# Concrete slab questions



## cpotts (May 22, 2014)

I live in canada...frost line code says 4' deep for footings

I am pouring a slab underneath my deck. The deck is crotched beside the house such that 2 sides of the deck contact the house. The deck is 10x10 feet. And sits with 5'5" underneath clearance.The deck design currently has only 2 posts on sonotube footings.
I am planning on doing 2 things to the deck
1. Adding a small top landing and a staircase such that the stairs run parallel the rim joist furthest from the house. To do this I will be adding 4 - 6 support post for the added structure.....
2. pouring a concrete pad under the deck and new stairs.

Questions
Is it ok to pour the pad using the house foundation as forms on the 2 sides?
Is it ok to pour the slab over the sonotube footings that currently exists for the two deck support posts?
Since I'm extending the pad to go underneath the new stairs, can I pour the new post footings and the pad all at once? Suggestions? 

Before you ask, I will be getting some pros to help with the job.....just trying to get some suggestions from some experienced DIY guys!


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## Msradell (Sep 1, 2011)

Sort of, no, yes to answer your questions. Actually, you should put some kind of expansion material between the slab and your home but other than that you can use your home as the forms. You shouldn't pour over the Sono tubes because in doing so you would encase the support post in concrete which is something you don't want to do. Posts should set on top of the concrete. You can pour the slab continuous with the support for the stairway without a problem. Since you're going to hire a contractor to do this he should know any other details for your area.


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## cpotts (May 22, 2014)

So for the continuous pour should I side sonotubes


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## Msradell (Sep 1, 2011)

cpotts said:


> So for the continuous pour should I side sonotubes


If you can dig the holes in the ground without the sides caving in I would just pour the slab in the holes in one shot without worrying about tubes. If you have to use them because the ground caves in just cut them even with the surface.


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## cpotts (May 22, 2014)

Ok I have the ground prepped mad the concrete forms are up. My contractor seems to think we should tie the slab into the house foundation with Rebar. He actually drilled the holes as you may notice in the pics. I stopped him because I thought the slab should move independently from the house. Any thoughts guys?

Speaking of move, I DO NOT want any heave at all since the slab will be connected to the deck piles, supporting my new deck stairs, and supporting the walls of the under deck shed. How can I do this so that the pad is rock solid? My contractor says we simply rebar the slab into the foundation and thicken the slab edge to 10 inches ( slab is 4 inches).


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## cpotts (May 22, 2014)

Another pic


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## cpotts (May 22, 2014)

Here's what I'm thinking.....
Tie the concrete slab into the foundation
3 piers In a continuous pour along the edge furthest from the foundation 5 feet deep
Slab won't move? Thoughts?


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## Nailbags (Feb 1, 2012)

you should put a expansion joint of asphalt felt board or something there. along the foundation wall. and yes the slab should float, i.e. be independent to the house . Just like a garage slab is a floating slab. Here is the reason why the foundation and the slab will contract and expand at different times during the year having it attached will cause both to crack in ways you do not want. Cement will crack no matter what but lets lessen the cracks that we can.


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## Canarywood1 (May 5, 2012)

cpotts said:


> Here's what I'm thinking.....
> Tie the concrete slab into the foundation
> 3 piers In a continuous pour along the edge furthest from the foundation 5 feet deep
> Slab won't move? Thoughts?


 
DO NOT  Tie the slab into the foundation, it needs to float.


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## Nailbags (Feb 1, 2012)

cpotts said:


> Here's what I'm thinking.....
> Tie the concrete slab into the foundation
> 3 piers In a continuous pour along the edge furthest from the foundation 5 feet deep
> Slab won't move? Thoughts?


NO NO NO! The slab must float! DO NOT TIE IT IN TO THE FOUNDATION!


