I am looking at two building sites. One appears to have ledge within a foot of the surface and has great views. The other appears to have at least 4 feet of soil before hitting bedrock and has a so-so view. I am being told that for the lot with the ledge at the surface, that I can put the footers on top of the ledge since the ledge is not impacted by frost. This is in NH with 4 foot frost line. I have always built by going to 4 feet below grade with the footers. Can the distance below grade be raised when building directly on ledge? How much?
Thanks for any thoughts
Replies
The footings can be pinned to the ledge provided the ledge itself is structually sound. No way to really tell till you get it scraped off. Don't forget you've got to bring water into the house and sewer/ septic out- that means blasting. Might be worth it to blast a little more for your foundation. If the entire footprint of the house is not on the ledge, you'll have a prime candidate for differential settling. Nothing a good engineer can't handle. More $ in any case.
Edited 8/9/2002 10:17:00 PM ET by jc
Another scenerio for concrete on ledge:
One thing that worries me is water which collects in depressions in the ledge right next to the concrete wall. Ledge is rarely completely smooth, so there are usually depressions or buckets where water can get trapped. Draining these areas would be impossible. If its well above frost line, can that water put lateral force on the wall, pushing it sidways as it freezes?
Can you get a "soil sample" tested? What I mean by that is can you get some engineering company to test both areas and give you an idea about what you are up against?
The reason for putting footers below frost is so that it won't heave the foundation out of the ground and move it around.
I've never seen frost move bedrock!
But as these guys have said, there are other issues like keeping your water supply lines from freezing and hwere will you place your wastewater system.
Thanks-
I will investigate the extent of bedrock further. You have verified that it is possible but not easy to build upon.
Ron,
I am going through the exact same dilemma right now. I am planning to build a heated barn/studio which will sit on ledge located between eight inches and four feet below grade. I've been asking as many sources as I can find about the prudence of pouring concrete so far above frost line (I live in northern Vermont) and have received many contradictory answers. Finally this week I called "construction consultant" to try to get a final word on the subject.
He says he has alot of experience in this area of construction and said to not worry, just pin and pour. I'm having him out to look at the site this weekend, I don't feel completely comfortable with his opinion until he looks at my particular situation. He also said to be sure to use a non-expanding grout to secure the pins in place, and to also seal the footing at the ledge using a tar product or rubber membrane.
Good luck, I know I'm hoping for it!
Thanks
I'm going to throw out one I've heard from a couple of concrete men around here. It may be one of those myths but it has enough of a ring to it that it's worth thinking on at least.
They say that concrete in solid contact with ledge will not freeze because the granite is constantly convecting and radiating the heat from the core of the earth.
Do with it what you will.
I have other reasons to believe that pinning and pouring is all you need to worry about.Excellence is its own reward!
"not freeze because the granite is constantly convecting and radiating the heat from the core of the earth. " ............ they must be from Hawaii near one of the lava flows ;o). Getting off the the subject ........ I imported some AutoCAD R14 drawings into IntelliCAD ....so far so good. When I get a chance I'll try some 3-D stuff.
We have built 3 or 4 houses on ledge. The ledge is easy to pin and pour and is much stronger than any conventional foundation. We have not had a problem with water and sewage, usually because there are dips in the ledge that are well below frost. One house had the well head in the basement. There are two problems. 1. The ledge may dip way down in one section of the house. This can lead to a very deep foundation, 13' in one section we had, or you may need to have part of the house on ledge and part sitting on soil. You will need a joint in the house that can adjust to the differential movement. 2. Water will enter the basement through crevices in the rock. There is no way to prevent this by sealing the footers to the ledge. The only cure is to put a vapor barrier over the ledge and cover this with sand or concrete to protect it. You may have to install a dehumidifier in an extreme case. Good luck.
Thanks for the info - I assume that you pinned about every 8 feet with #4 rod about 12 to 18 inches into the ledge?
Thanks
Ron
We pinned every two feet but we are less than 4" into the rock. Our most recent venture specified 12" into the rock and grouted with epoxy. The owner nixed this when he found out the cost. Unless you are in an exceptionally active earthquake zone, our usually procedure is sufficient for residential construction. You can tell quite a bit by looking at the ledge itself. The ledge we have built on is millions of years old and is assuredly the most permanent part of the house.