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Community and Q&A

Building on a rock ledge

Steve Knapp CZ 3A Georgia | Posted in GBA Pro Help on

Building a foundation is usually a pretty straight forward process in my part of the country. But I am considering a small lot that is likely more rock ledge or boulder than soil. The lot has great views in several directions, but I suspect the rock has deterred at least two previous owners from moving forward with their construction plans.

One of the community’s regular builders lives next store to the property. Based on his experience building his current house, he thinks it will take one to three weeks to chisel out a spot for my planned 1,800 square foot one-level house. Blasting is not allowed, so the work would have to be done using a hydraulic breaker or similar heavy equipment.

I imagine rock is a common complication for builders in the northeast. Any suggestions for how to approach this phase of the site work. Building in this lot will max out our budget. So I really want to avoid surprises.

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Replies

  1. Malcolm Taylor | | #1

    Steve,

    Sloped sites with a lot of rock are very common here on Vancouver Island. The complexity and expense of building are almost entirely dependant on whether you want to fit a pre-planned house on the lot or have one designed to fit the site.

    Designing for the existing conditions, or with some very minor rock removal, often results in a very cheap foundation, without footings, pinned to the rock.

    The extent of the rock and slope can mean a few things. Changes in level can be taken up in the crawlspace, or in the living areas above. What is often precluded is slab on grade construction.

  2. Steve Knapp CZ 3A Georgia | | #2

    Hi Malcolm,

    Thanks for the feedback.

    This lot is one of the few on the mountain that is relatively flat. That feature was attractive to us because we want to avoid the cliffside basement look as well as the expense of piers into bedrock.

    The house will be designed to fit the site rather than trying to force a preexisting plan on to the land. An elevated slab or a crawlspace is actually our preference. (I'm attaching a photo for reference.) A hump of stone along the front third of the lot. The builder suggested notching this area out to create the homesite and drop everything closer to the street. (The community requires detached garages, so this would reduce the steepness of the driveway.)

    I read about a product called Ecobust for fracturing rock and wondered if it would help with cost or speed of construction. I had wondered about forming the foundation around "the hump," but that approach would require the house to sitting much higher on the lot.

  3. Malcolm Taylor | | #3

    Steve,

    From what I can see the site looks very build-able. I'd get a quote on removing the ledge with a hydraulic rock breaker and see where you stand. I've used expanding power to remove rock with very poor results. There is a lot of drilling - days of it, much more than with blasting - and the results are unpredictable. There are circumstances where it may be appropriate, where you cat get machines in but it sure isn't cheaper.

    I hope you blog your build for GBA. The expectations are high!

  4. T Carlson | | #4

    There are 4 built foundations around you, get one of those excavation contractors to do yours for an accurate budget number. I would see if they could try and tooth it first before they go crazy with a hammer or start drilling. The house to the left looks like it really drops in elevation, is there rock outcropping on that side?
    Is poor roof design in the covenants?

  5. Malcolm Taylor | | #5

    Steve,

    I completely forgot to mention buried services. The location of the rock in relation to the stub-outs at the street will make a huge difference to how much the excavation will cost.

    Still laughing at T's comment about whether "poor roof design is in the covenants"!

  6. Steve Knapp CZ 3A Georgia | | #6

    T.

    I agree on the excavator and the rooflines. (The designs leave a lot to be desired.). Can you explain "tooth it?"

    Malcolm,

    Water and sewer follow the street directly in front of the lot. The builder I spoke to said they had to blast that area when installing the utilities. But I will ask the excavator to price in the worst case for completing connections between the house and street.

  7. T Carlson | | #7

    Instead of a bucket they put a tooth on the hoe, looks like a giant canine tooth which they will fracture rock with. Allows to get in nooks and crannies where a bucket tooth cant. Depends on the type of rock of course.

    Looking at your picture again compared to tree line/grade it looks like the houses are on half partial excavation and half fill, Im assuming there are no basements hidden under the lower level walkouts?

  8. User avater
    Michael Maines | | #8

    Steve, sometimes ledge or bedrock is "foliated" or "rotten," and can be broken up with an excavator's bucket. The next level up is a hydraulic hammer, aka jackhammer, aka "dinker" around here. That can often get ledge flat enough to drill and pin frost walls, which can then be backfilled and topped with a slab (or foam and a floating wood subfloor).

    That's an impressive view! I imagine you'll have the nicest looking and highest-performing house in the neighborhood.

  9. Steve Knapp CZ 3A Georgia | | #9

    T.

    Thanks for the clarification.

    Most of the homes at this elevation are on basements or combined basements with subbasements. The hillside lots are steep, and, in most cases, the builders have pinned the foundations to bedrock.

    Malcolm,

    Yes, the views are something for a lot that is just 30 minutes north of Atlanta. I noticed excavations that had exposed decomposed rock but much of the mountain appears to be granite. (There is a lot of that rock in this part of the state.)

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