Deck support alternatives to sonotubes?

[ml5299]

Hey everyone,

I am working on a project that involves attaching a 2nd story deck to an existing building (residential). The existing building has a poured frost wall in northeast climate (NY).

For the deck, current plans are to dig footers for 4 sonotubes using bigfoot bf28 footers and then use 12" sonotubes over the bigfoot. There will be a patio concrete slab poured on the ground level, which originally we were planning to isolate from the sonotubes (small joint around top 5-6" of sonotube and fill in with caulk). We are not fans of the looks of sonotubes and the potential cracking around the area. As an alternative to this approach, the concrete contractor suggested doing a "trench pour", the entire length of the deck + 4 ft on each end. This is where we'd fill in a 24" trench along the length of the front deck and then the concrete patio slab would tie into via rebar and sit on this 'footing'. The slab would then be "floating" everywhere else, separated from the existing foundation by expansion joint.

This approach was interesting as it would allow the entire slab to be poured without visible "sonotubes" at the surface. The bracket for the posts would be right on the slab more centered around the 24" footer. And I also liked the idea about the weight from the deck onto the footer would be more evenly dispersed opposed to sitting on just 4 sonotubes.
The contractor also said they did a lot of homes this way in the area.

Has anyone heard of this before to support decks? Any idea on the pros & cons? Should I just stick with conventional sonotubes separated from the floating patio slab?

Thanks in advance..

 

[jayrod12]

The potential issue is differential movement having the deck foundation bearing on different subgrade than the main residence.

Does the raised deck connect directly to the existing residence? If so, that is my biggest concern. If it's a fully floating deck with its own lateral system, then no issue on my end.

 

[ml5299]

Hi Jayrod12, thank you for the response. Yes - the raised deck would connect to the existing residence with a ledger board (deck is 38' long). But the slab would not tie into the residence, just tie into the footer "trench" to support the posts for the deck, which would be below the frost line. Here is a rough mockup:

 

[jayrod12]

And that would then concern me. You said the existing residence is on a frost wall? How deep?

I would want the deck foundation to bear at the same elevation as the house foundation. I doubt that a thickened slab would be bearing at the same depth.

You said frost wall, is freezing a concern? How deep is typical frost depth?

Helical piles could be an alternative.

 

[ml5299]

The existing structure is on a poured wall foundation that goes down 4'. So the options are either:

1) 4 sonotubes going down 4' to footer. Deck posts sit on sonotubes. Slab is floating independent of sonotubes.

2) Trench pour (2' wide trench that goes down 4') filled with concrete. Slab is tied into the concrete trench and sits on it. Rest of slab is not tied to anything or to house foundation. Theory behind this is the slab won't move where the posts come down to where the 'trench' is because of it going 4' down (same as residence). But the rest of the slab/patio area can flex a little if needed.

 

[jayrod12]

As long as they bear at the same elevation, and have the same level of frost protection, then I don't see an issue with that proposal. I would however ensure that the width of the trench is such that the actual change on subgrade bearing pressure is minor once you include the new load from the deck. That way you avoid as much settlement as possible by maintaining the current pressure on the subgrade at that elevation.

 

[BAretired]

Sonotubes are cylindrical cardboard forms. They do not support the deck. The concrete piers inside them might continue up to support the deck, but it is more likely that concrete would terminate at or about grade level and the deck would bear on slim steel columns which, in turn, would bear on the piers and bigfoot footings. Any sonotube exposed to view would be removed after the concrete cures.

Depending on soil properties, drilled concrete piles or screw piles could be considered as an alternative.

BA

 

[ml5299]

BAretired, you are absolutely correct about Sonotube's being cardboard.. hardly a circle shape when you get them from the store.

What I meant was that the deck would be supported by columns/vertical beams attached from the deck beam to the top of the concrete. In the first example, the sonotube would be filled with concrete to the same height as the finished top of patio concrete slab. The slab would be poured separately and you would see the circle of the sonotube at the surface, filled with caulk. To make it more interesting, the concrete will be colored and textured by having separate pours, they likely wouldn't be exact same color and texture.

I don't mind spending a little extra money to use more concrete if it meant a better frost protection for the slab and more dispersed weight from the posts to support weight of the deck. The bigfoots already are probably the minimum size to support the weight and height planned (12' high) and the deck will also be supporting a hot tub.

I do prefer steel I beams over 6x6 posts, which probably are undersized anyway and a 8x8 should be used if wood was chosen. But I think a steel beam would be stronger and take up less room.