Anybody who knows about house foundations?

[cooking]

I'm looking at a residential building lot that appears to have some low outcroppings of what the realtor calls ledge rock.  I would be open to any kind of foundation build that keeps my building budget low.  Does anyone have experience with this?

For instance, would a slab be feasible (and a reasonable price to build) where there is ledge rock?  I have read a brief mention of something that I think is called "pinning into the rock" by drilling into the rock in certain places and anchoring the foundation to it.  Does this end up being very expensive?

Or would a pole-type or timber frame building be more compatible on this type of site?  Not to confuse the issue even more, but would helical piles work better?

I don't even know which kind of expert opinion to seek on this, as I've heard that most foundation contractors specialize in a certain type.

Could those who have any experience or knowledge on this please weigh in?

[BudgetSlasher]

Where are you in the world/country?

If what is being called "ledge" truly is bedrock, then my understanding is that you can build a slab on grade pinned to the bedrock in almost any climate. Without any documentation by someone who knows what is going on I would not rely on a realtor to know the difference between a bedrock outcropping and a partially exposed large boulder.

Where I live a standard foundation needs a footer to below the frost line (about 4 feet). Any form of slab on grade would likely be cheaper; but then again basement are expected here (to the point where people will blast a basement).

I hate to tell you to spend money, but you might want to consult a professional. Since you mention 2 foundation types (slab on grade pinned to ledge and helical pile) each suited (as I understand) to different soils and behave differently in different soils. I doubt a helical pile would go into bedrock/ledge, but a slab on grade might very well work. That's a long way of saying the suitability of a foundation depends on the site and what is under the site. If you like the site, I would find a local expert who can tell you what foundations are suitable for the site (after all without a good foundation a house is off to a very bad start).

As an aside I am fascinated by the idea of the shallow frost protected foundation.

[bacchi]

You need a soil engineer, which is a specialized civil engineer. When I built, I dug my own soil samples and dropped them off but the engineer can also visit on site.

[cooking]

The lot is in the northern reaches of NJ, pretty much in the middle of the state east/west-wise.  I think it's in the 5 zone for climate .  It's an infill lot, most of the neighborhood was built in the 30's and 40's.  So there are probably not any living builders to ask for that particular spot in the town. 

I thought soil engineers were more attuned to the drainage characteristics of the ground, like for septic systems, etc.  This lot has access to city sewer, so drainage for a septic wouldn't be an issue.

I too have long liked the idea of a shallow frost protected foundation, but I don't know if they're suitable for a soil with ledge rock.  In my readings on this subject, I read that Frank Lloyd Wright declared that they were suitable for most soils unless they were what he called "treacherous soils".  But everything I've read on them calls for them to rest on gravel, which is what makes me wonder if rock would work.

[Fishindude]

In New Jersey you occasionally have freezing conditions, so you need to have a footing that goes below frost depth.   Don't know what your frost depth is, but where I live is 36", would guess yours to be similar.

Knowing the above and forgetting about the rock, this means that you need a foundation around the outside walls of the house that goes 36" below ground surface level and bears on good, undisturbed soil.   A very typical foundation for a house involves excavating to the 36" depth, pouring a spread concrete footing at that depth, typically approx. 24" wide x 12" thick.   On top of this footing you then form a wall of poured concrete, or lay blocks to get you up above ground level to the elevation you want your floor system to be at.   The floor can be either standard framed lumber with a crawl space, or you can backfill the interior with sand or gravel and pour a slab, but you still need that perimeter foundation down to below frost level.

If you do have bedrock on this building site that can present some challenges.  If it's close to the surface of the ground you can forgo the 36" depth requirement as bedrock isn't going anywhere with freeze / thaw. and can build the footing and walls right on top of the bedrock.   You typically "pin" your footing to the bedrock with rebar dowels to help hold things in place.

Some serious challenges with building on a site with bedrock include:
a. You probably can't afford a basement.
b. Septic systems won't work.
c. Trenching in utility lines is extremely difficult.

NOTE - You have similar challenges with pole or helical pile construction.  Can't effectively auger holes in bedrock and metal helical piles won't drill into it.

First thing I would do on a site like this is to do some test excavations around the site to find out how deep I could go before I hit bedrock.   This would tell me what i could or could not build and how difficult it would be.   Removal of bedrock for excavations can get very costly.