New slab connected to fondation


Hi guys,

I am working with a client that has a specific idea. He wants to build a new addition to his current cottage, however he thinks the best way to do it is by building it on a slab. It is the most cost efficient and he consider it to be the smartest idea.

I am doubtful with the idea, I don't like the idea of having a slab connected to the current fondation. I feel like it will be problematic in the futur if the cold get the slab to move. 

I suggested him to make complet fondations 5' in the ground, and connecting it to the current fondation. That way, the new fondation would make the entire building monolithique and it would be 100% safe.

I was wondering, what do you think about adding a slab to an existing fondation and how are you working it to make sure that a -40 degree doesnt make it move?

The best way I know for isolating a slab is by using the rule of inches (5' - the height of the footing = the amount of feet (lenght) and the amount of inches of thickness) Ex: 2' footing would force us to put rigid insulation of 3' long x 3'' thick on all the perimeter of the slab.

Thanks for your help

Feb 17, 20 12:59 pm
Wood Guy

This publication covers what you need to do for a slab that is safe from frost movement when it does not have perimeter walls that extend down to the frost line.

There is an abbreviated version here:

In either case, if the sub-slab insulation has significantly higher R-value than the building code requires, you need to work with an engineer who has experience with frost protected shallow foundations and understands how the heat flow works.

If you are still concerned, then do a raised slab, with perimeter walls that extend to the frost line (or deeper; frost goes deeper than the frost line we use for design purposes), then backfill, insulate and pour a slab. I use this system a lot. I cast a shelf in to the wall so it's only 5-6" wide at the top, to allow more space for an insulation gap between the slab and the wall. 

There is no need to be concerned with a frost protected shallow foundation, though. If the ground below the slab can't freeze, and it's properly compacted, it won't move. 

Feb 17, 20 1:52 pm

It can be done if everything is in your favor.  But, if the existing system works, don't reinvent the wheel.  The cost is negligible by the time you subgrade prep, add the vapor barrier, provide expansion joints, and add the insulation.  Not to mention what you'l have to do for finishes and how the floor performs won't feel like the rest of the cottage (assuming crawlspace / structural floor).  Basically, you'd have to twist my arm to accept the concept and I sort of make a living looking at failed buildings. My gut always defaults to what works, and if the system in place is working, then don't mess with it. 

Feb 17, 20 3:54 pm

My gut says hire a structural engineer.   Where are you in the USA  that you are allowed an architect to design and stamp foundations?   That is not allowed in the communities we work in.  

Feb 18, 20 10:42 am

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