Insulating Slab on Ledge floor

[archie tect]

Doing a one story addition on a site with a lot of ledge.
Think the best way to go without having to hammer is to pin the slab to the ledge which is basically at finish grade.

Typically, I'd put rigid foam under the 4" slab, but I'm assuming that would be difficult if I need to pin the slab to the ledge. The only way to get enough depth for 4" of gravel plus 2" rigid would be to take everything down a few inches with a hammer.

In this case, would all advise insulation on top of the slab? For a floating wood floor I wouldn't hesitate, but the client wants porcelain tile and I'm afraid of cracking.

Thanks,

CD

[Mark Parlee]

CD
What are you doing to isolate the tile from the concrete floor?

[archie tect]

I'm going to use an isolation membrane, probably
something from Schluter.

[Mark Parlee]

CD
We are not getting any replies to this thread.

can you provide a section drawing of this ?

[archie tect]

See SK attached. Can you have rebar dowels AND insulation under your
new slab?

[Mark Parlee]

CD

still not getting it.
can you post a picture

[Lavrans]

What's "a lot of ledge"?
Is that a ledge on the existing foundation (what I would assume).
And I'm not sure why you'd want gravel under the foam in the area of the ledge, if it's part of the existing foundation... or even if it's bedrock- just so you'd have equal compression of the gravel over time? The purpose of compacting the gravel layer is to prevent it from compacting and leaving the floor suspended.

If you're doweling into the ledge, is it really a floating floor? Anyhow, I can't see why there would be a problem with just running the dowels through a couple inches of insulation, being as they aren't supporting the floor, right? And you'll get a thermal break, which may mean your walls are slightly colder than your floor. but if it's going to have a radiant floor, you'll have better efficiency.

But then, you might be doing a thickened slab on grade for the addition? Again, not sure why you'd need gravel there, especially if you're pinning to the foundation. And not sure why you'd be concerned about insulating that area, being as it's going to be attached to the existing and, (I'm assuming) uninsulated, foundation, to which it's going to be firmly attached.

But then, maybe it's some other ledge. As Michael Collier writes:

...Ledges can be jagged outward projections, or they can run in long narrow shelves from mountains, cliffs, or buildings. Standing on a ledge, one feels both the precariousness of being suspended above and over a space and the thrill of looking out and being drawn to the beauty and mystery of an expanse. Certainly this is what one experiences when standing on the brittle and crumbling sandstone ledges along the top of Canyon de Chelly in Arizona. And when William Meredith writes, in “The Wreck of the Thresher,” “I stand on a ledge where rock runs into the river/As night turns brackish with morning, and mourn the drowned,” one fathoms not only the depth of the sea that waits beyond the ledge but the depth of grief that leads the poet to elegize the lost submariners.

[archie tect]

1. A lot of ledge means that after scraping off a few inches of topsoil, all you're left with is solid ledge.

2. Talked about gravel because I thought it might be important/ necessary to have a level surface to lay the insulation on.

3. If the dowels go right thru the insulation, are they drilled first, and then you "stab" the insulation down, or do you lay the insulation down first and then drill?

Thanks,

Archie

[ThingOfBeauty]

Lav,
"Ledge" is what New Englanders call bedrock or 'so much rock you can't really get rid of it'.
It's often pretty close to the surface. My neighbors when I lived in Vermont had a 4' deep in-ground-pool because that was about how far they could go.

I don't 100% understand how to answer the OP questions. We have pretty much no underground rock around here, so we never pin to anything, everything is floating. Also there are innumerable houses built less than 12" into the ground with no discernable frost issues! not much like New England.

[dgbldr]

Haha, thanks Doug, I had no clue what "ledge" means.

Depends on local code and AHJ requirements. Check with them. Here we can't do an addition without a full frost depth foundation wall and pinning to existing house foundation. We don't have rock, but if we did we probably couldn't do that addition without tearing out the rock.

If they let you, I would do as you said. Fill with gravel to get a flat surface, compact, put foam down, pour slab. Depending on how many dowels (talk to your engineer), I would drill and place dowels before gravel and then impale the foam. I would also try to put some foam on the edges of the slab.

[ThingOfBeauty]

FYI no frost protection is required when you build on ledge, per the code book anyway.
As I mentioned I have no ledge around here so I don't know what works and what doesn't. Happened to see a note in the 2012 code that you have to go to frost depth or ledge.
Anyone up north care to school us? Even though code allows you to build directly on ledge, is it an OK idea?

CD, I would guess you will be better off with foam under your slab. That is not the same as knowing how to get that to happen though.

[aptbldr]

Isolate building fnd/slab from ledge with the insulation and aggregate fill. Allow for differential movement, settlement, of fnd/slab where it rests on earth and rock. IMO.

[gary robertson]

you will need the slab eng.so start with that.
i have done about 8 homes on ledge and all of them start with eng.
one thing to think of is if you hire the drilling of pins have them do the ones for the slab frame at the same time.