Durability of FPSF vs basement: opinions and experiences
I was thinking about the pro’s and cons of a frost protect shallow slab vs a basement foundation.
I’m in Toronto, we don’t see many slabs on grade so I don’t see it much or have other people’s experiences to lean on. For instance, I reviewed a a set of drawings from an architect for a slab on grade and they still included a 4′ foundation below the slab to stay below the frost line. But I know it’s possible., I’ve built a small shed with a FPSF and it doesn’t heave or move, but, it’s very small and inconsequential.
FSPF is cheaper to build, but for larger houses, I wonder about things like house movement (regular cracks in walls or door frame movement) or call me crazy, major weather events where things can really move. This compared to a deep foundation where I know for sure there isn’t much movement, and a good chunk of the house is surrounded by the ground to keep it in place.
I’d like to point out the assumption of resilient building practices like tieing roofs to walls and walls to foundations.
So does anyone else have any local or otherwise cold climate experience to share? Any issues I haven’t thoughts of?
Jamie
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This is an interesting issue. I've got similar questions. My real concern is over the long term durability of the system, especially with regard to sub-footing foam being attacked by pests of various kinds. I've heard of cases where foam lost its structural capacity because of this.
But in searching online I can find nothing. I've tried a variety of search terms but the same dozen or so websites come up, all with useful information about the design of FPSF, but no long term studies analyzing actual performance.
I know they've been building these foundations for several decades in Scandinavia but I can still find no long term studies. If anyone else can I hope they will post here.
As far as structural issues independent of foam failures, I have a master's degree in structural engineering and my belief is that if slab on grade foundation was properly designed at the outset (and properly constructed) it should perform equally well as any other foundation type. There should be no long term issues or movement.
One alternative to foam I've been wondering about is the use of AAC blocks below footings, as insulation. This would be an interesting alternative but as far as I know no one has done it yet. AAC would completely eliminate the pest vulnerability.
I only have a BS in structural engineering but I've been building and designing homes for a long time and have tried many different techniques. There are pros and cons to both full foundations and slab-on-grade systems. Full foundation walls are probably better able to handle improperly prepared soil, because the walls are basically giant beams, and the talled/deeper the beam, the stronger it is. But if soils are properly prepared, there is no reason why a slab-on-grade system shouldn't work just as well.
I’ve worked on 100+ commercial metal frame buildings on FPSFs and my family owns two buildings constructed this way. They are very common in our area of Wisconsin. We’ve also worked on many FPSF buildings that later had additions added on.
Without exception, the foundations hold up great. I don’t recall any major issues. One of our buildings 72’ x 192’ unbeaten building has no sub-slab or perimeter insulation. Despite being built on highly frost susceptible clay soils with only 6” of stone under slab, there are only a few hairline cracks in the slab. The other building (heated year round) has no cracks.
The most difficult thing with these foundations is getting underground rough in’s accurately located before pouring the floor. They are also a little less flexible for future additions.
Either system should hold up well if constructed properly.
I would personally not place foam under the footing section of a floating slab, I would be concerned that in some point of the long life cycle of the building something could deteriorate the foam leaving a major structural issue. But I am quite risk adverse.
The thing I do not like about structural slab foundations is the amount of plumbing encased in the slab. If there is every a serious problem the fix is a very big deal.
The thing about them that I do like is the lack of a crawl space or basement, which avoids the many potential water issues that can occur.
Thanks all,
What about the issue of tree roots getting under the slab? Has anyone encountered this? Or know of issues to look out for?
Obviously, the typical response is "don't build near a tree". Well, welcome to heavy urban/high density home building. Trees are inevitable.
Thoughts?
That's a really interesting point about tree roots. With a basement the worst that can happen is that they clog your perimeter drains. Around here roots regularly undermine and heave carport slabs and driveways.
I'll defer to Akos and DC on the value of garages in their markets, but disagree about price. Under our code, by the time you have done what is necessary to get approval, the spaces are just about fully finished, and the only thing that distinguishes them from space you paid to build above ground is that the cladding is concrete, not something else.
Here where the frost depth is not deep, you don't see basements except on sloped sites where the walls are necessary to retain the fill. I think they provide a poor quality of living space, and that comes with the potential problems of water infiltration. Many of the major flood events we have been seeing here in Canada only affected basements and crawlspaces. If they weren't present, the damage would have been much less.
The market in TO has spoken, a house with a basement especially a finished basement is worth much more. It is the cheapest way to add sqft and doesn't count towards your GFA. I would not build without it.
I sometimes tease my wife that if we ever get to underpinning the house, since there are no zoning rules limiting it, I'll add a 2nd basement under it.
Oh I agree, go as deep as you can, basements in TO are 'free' from zoning.
However, sometimes, cost comes plays a factor, especially with smaller budget builds like ADU's. It's cheaper and faster to do a slab on grade. I'm just scared of them.
I that case, I would go for standard foundation down to frost line and either slab on grade or crawlspace. This also puts you back into the prescriptive part of the OBC so you don't need an engineer's stamp.
Thinking about it some more, it might be worth it to bring the foundation down to 7' or 8' even with a slab on grade. This way if you ever want to dig it out for a full basement, you don't have to underpin.
The other option is screw piles.
Akos,
I can see that with a framed floor and crawlspace, but would there ever be a situation where digging out a basement under a slab on grade, with all the services and interior walls, etc. was feasible?
This was just me thinking out loud.
In some way I look back to our forefathers, if they would have made most basements here 6" taller it would have saved a lot of headache for us.
The reality is if you are already digging down that far for footings, removing the dirt and having a full basement from the start is not much extra cost. When you look at the full cost of something like an ADU this does add to it but you get 50% more sqft. For most lots, ADU sizes are very limited by zoning, that bit of extra sqft is a great deal.
Akos,
It's a uniquely Canadian thing. Many of my family and friends grew up mostly in their basements, and only ventured above ground occasionally. For some reason I never saw the appeal.
In DC, not only does below ground space not count against square footage limits, they don't count for setbacks either; you can dig all the way up to the lot line, or as close as you can get without your neighbor's house falling into the hole. So the trend in high-end houses is to have the basement be larger than the footprint of the house. Sometimes it's the foundation for a deck, which doesn't count toward square footage either, but sometimes it's just buried.
A guy near me built a basement 21' deep during the pandemic, he wanted to be able to shoot baskets. I figure once the novelty wears off it will be pretty easy to divide it into two floors.