Okay, So I'm ready to go. I just took the Cob Cottage Company workshop, but we didn't get very deep into floors and foundations (no pun intended). Here's the deal. My site has about a 17% slope. It loses about 41 inches from the west wall to the east wall. In other words, the west wall is 41" higher than the east wall. My thought was to take advantage of the slope and just plan for floor level changes that go with the natural level of the slope; a raised kitchen on west side and a sunken living/dining room on east side. I thought that if I could start with these floor levels before I start the rubble trench and foundation, it would save digging down 41" on the west side, which is what I'd have to do to get the whole site level as a starting point. Am I making sense? But if I start with floor level changes from the beginning, then I would have to build a foundation/stem wall on a slope, or tiered. I'm not sure that's a good idea.
So, question: Do I have to level the site down to the lowest level, then build my foundation on that level surface, then build the floor levels back up with fill dirt and tamping? Or can I start with the floor levels, save digging and take advantage of the already compacted dirt and find a way to build the foundation around that? In other words, does the foundation have to be level or can it be sloped/tiered to conform to the slope of the hill? I hope this makes sense. It's hard without a visual I know. Any help would be appreciated so much. I'm ready to go once I figure this out. And summer's a dwindlin'! Thanks . . . Clay (yes, real name)
I am currently building now and have the same issue to contend with. What I have is a change of about 6' from my west to east walls. What I have done is to dig it all out. The reason is it will simplify the build as well as I wont have steps all up in my hizzy. If you dig out the floor to grade you will need to build a gravity wall on your west wall, the high side. If you look up gravity wall it will explain to you exactly why and how you need to do this, and you should find a good calculator to determine the proper engineering for it. One other step I took to simplify the design and build is to do post and beam rather than footers and stem walls. This requires significantly less concrete. I put in a crushed stone trench with a grade beam that has a french drain at the bottom to hold the straw bales. This trench does not support any of the actual structure of the home so it only supports about 120 lbs per linear foot rather than several tons. As well my posts piers will have excellent frost protection as they are all just inside my straw walls. I highly recommend you dig out the site to one grade, and do put in a gravity wall.
I have built a traditional/conventional style home that stepped in tiers down a very steep hill. Concrete stemwalls etc. Each level was excavated to the level of the footers, instead of backfilling the lowest part of the basement, we created a sunken living room with a 14' ceiling on the lower level - the client was an african big game hunter and this allowed him to display his cape buffalo mounts. The home spanned 26' feet of soil elevation from top to bottom by the time it was complete. We also literally saved money by installing core-floor (giant cable tensioned hollow core concrete slabs) over the garage excavation. Although mostly used for parking garages and banks, the core-floor package was much cheaper than the fill for a 30'x33' hole 10' deep. Fill would have been over 9k, the core-floor, crane, and lower level slab ran just under 8k. I never cut the top level slab for expansion, but seven years later it is flawless - not even a hairline crack. The only real hassle was that I installed a 4' rollup door that did not seal at all at the edges, and so ended up going back and putting in a standard exterior door.
My little cottage is multiple levels, tiered foundations under the cob, but I am on top of a dry hill in a dry climate. I generally reverse the slope of the tiers into the slope of the hill. I would highly recommend going with Dan's advice and excavating to one single grade as staging the heavy materials on my site would have been very difficult without the equipment that I have (all terrain forklift can boom out a 1 yard load of cob around 15' for placement). Drainage issues will be greatly simplified as well.
we finished our house last year and dealt with this same issue, almost with the same slope (but ours faces west rather than east). and we chose to do the step downs as you're thinking. You can see our whole blog at www.ideamountain.com which covers building from start to finish.
Our main concern was the retaining wall on the uphill side - if we went down all the way to have one flat floor, we'd have to go deep into the ground to build an appropriately sturdy retaining wall. By only going down 5 feet, we didn't have to go massive with the retaining wall footing.
(another part of our design includes leaving a 'hallway' between the house and the retaining wall. Not sure it was necessary, but we were concerned with the possibility of water/force of gravity affecting that retaining wall in the long term, so we didn't want it to be the bedroom wall that weeped.)
another part of our 'step down' design allowed for a loft over the kitchen without having 14 foot high ceilings in the whole house. (that is, the west side is taller than the east, which adds a very nice dynamic to the design)
Looking back on it, I can't decide entirely if it was the best solution or not. The challenges include:
-when we're old, we'll still have to go up and down the stairs to the bathroom.
-putting in the radiant tubing for the hot water heating was a pain, and might create more trouble for us down the line
-my stair design included wood runners on top of the mini stone retaining walls that are the stairs. that was really hard to do (in part because of the wood I chose) and the connection between the wood and the flor and wall plaster continues to be a bit of a design flaw. (remember that if you make multi-levels of cob floor, you'll have to have some sort of edge at the stairs... a cob edge will crumble)
-I'm a bit concerned about having a cob house that has much bigger - heavier- walls on one side than the other... and am wondering if there might be some settling differences over time. I have seen a few cracks in my plaster, but as of now, they're just the kind that should be hidden when we whitewash again.
-the above mentioned retaining wall issue
-the easier digging at the time (which is NOT a good enough reason to do it)
-the sense of flow and design that comes from the multi-levels. this is actually a big deal.