Building on a Hillside. What do I need to Consider?

Hello!

I am attempting to build a house for the first time, and I'm doing so on a hillside. Please, please, please give me your wisdom.

House Details:

I am in Southeast Tennessee

The soil seems to drain well

24’x24’, 1.5 stories

All four corners of the house are at different elevations.

Maximum slope is between one of the diagonals, at most 20%.

I'm digging by hand, and so I would like to avoid excavating out level ground.

The footing will be a rubble trench, 16" wide, 16" deep.

Stone foundation. Stepped or not? I'm not sure. The foundation will also be 16" wide. 6" below ground, and 16" above ground.

Post and beam construction.

First floor will be an earth floor. I might split the floor at different heights.

Is any of the above foolish?

Some of my concerns:

What’s to prevent the house from simply sliding down hill?

Is a rubble trench suitable for such a slope?

Should the rubble trench match the slope of the hill? Should I dig the trench to be stepped?

How high should I raise the earth floor from the ground?

Please tell me how I'm dumb.

Also, if there are examples of this kind of building, please point me to them.

Here's a picture of the trench I've just begun to dig.

Hi Stef,
The picture does not look as steep of a hillside as I imagined when I was reading your description, but you still want to be extra careful. I am not a building expert, but some questions come to mind. The first is what climate are you in? I'm not familiar enough with Tennessee to have a sense of whether or not cold weather and rainfall would effect how you should build.  You mention the foundation only being 6" below ground. That seems quite shallow to me.

Hi Stef;
From your post, I envisioned a steep timbered hillside.
And then I got to the photo...
That does not look seriously steep. I think you will do fine with your plan.
Myself, I would use a wood floor but that is a personal choice.
I could see my doggy getting bored and digging a hole in the floor...
Please keep us posted with plenty of photos as you build.

My home is constructed on the east side of a hill.  More than once a tornado has skipped over my house. Most spectacular was when a settlement near me was largely destroyed, and my home was missing a handful of shingles.

I wish I could take credit for selecting the location, but the house was around 25 years before I moved in.  

Matt McSpadden wrote:Hi Stef,
The picture does not look as steep of a hillside as I imagined when I was reading your description, but you still want to be extra careful. I am not a building expert, but some questions come to mind. The first is what climate are you in? I'm not familiar enough with Tennessee to have a sense of whether or not cold weather and rainfall would effect how you should build.  You mention the foundation only being 6" below ground. That seems quite shallow to me.

Thank you, Matt. It's hard to see from the picture, but there is about 6' height difference between the high corner and the low corner, over the ~35' diagonal. If that's nothing to worry about, I'm relieved!
Where I am, when it rains, it pours. I'll ask the local building office about the frost line. It should not be very deep here. Should the foundation itself be below the frost line, or does only the footing need to be below it? You made me realize that I assumed only the footing needs to be below the frost line. Thanks!

What is the frost-line in your area?  You will want the footer to go below the frost-line.  I would think 18" would be appropriate?  Do it right the first time.  It's not like you can go back and improve on it later.

Why rubble and not a standard concrete footer?  With a cement mixer and some help, I poured the footer for my 40X20' workshop/carport in one day.  Mixed, poured, load after load till I went around 360 degrees of footer.  I can envision windows shattering, and doors not opening once your rubble foundation settles in the next year or two.

Building on a hillside does not sound like a new concept to me.

I feel folks have been building all sorts of buildings on a hillside since people started building log cabins.

The challenge is trying to get everything level.

That can be accomplished by using a tool called a level.

Stef said, "Should the rubble trench match the slope of the hill? Should I dig the trench to be stepped?
How high should I raise the earth floor from the ground?

These questions will be answered by the level.  Someone cannot look at your picture and answer those questions. Except for the question about the stepped. And I don't have the answer if that would be the best way.

I feel you might want to make rubble trench footers at each corner with string to make your first floor level.

Michael Qulek wrote:What is the frost-line in your area?  You will want the footer to go below the frost-line.  I would think 18" would be appropriate?  Do it right the first time.  It's not like you can go back and improve on it later.

Why rubble and not a standard concrete footer?  With a cement mixer and some help, I poured the footer for my 40X20' workshop/carport in one day.  Mixed, poured, load after load till I went around 360 degrees of footer.  I can envision windows shattering, and doors not opening once your rubble foundation settles in the next year or two.

