Building on a mountain slope

[db4570]

Hi, guys. It has been some years since I have posted here, and am glad to have an excuse to be back. 

We just bought a small piece of land in the mountains of Arizona, which has been a bit of a dream spot of mine for a while. We plan to build a small seasonal/vacation/retirement house there. I may GC it myself.

The lot is sloped about 15% to maybe 20% max (percent, not degrees). This, to me, is a moderate slope that is easy to walk up. I have always pooh-poohed when people seemed nervous about developing a piece of land to build a house on anything except a perfectly flat building lot. To me, a lot with some elevation change adds a lot of beauty and character. But now, I am talking to builders and excavation contractors and they are saying that between driveway (under 100'), septic (which may require an alternative or "engineered" system, or maybe not), and digging a level pad for the house I could be looking at $100-150K just for excavation, which seems insane. They are nervous about big rocks, and not knowing where the granite bedrock is. 

The two builders I have talked to want to build on a slab, which seems like the worst of all choices to me. A slab requires a lot of excavation to get a slope level, has inherent moisture challenges, and doesn't offer any the benefit of a basement for cheap storage or work space. 

So I guess the other two choices are a full basement, which requires even more excavation, or building on poles or columns. Or perhaps a combination of the two, with the part of the house facing downhill up on poles, and the uphill part dug into the slope a bit, providing some basement space.

I like the pole idea the more I think about it, and I rather like the look, in a mountain setting. It seems like it would require far less excavation. In fact, it has gotten me thinking about why I need to dig at all, in theory. If there is an enormous granite shelf, why not just bolt to that sucker?

This is a completely new topography/geography compared to what I am used to back in the Northeast, so information and ideas are welcome. Thanks!


(This is much steeper than our lot, but you get the idea of what I have in mind.)

[OlJarhead]

I think there are challenges to building like that but if there is granite close enough to the surface bolting to it might be the best option though I don't know.

I can tell you that I am still planning on building a wall foundation under my cabin which is on a post an pier.  I've done one section of wall but my body doesn't like me digging and lifting much anymore lol so the progress on the wall has slowed to a halt (I hope to change that in the next year).

If it's just a vacation cabin and not too big I think well done (and braced) piers can work though county permits might not allow.

[MountainDon]

First I'd like to say that here in most parts of the southwest slab foundations are the norm. The frost depth makes deep foundations unneeded. Make them a luxury.

For sloped terrain much depends on what the local building code rules and enforcement are. The mountainous area where our cabin is located uses the state-adopted IRC and the county inspects it. Anything that involves licensed contractors needs permits and codes. Small projects that are way off in the boonies and are DIY often slip by inspections, but can then be a problem to insure.

From what I have seen in our mountains slopes need engineered plans. And I'll add that we graded our building spot on our lot flat to build on. That was easy and took very little time. The terrain is well drained and packed pumice that was laid down 1.25 million years ago.  I did not want to ne on any tall piers. My preference.

[db4570]

Thanks for the replies. 

Good to hear from you, Mountain Don. I thought you might chime in.

Where we are building is in an area with strict county building code requirements, so we we obviously will need to comply with those. I'm planning to do it correctly. 

I'm basically just wondering what the advantages of one method over another are, or potential pitfalls of either. What are disadvantages of post/pier? The obvious ones are insulating the floor properly (which seems straightforward) and lack of enclosed storage space.

I like the idea of a nice big basement, but apparently the rock there is granite, which I am learning is about the worst to dig (or hammer, actually) through. I've watched a few videos where they show blasting with dynamite, after laboriously drilling holes, only to remove about a foot of depth after a day or two of brutal and very expensive work. It's easy to see how the cost can get astronomical, fast. 

So this is what got me thinking: why fight it? Why not embrace the bedrock as part of the natural surroundings and incorporate it into the structure. Make the house fit the land, not vice-versa.  

[MountainDon]

I believe an engineered foundation with some or all of the structure elevated would be fine. The key is being engineered for the terrain and weather. If some of the area under the elevated floor can be used for storage  so much the better. Maybe an enclosed stairway from underneath up, or if not enclosed covered against weather {rain, snow, sun}.

[Don_P]

I live in the Blue Ridge, our "basement" rock is gneiss, metamorphosed granite. The most common foundation here is probably a crawlspace with stepped footings as you climb the slope. I'll attach a graphic from Rob Thallon's "Graphic Guide to Frame Construction" that shows a basic idea of several footing types.

Codewise, a continuous perimeter foundation is "prescriptive", braced walls go over a bracing foundation walls,  no engineer required... in most places. A pier supporting a braced wall does require engineering. 

