Simplifying hillside construction. Continuing a discustion from another topic. Starting a new thread.

In a discussion on another topic I wrote:

The only reason a basement makes sense is if you have a deep enough frost line to justify the excacvation. So slab on grade makes the most sense. Lately I’ve been dealing with slopes most of the time. I’ve found that it’s often less expensive to build foundations on the existing grade and used a raised subfloor than do the engineering, grading and compaction to do a slab. Soils work has gotten quite expensive. Often access for machinery is an issue. A couple of years ago I built a garage with a zero stem wall on the high side and an 8′ cripple wall on the low side. It was alot cheaper to engineer an build the subfloor for vehicle loads than to build an 8′ + retaining wall and bring in god only knows how much fill.

Oops replied:

I’ve thought of doing that, but it always seemed so out of the box for single family homes, I discarded the idea.  Do you run into any code problems?

Here the developers cut and fill, stepping down the slope, to create flat lots. If it is sever enough, they have to build retaining wall at the property line.  Basiclly making the lot fit the house instead of making the house fit the lot. Of course they wipe out all of any trees on the site.

It is quite a bit more economical to grade the lot, but it would make for a more architecturally divirse neighborhood to fit the house  to the lot.

Too eliminate the cost of compaction etc, when necessary, they set the slab on piers down into understurbed bearing soil.

I reply:

Here in California building slab-on-grade foundations has been the norm for many years. Just grade a flat pad on the side of a hill; cut the top fill the bottom. I built like this for years. There are lots of advantages. I like slabs. They have a lot of thermal mass. I like tile floors and stained concrete finishes. But the cost of building like this has risen dramatically. Local jurisdictions have gotten very picky about grading and compaction, so soils engineering is often required. Compaction testing is almost always necessary. Environmental costs have also risen. California now requires construction projects to observe “best management practices” or BMPs. This involves creating a detailed plan for controlling the direct environmental impact of grading, construction and the developed building site. This involves silt fences, wattles, sand bagging, drainage plans, retaining walls, swales, etc. You can no longer just grade off a pad and build. It’s become very expensive. I’ve also come to believe that preserving natural features and vegitation of a site is desirable. Removing established trees should be avoided. Most owners agree, especially when this is the less expensive alternative. Three examples.

There are options for building on sloped or severely sloped sites. Grade beams, concrete piers and retaining walls are options but these are not conventional construction and/or involve extensive engineering and soils work. Stepped footings with stem and cripple walls and wood framed floor systems are conventional construction and often covered in building department hand-outs. (See the attached detail.) Excavation is labor intensive (and I mean Labor with a capital L) involving shovels and jack hammers, and many contractors seem to be afraid of hand digging, trying to avoid it at all costs. I don’t mind busting my butt and I’m almost 70. Clearances to daylight must be maintained and this often equates to some deep footings. I’ve never had to go more than 5′. With careful use floor framing lumber can be used for stem wall forms before it is used for the floor. I have a supply of 2x form lumber left over from decks I’ve demolished to supliment this. I use Simpson’s wedge ties. I’ve never had to buy form lumber.

The first hillside project I did was an extensive residential addition on the downhill side of an existing residence. The top floor was at street level, the bottom floor had a retaining wall on the high side. Access was through a 4′ side yard. The owner had been told that it couldn’t be done. I used stepped footings with a couple of post and piers. The only soils work was a bearing test. The structure was engineered, but conventional construction. All excavation was by hand.

I did an artist studio on a hillside. Site options were extremely limited. Two walls of the structure were right on the setback lines. Because this was a single story no engineering was involved. Excavation was by hand. I probably could have used a mini excavator for some of it but access was over a 4′ drop. Photo and drawings attached.

I did another studio, which for reasons I won’t go into was permited as a garage. We had only one possible location on an already developed site. I could have used an 8′ + retaining wall on the down hill side and lots of imported fill. It was cheaper to engineer and build a conventional subfloor for vehicle loads, than do the engineering and construction of the retaining wall. If the subfloor area is not intended for any additional use, engineering is easy because mutiple piers can be used. In this case they were 4′ oc in one direction 4′ 6″ the other. The only other requirement for the garage was that the floor be non combustible. This was accomplished with a light weight coating. No soils engineering was required. Excavation of the wall footings was by backhoe. The posts were dug by hand after the walls were poured. Photo and drawings attached. BMPs for this project involved a silt fence and nothing more.