Thanks, Michael. The building office tells me "The bottom of footings are to be 12" below grade, Water lines will be safer at 16." I may dig deeper to 18" just to be safe.

How much did the concrete cost you for your workshop, if you don't mind me asking? There is a quarry down the road from me that will dump 22 tons of rubble for $600. I'm not sure how the price compares to concrete. As for settling, thanks for scaring me. I've seen videos where people fill the trenches with water to 'pre-settle' it, but I do not know how effective that is. I'll definitely consider concrete more.

Hi Stef,
Keep in mind that the 18" below grade that was suggested would be for the lowest corner. The other corners will be much deeper.

Matt McSpadden wrote:Hi Stef,
Keep in mind that the 18" below grade that was suggested would be for the lowest corner. The other corners will be much deeper.

Woahh! Thank you, I really did not understand that (hence the title). So the footing should be an even level all around, am I understanding that? I've got all these books about building a house, and they all assume flat ground…

Yes. your footer should be level all the way around. The vertical foundation wall can somewhat follow the lay of the land, but its going to need a flat surface to start.

Generally after the footer is in place, you would build the vertical part of the foundation wall. And it is common to build it so that the top is roughly the same height above the ground level all the way around. This means part of the vertical wall might be 6ft (5ft to get to ground level and 1ft above) and at the lowest part it might only be 2.5ft below (18" to ground level and then 1ft above ground level). So the foundation wall could stair step down from the higher side to the lower side, but you would want the footer flat. Does that make sense? Then you build on those stair steps for your house wall. I included a really bad drawing. The footer will be wider than the foundation wall. The exact sizes of everything will vary depending on what sort of foundation you make.

Stef, I'm not sure what the typical snow load is in your area, but I've experienced trouble with my existing home due to snow shed.  The steel roof dumps the snow load all at once on the uphill side of my cabin.  As mine is on posts and cinder blocks (added after I bought the place), the huge snow dump thrashes my cabin downslope every season. In time, I'll jack the cabin up and turn it so the snow no longer moves the building.

I might recommend pitching your roof to the slopes, rather than high side/low side.  If the budget allows, I'd add drainage pipe to the trenches as well.

Cheers and good luck!

Stef Hoff wrote:
How much did the concrete cost you for your workshop, if you don't mind me asking? There is a quarry down the road from me that will dump 22 tons of rubble for $600. I'm not sure how the price compares to concrete. As for settling, thanks for scaring me. I've seen videos where people fill the trenches with water to 'pre-settle' it, but I do not know how effective that is. I'll definitely consider concrete more.

That's a bit hard to say, because I bought my cement, and the sand & gravel, in bulk for mixing on demand.  I started out with 16 tons of sand/gravel mix, and 80 94lb bags of cement.  BTW, I had to carry only 40 bags at a time with my Ford 350 up the mountain road.  Bought all the cement on my HomeDepot credit card (interest free for 2y), then drove up with 40 bags, unloaded myself, then drove back down the hill to get the second load.

The sand/gravel was mixed at the yard, and then delivered to me via dump truck.  It was a smaller 2-axle truck, so he made two trips also.  It was all dumped on top of a plastic sheet spread out next to my construction site.  Once the footer was poured, we continued to pour the pads inside the footer rectangle.

By the time I was finished, I think I still had ~4 bags of cement left, and enough sand/gravel left for little projects, like putting the clothesline posts in the ground.

Matt McSpadden wrote:Yes. your footer should be level all the way around. The vertical foundation wall can somewhat follow the lay of the land, but its going to need a flat surface to start.

Generally after the footer is in place, you would build the vertical part of the foundation wall. And it is common to build it so that the top is roughly the same height above the ground level all the way around. This means part of the vertical wall might be 6ft (5ft to get to ground level and 1ft above) and at the lowest part it might only be 2.5ft below (18" to ground level and then 1ft above ground level). So the foundation wall could stair step down from the higher side to the lower side, but you would want the footer flat. Does that make sense? Then you build on those stair steps for your house wall. I included a really bad drawing. The footer will be wider than the foundation wall. The exact sizes of everything will vary depending on what sort of foundation you make.

