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Porch foundation considerations

8K views 65 replies 5 participants last post by  HotRodx10 
#1 ·
I'm in the process of replacing the "temporary" wooden stoop I built to service our new home six years ago.

We'll be replacing the stoop with a 12x8 bluestone porch, and I need to finalize the design.

The ICF basement foundation is a monolithic wall and footing pour with a height of 9'4" from the top of the footing. The top two feet of that wall are above grade.

A contractor who is doing some siding work for me thought we should dig footings for the porch no deeper than the 42-inch frost line and build a CMU foundation for the porch. He proposed to fill in the foundation with seven or eight yards of gravel to support a slab for the bluestone.

My plan was to escavate the entire area 8'4" (4" below the current footing), install four inches of stone, do an ICF monopour for the foundation, add another 8-inches of stone, place a 4" slab (future basement expansion), then build an Advantech deck supported by lumber for the bluestone tile.

My plan involves more material, but I feel like I'll avoid any issues related to soil compaction, the ICFs and the monopour will be easier than separate footings and CMU, and I'll have the benefit of another seventy square feet of basement.

What do you think? Am I overestimating the potential for differential settling?
 
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#2 ·
If you get below the frost line with footings of adequate size, you shouldn't have settlement issues. However, if you want the additional basement space, the ICF wall certainly works. You're obviously familiar with the ICF wall system, so that's a plus for that option.

I'm not familiar with what the bluestone is, but a concrete slab directly on the walls would seem the most straightforward and best support for a stone floor. You should consider precast concrete panels (hollow core, maybe?), especially if there's a precast supplier nearby, or a reinforced slab, cast on-site next to the porch and slid into place. It takes a fairly beefy wood framing and decking system to provide the stiffness required for rigid flooring.
 
#6 ·
I know alot about corrugated steel decking. We check the shop plans for it on practically every one of our new highway bridges. We refer to it as stay-in-place formwork. Wow, I can't believe I didn't think of mentioning that before! Yes, that would be an excellent way to form up the slab in place.

all you need is a flat surface for a couple inches along the edges of the walls for the ends of the pans. The supplier should supply the installation details. I think they'll do a design for you as well. For a 10'-3" span, with an 8" slab, I would expect something more in the 20 or 21 gauge, for a 3" deck pan. Since the depth of the form doesn't seem like it would be an issue, a 3 1/2" deck form might be more efficient.
 
#4 ·
In any case it would be better to lay the bluestone on concrete rather than wood.

If you do not want long-term settlement, you just have to get the footings past the backfill to undisturbed soil. Two scenarios are either take the entire 3-wall perimeter down to house footing level, or step up the 2 sidewalls from the footings to frost depth. They should be rebarred into the basement walls.

If you go full depth and capture the space, that's probably much easier to form with ICF. Your assessor would probably want to talk to you, and you have to deal with waterproofing, rerouting footing drain tile, and cut a doorway. The composite metal deck just needs an angle along the house wall. You also probably want to detail the wall/top of slab correctly with flashing to prevent water seepage.

Just frost footings (with steps down to house footing) the slab is 'on-grade' and much less detailing. Hard to form the steps with ICF. At that point though there is no gain with ICF, its a bit pricey to begin with. *talking about foundation wall step not people step
 
#5 ·
The extra footage for the basement adds a lot to the expense

Normally this is what you would see unless a geo tech engineers say that the refill soil can support it, which is doubtful.

The bridge across the missing footin is done with extra rebar.
 

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#7 · (Edited)
I have only about 1-1/2 inches of footing projecting beyond the ICF form, and it would provide basically no support due to the sloping shape (FabForm Monopour). Since the overdig was <3 ft., could I not use number 6 rebar to bridge the potential soft spot? I'm thinking I could run my jumping jack in the rest of the trench. I would love to avoid working on a stepped footing while at the bottom of an 8-foot hole.

HotRod, would I really need an 8-inch slab?
 
#12 ·
My wife wants me to put down piers and build a wooden substructure. Is that what you're suggesting? I guess I could put lath over wood panels to attach the stone, but I'd be concerned about longevity. Could I let the stone overhang the plywood and contact the soil, or would I have to leave a weird gap?
 