[db4570]

That Thallon book is great. Thanks for bringing it to my attention. It shows a lot of different methods and options. I am looking forward to going through it in depth.

For some reason, I have a soft spot for post construction (or is it pier?). It just seems so simple and rugged, and eliminates a lot of potential headaches. 

Is the only disadvantage to post construction, assuming it meets code, is not having the extra basement storage space?

[Don_P]

Consider all potential loads, most of the time I'm thinking "clear sight is not the same as clear vision". Just because we see something doesn't mean we understand it. If you plan to build it, it is sure better to understand it.

A foundation needs to support the structure under local expected gravity, uplift and lateral loads

Cons;
With post frame, load is concentrated on the soil using several small independent footings. Is the ground up to the load (as in calculate and confirm). will each settle equally? (the footings in the middle of a row experience ~twice the load of those on the ends... has this been considered?)

Posts supporting a structure under lateral load vs a perimeter wall under lateral load... no contest. What is keeping the posts upright, will they bend and break, will they slide or overturn. Resist all of these by quantifying load and resisting it. That is the engineering that is prescriptive with conventional construction. The engineering is baked in to the tables and conditions of the prescriptive code. You can step outside to the engineered provisions, this is one of those instances and some of the why it goes to engineering.

Some more in that line of thinking;

Perimeter beam, you need one capable of supporting loads from above. If continuous foundation support goes away you need beams between posts.. Look all the way up, did you line up loads above over support or do they land in the span of the beams between posts, calculate and size beams accordingly. Do you plan on a porch or deck or addition in the future, size the beams, posts, footings accordingly. Conventional construction has continuous perimeter support, not so much of an issue. Identify and support loads more carefully with post frame, or timberframe for that matter. Construction has evolved to more distributed uniform loadings and more redundancy. Lacking that uniform foundation wall under the floor you need to size the beams between posts to support the floor(s) walls and roof. 

If post frame (there's the term to search and educate on, NFBA has publications online)... if post frame were the magic bullet, why is it that in a competitive environment it has not dominated? It is faster and initially cheaper. It is not "better". In many senses, it struggles to be adequate, which is why it has become engineer required for habitable construction in most places. It is also superior in some cases, if done correctly.

One of the things I'm looking at on our slopes is the trees. Do you see "pistol butts", trees with a rolled base, belly downhill. The sign of sliding land and trees trying to remain upright.

[db4570]

I am open to considering a perimeter foundation. The stepped footings is an interesting idea. 

It makes sense what you say about a perimeter foundation supporting the house more evenly, and I like the redundancy. 

I see some houses in the area with a hybrid foundation. Part perimeter, and part post. 

I guess I assumed that if the design met code, and an engineer's approval, it had to be sufficient. But if it may be cheaper to do a masonry foundation and save engineering expense, and get the bonus of some basement space, why not?

In a stepped foundation, what is the basement/crawlspace floor? Just the natural surface?

[Don_P]

I guess I assumed that if the design met code, and an engineer's approval, it had to be sufficient. 

That is the intent. Pier type foundations do not meet code without an engineer's approval and most of what you see done along those lines by DIY'ers they would not be able to approve.

I see some houses in the area with a hybrid foundation. Part perimeter, and part post. 

I've built one that way and it has held up fine. It is tucked down in a very protected location. They wanted to build under radar on piers, uh yeah, that wasn't happening. This was as far as I could get them at that time. The handyman bench, bee suit, bikes, solar inverter, etc ended up in that "basement" bracing room. He is now putting in a full perimeter foundation the hard way. One engineer noted that it is frustrating to see people's light flicker on too late time and again.  Let's go back to the assumption you would like to make... Code requires a full perimeter foundation, or, another way to think about it is that bracing walls above (generally the exterior walls), need to sit over and be tied to bracing walls below. There are cantilever tables that will buy you a foot or two. Going to a hybrid is an engineered design.

In a stepped foundation, what is the basement/crawlspace floor? Just the natural surface?

If the soil is dry and not shifting I believe code is silent on basement or crawlspace floors if the foundation is masonry or concrete. Clean subsoil is common for the floor, remove the topsoil and all organic material to avoid attracting termites and ants. Sand is more comfortable if the soil is knobby. Then cover the soil with at least 6mil plastic to control moisture above. Study this chapter, if it is in red that is changes for my state only;
CHAPTER 4 FOUNDATIONS - 2021 VIRGINIA RESIDENTIAL CODE (iccsafe.org)

Also check the section around 403.1.7.1 on foundations near slopes.