Thank you for taking the time to explain and draw that! I'm realizing that an implication of the flat footer is that, at the high end of the hill, I would need to dig a 6 foot deep trench, squeeze myself in there, and stack a 6 foot high stone wall underground. I would definitely prefer concrete at that point. I've got to keep mulling over this… Thanks for all the help, Matt.

Footings are generally kept on a level base where practical, but a 6' difference is not practical. There are standard details for stepping concrete footings; I am not sure of the best practice for stepping rubble trench footings. I would step about 2' at a time at most, keeping the base nearly level but slightly sloped to drain. Run a French drain downhill to daylight from the bottom corner of your trench. A perforated pipe at the base of the trench, with filter fabric and sand or fine gravel for a couple of inches all around it, would be excellent insurance for cheap. (Don't use the flexible corrugated drain pipe, that is weak and may collapse if not installed perfectly.)

Since you are on a distinct hillside, you need to make sure to have good drainage around the upper sides of your foundation. Flatten the ground for at least 3' on all sides, sloping slightly away from the foundation, with a gentle swale that will guide surface water positively around and away from the house. I think I would do a combination of digging down at the uphill corner and building up at the downhill corner so you still have good drainage around the house without having to bring in tons of fill dirt from elsewhere for a level floor.

You will need to contain the rubble trench material on both sides, unlike laid-up stone or concrete/block/brick. This means the ground around the downhill side will need to be built up close to floor level. The more flat or angular rock you have available from your rubble or locally occurring, the better for the upper part of your foundation. Set good building stones aside as you are filling the lower bulk of your trench. The top of the foundation will need to come to floor level all around, unless you make a basement or root cellar at the downhill corner of the house. Obviously that section would have a wood floor rather than an earthen floor, and you would still have to make foundation walls around the interior sides of the cellar space. They could be stepped back so you have shelves for storing produce and the walls would be more stable.

If your soil at the base of the trench is loose enough that "pre-settling" with water would make a difference, the soil is not compact or dense enough to be a good base. Dig down at least below topsoil everywhere; you should get to hard, solid soil within a foot or two at most if you are on a hillside. A flat flood plain with deep rich soil is not likely to be as good a footing base as upland subsoil. If you need to chip or hack at the ground to dig farther down, you most likely have a good base. If you can sink a shovel in just by stepping on it, it is too loose.

I would keep the top of your foundation wall at least 8" above finished grade, with good roof overhangs all around to keep water from splashing up onto the wooden parts. More clearance above grade is better. This will put your finished earthen floor 8" or more above grade. I would put a thick layer of gravel/rock (with minimal sand/dust) under the earthen floor to help prevent water from wicking up. I don't know how well-drained your soil is in general. If it has much clay content, the anti-wicking becomes critically important.

You can definitely do stepped footings. We live in a mountainous region and people build in steep slopes all the time.

My husband's a structural engineer. There's one town where he works that has a lot of houses built on rubble foundations on steep hillsides. He sees a lot of major issues, and many (most?) of these foundations were built by stonemasons. It's hard to do well. I'm going to qualify that by saying these houses are 60-80 years old. But for every house still standing today, you might have ten that fell down.  I think concrete would be easier.

@ Glenn and @ Jan

I stand corrected. I have only ever seen a flat footing, but it is good to know you can step that as well.

I am coming off a year of building my house. I used a foundation product called Fast Foot by Fab Form for my footer, would recommend. It is plastic, it was one of the few places I used plastic in the house, and I feel good about it. It also takes the place of any water proofing that might be necessary for your foundation. It is a “bag” concrete form, where the bag is the form for the concrete and remains in place once the concrete cures. No bulky forms to move, rent, clean. I made a rubble foundation for the footer, then poured the concrete over that. It worked well. Accommodates stepped footers.

If your house is 6ft into the ground on one side how will you protect the walls from caving in and also from ground water from seeping in? And how will you protect the earthen floor from the ground water?

Why not do a "post" and beam foundation. And build above the ground vs into the ground. I also recommend building a 24ft by 36ft house vs a 1.5story house.


All that said I do like the idea of a green roof, earth-berm walls and living walls for a house with radiant floor heating (and cooling with a dehumidifier and ERV). But for that I would make quite a few changes.