#13 ·
HotRod, I think I was working on my response to Neal when you posted your message. I found a precast place that's only an hour away, so I think I might get a quote. It would be a simple solution.

If I don't excavate to the basement footing depth, couldn't I bridge the 2-3' overdig area by pinning the new foundation to the old with ample horizontal 6 rebar?
 
#14 ·
I'm not really envisioning what you mean by bridging between the foundations. It's possible to support the top of the wall by using pins that go into the slab, if the house foundation can brace the the 'inside' end of the slab. Basically, the slab acts as a compression strut to carry the force from the soil pushing the top of the wall in, across to the existing house foundation.
 
#25 ·
I would think that would be fine if you are just bridging 2 feet
If you go the 8 ft direction with the steel most of the weight will be on the foundation and the outside wall, missing part of the footing would not make any difference as long as the bottom of the wall there is below frost.
 
#28 ·
The decking supports the weight of the wet concrete until the reinforced slab has sufficiently cured to be self supporting and able to support the loads that will be added afterwards (flooring material, furniture, people, etc.). If it's spanning the long direction you may need to add a temporary (or permanent, depending on the design span of the slab) beam across at the midspan.
 
#29 ·
We have a 1-1/2" sill, a 9-1/4" rim joist, and 3/4" subfloor, putting the underside of the threshold at 11-1/2" above the top of the basement wall. The total floor system thickness will likely be 5-15/16" to 7-7/16", depending on the decking and slab thickness I choose. I'll have to consult the steel supplier's tables. I'm trying to avoid a step at the door. Unfortunately, I'm allowed only three risers without a handrail, so I'll probably have to bring up the level of the walkway beyond/

With 18 gauge 3-inch decking, I believe I can actually span the 10 feet without any temporary shoring. I'll probably use a 5000 PSI mix.
 
#39 ·
I called the steel place again for price quote. I would pay about $285 for 18 gauge 3-inch composite and $315 for 16 gauge.

I was thinking all along that I would tie the slab into the walls, but the the video posted further back recommends a floating slab. What do you think? I was going to give the decking four inches of bearing at each end and bend the rebar from the walls into the slab.
 
#41 ·
That area was probably excavated and backfilled when your house was built. That means you are building on fill that was probably not engineered and compacted. I would excavate to the original foundation depth and start there or put in engineered fill properly compacted. When I excavate with my backhoe, I can tell when I cross a trench that was dug decades earlier.
 
#42 ·
I was present when the basement was excavated. The area that was backfilled extends only about two feet out from the foundation.

The only detail I'm trying to iron out right now is how to incorporate the steps. The stairs will project approximately 32" (two 16-inch treads or stone slabs) and they will be 72" wide.

So, my plan is to wait a week before I backfill against the inside of the porch. I'll then place the concrete for the deck and the porch at the same time. I would like to form the steps by doing a monolithic pour down to the frost line. I know concrete's expensive, but this plan seems like it might minimize labor and avoid problems with sediment and water infiltrating a gravel bed and moving the steps. I would bend a few pieces of rebar in the porch through the ICFs and into the stairs.

Opinions?
 
#54 ·
I have excavated the trench. I'm waiting for the rain to stop so I can jump in there and dig out some of the clay that washed in.

At any rate, I'm to the point at which I'm going to cut the dimpled drainage membrane and the adhered waterproofing membrane. Since the trench is not full depth, I won't be able to neatly wrap the cut membrane, and I'm afraid of creating a spot that will be hard to detail and that will represent a water intrusion issue.

Do I really need to cut back the waterproofing and the ICF foam to have concrete-to-concrete contact? I was thinking I could instead just run the dowels through the 2.5-inch foam and place the concrete right against the waterproofing. Would four pieces of #6 rebar per side (including the two in the footing) suffice to tie old foundation to the new. I'll be bending the vertical rebar in the new porch foundation into the suspended slab to help support the top of the wall.
 
#55 ·
It's been a while since this thread was active, so I don't remember what your configuration is. I'll try to find some time to go back through the thread and look at what you're doing. I can tell you from a structural standpoint, if you separate your concrete by a couple inches of foam, you might as well skip the rebar, because effectively you'll have 2 separate concrete sections.
 
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