flower beds will be right against the house siding!

This shouyldnb't be an issue as the siding should come no, or little, lower than the ground floor level, which itself should be above ground level by 6 inches or so. You should insulate the outside edge of the slab and cover this with something durable such as metal sheet of the required width - valley flashing would be suitable and available in appropriate colours: black, grey and brown.

Useful article from CMHC:
http://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/2000-127E.html

I am trying to decide whether to build our foundation as a basement, crawl space or slab on grade.The water table is 3 feet below grade. We are beside the Trent Canal. We would have to have much of the basement wall above grade and this would mean steps to get in. We are in our mid 60s and do not like the idea of steps. Any suggestions?

Hi John

With such a high water table I'd be reluctant myself to go with a basement (of course I'm reluctant in dry soil too), and none of us around here are big fans of crawlspaces. If you don't feel like negotiating stairs, you likely won't want to crawl around on your hands and knees below your house either. If you found this page you must be at least exploring the option of a slab, assuming you can get it approved by an engineer on the site, its likely your best option for avoiding flooding but also for avoiding stairs. Please let us know how you decide in the end, we'd very much like to get feedback on cost, design and performance for all types of terrain so we can better pass on info in the future. With a slab and a gentle slope or ramp you could avoid stairs altogether.

Hey there,
I am ordering a house kit that is designed to get me to a 'lock up' and 'weather tight' scenario. Builder's must put in their own base/basement before assembly of the kit begins. What is an approximate cost of having a basment/base for the home in British Columbia. It's on private rural property and mostly clay. Would be about 936sq ft. Not sure if this is the type of information shared on this site, as this is my first time on it, but hopefully you can help me get an idea.

Thanks!

Hi Jayme-Lyn

Pricing is really tough to estimate, as that is subject to change by region. What will also affect the total cost is the design of it, and how you have to plan drainage. So your first step to determining an accurate price is speaking to an engineer to see what you would need on your particular site. The amount of mechanical infrastructure and insulation will also affect cost. Don't skimp on insulation, a lot of heat can be lost to the ground under a poorly insulated slap, Our engineer has figured that where we are in Central Canada, going from R5 to R10 under the slab will pay for itself in only a couple of years, and he recommends far more. One home in our Net Zero Heat program has R60 under the slab. All that said, you will probably save quite a bit of money doing a slab over a foundation, as long as the home you have ordered has enough space for you. Costs in this area are often estimated at about $13 a square foot, though that is only rough I hope it helps slightly. Please let us know what you decide, and how prices end up comparing.

Having it built, I should say* Sorry.

Can you use a slab on grade foundation on a waterfront lot located on a flood plane with 10 feet of peet moss.?

Hi Mike,

That doesn't sound like an ideal building site regardless of what you put there, so I'd check with an engineer before getting too far into any plans, be it foundation or slab. We did do a story some time ago about a home that was built on a site with a high water table, they put it on screw piles to completely avoid the risk of flooding. This may be something you want to look into. http://www.ecohome.net/news/latest/high-performance-home-shoe-string-budget Best of luck.

It's not my first time to pay a quick visit this website, i am visiting this web site dailly and obtain nice data from here all the time.

Thanks Rochell, that's nice to hear. We are always adding pages, so if you don't find what you are looking for let us know and maybe we can help out. If you are here on the slab page, please come back next week we have a couple of pages on slab-on-grade construction that we will be adding, including a step-by-step construction guide.
Best Regards.

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