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1. 
   piffin_ | Aug 20, 2001 11:07pm | #1

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   You'll need to drill and pin to the rock where you sit on it. Then use extremely well drained, compacted fill under the slab. It woiuld be definitely besst if you run the footings all down to ledge. I have repaired homes that were half and half and the side not on ledge moved.

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   JohnD_ | Aug 21, 2001 12:23am | #2

   *I visited relatives in Norway about 10 years ago and they were building a new house on a hillside. (If you have ever been in Norway, EVERYTHING is built on a hillside!) They blasted away the rock to give a sort-of flat base for the cellar, and cleared away all of the loose rock.They then set up a level line and mortared in a first course of cement block directly onto the bedrock. They simply chipped away the bottom of the block to get it to fit, and used the mortar to hold things at the right height.Incidentally, the block is made from an expanded synthetic material (perlite?) that is round in shape. Those blocks are half or less the weight of an ordinary block. They also are VERY porous. They are used for thermal insulation.In this specific house, they then waterproofed the house by putting 3 mm polyethylene sheathing around the house, and a drain tile around the three sides of the house inside the hill. The last side was a stepout (and watch your step outside as the ground sloped at about 30 degrees!) so the drain tile simply opened up to the hillside.The house is still there, and still waterproof as of last June.

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   Thaddeus_Raven | Aug 21, 2001 08:31pm | #3

   *JohnD--Copied from http://www.buildinggreen.com-Autoclaved Aerated Concrete. It was discovered in 1914 in Sweden that adding aluminum powder to cement, lime, water, and finely ground sand caused the mixture to expand dramatically. The Swedes allowed this "foamed" concrete to harden in a mold, and then they cured it in a pressurized steam chamber--an autoclave.

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   JohnD_ | Aug 21, 2001 11:29pm | #4

   *And here is a USA source. It describes foamed concrete blocks, non porous, with excellent insulation values. http://www.aacpa.org/ But the stuff I saw consisted of round spheres about 1/4 inch to 1/2 inch held together by a concrete paste--exactly like our concrete blocks are made. They even had the two holes in the middle.

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   jim_"crazy_legs"_blodg | Aug 22, 2001 02:43am | #5

   *Lew - You might try asking at JLC if they can sell you a copy of their March, 1997 issue, or maybe a reprint of the cover article - "Foundations on Ledge". Sounds like just what you could use.

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   Lew_Shelley | Aug 22, 2001 02:44am | #6

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   Anyone have any experience with placing house foundations directly on ledge? I am preparing to work on designs for my house and the site is on ledge. There is room for a partial basement and the rest would be on slab. Prefer to design for solar gain which makes the slab worthwhile. What advice is out there to prevent problems with frost, earthquake potential, leaks, etc. I'm in SW NH.