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## stadry (Jun 20, 2009)

vigorous YES to woody & 'bags - NEVER tie the slab to a structure,,,that's why they make expansion jnt 

irc


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## cpotts (May 22, 2014)

Getting mixed advise on this....I talked to a local builder, and a local engineer. Both suggested doweling to the foundation....and so did my concrete guy. I don't mind doing it as long as I can properly prevent the slab from heaving. I've got the engineer coming by on Thursday. An old teammate from university days. I will post again then. In the mean time, any great ideas on preventing the slab from heaving?


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## Nailbags (Feb 1, 2012)

Lets look at this way your slab is tied in to the foundation and it heaves and guess what your foundation does? It heaves and guess what your house does? doors and windows don't open or close right. My family is from Alaska no way would they do what your thinking.


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## landfillwizard (Feb 21, 2014)

I agree with the others that the slab needs to float! You do not indicate where you live. If there is any chance for frost or the ground freezing or extreme changes in temperature, the slab needs to float. The only time I know of that concrete is pinned to concrete is in footers and walls. From the pictures you will need to compact your base as much as possible to try to keep settlement to a minium. It would also behoove you place drainage stone with an outlet for water below the slab.


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## jomama45 (Nov 13, 2008)

I wouldn't dowel it into the foundation, they're two completely different elements, and perform differently.

There's only two effective ways I know of to control frost heave, either remove all of the water below the slab, down to the frost line, or insulate against the freezing of the ground.

Obviously, lowering the water table that far down is complicated in most scenarios, so insulating is the easier approach. On a regular basis, when we feel that there's a strong chance of severe frost heave (like under a patio at a full exposure walk-out basemen) we will install 2" foam insulation under the slab, and extending 2' beyond the slab's outside perimeter. We generally pour directly over the foam, but the concrete finisher's need to adjust the concrete mix slightly to compensate for the excess bleed water created by the foam base.......


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## cpotts (May 22, 2014)

Ok jomama ....you are talking about something like the image below? Do you use the regular rigid foam insulation or is it a special kind? And will this eliminate heave, or just limit it? I was told that this method only works for heated structures.... Also I did mention that I live in canada where the frontline is 7 foot deep ( but deck footings only need to be min 4 foot by code....go figure...


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## cpotts (May 22, 2014)

I wanted to share a few more details with you guys...
The base is compacted clay with 5 inches of compacted gravel. The pad will be 4 inches thick with edges reaching 12 inches (blue lines)
I tool a line in the concrete are the red mark to control cracking. 
Ultimately, I will be enclosing the under deck area for a shed, then adding a top landing with stairs.


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## cpotts (May 22, 2014)

So the engineer came by today....

Dowelling into the foundation is fine according to him. He says since the foundation is 8 inches of reinforced concrete, this slab is very Unlikely to crack it.

He suggested I could put a layer of rigid foam like jomama said. But he said he really only does it I'm commercial applications

I will keep posting until it's done....


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## jomama45 (Nov 13, 2008)

cpotts said:


> So the engineer came by today....
> 
> Dowelling into the foundation is fine according to him. He says since the foundation is 8 inches of reinforced concrete, this slab is very Unlikely to crack it.
> 
> ...


This makes no sense to me, unless for some reason the cold weather and frost is somehow selective, and is too intimidated to have an effect on commercial applications????


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## cpotts (May 22, 2014)

Not sure why exactly jomama ......I figure backyard patio slabs don't usually get the royal treatment! I will find out why and post...concrete guys are supposed to be here today but were delayed....


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## jomama45 (Nov 13, 2008)

You're asking alot out of a slab on grade patio in Canada: You don't want it to move at all in a harsh freeze climate. Perhaps you should design the enclosure to allow for some kind of movement. Best of luck to you...........


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## landfillwizard (Feb 21, 2014)

2 things to look out for. 1. Make sure there is drainage away from the slab and make sure the gravel under the slab has a way to drain. 2. Make sure the contractor places expansion around the peir so the slab has some here to expand when it get cold.


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