Slab foundation vs Friction Piers
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Hey!
We’re moving forward on our high desert house (elevation 4500). We did a bunch of work on the pad a couple of years ago in anticipation of putting the house in…having flattened a pad by cutting into a slight incline. We’ve excavated and repacked (there’s a more tech term, I know) the sandy soil (mostly Decomposed Granite) and it’s compacted over the past couple of years.
Our Geotech engineer just dug some test holes with an auger and the soil is very solid and we can go down between 4 and 6 feet before we hit rock. He’s quite happy recommending we put the house on piers…which feels like it might be more economical given it should take less concrete. My only issues with a lot of concrete is that it’s not all that great for the environment and we’re at the end of about 4 miles of one lane dirt road.
I was planning on an insulated slab…but would be totally fine with piers. Is there any preference in the community given green building/air tightness/practicality etc. No one in the field seems to express a preference.
What do you guys think…why a foundation vs. piers? Or why piers vs. a foundation? It’s a 25’x65′ footprint with a second story bedroom.
Thanks,
Mark
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Replies
The thing that tips my preference to a slab is that the concrete can be the finished floor. I've found in the resale market, however, it's a bit of a liability.
Slabs are nice with a little bit of passive solar, if you have that available.
To me, durability is the best feature. Here's a photo of my slab at ten years, and it still looks perfect at 16 years: https://greenbuildingindenver.blogspot.com/2014/10/slabs-vs-crawlspaces.html
Dang, that beautiful slab is a real selling point. but I'll consolidate my responses because all these notes are super freaking helpful... more below! Thanks Kevin.
Mark, here's my typical decision tree:
Is lowest cost a priority and client is ok with or prefers an exposed concrete slab as the finished floor? It's hard to beat a slab on grade, especially on a flat site where you can use a frost protected shallow foundation. Specify reduced-Portland concrete to reduce climate impact and improve performance. On a sloped site, use a raised slab--poured inside frost walls.
Is it important for the main floor to be close to grade? Piers raise the floor significantly so go with a slab (or crawlspace or full basement).
Is a low main level and low cost important, but the finished floor has to be wood or something else that requires extra effort? Use a concrete-free slab with a perimeter grade beam or frost walls. A bonus is lower carbon emissions.
Is low embodied carbon important, a concrete floor is not desired and a raised floor level is acceptable or desirable? Then build on piers.
There are more options and fine points, of course, but for me those are the big ones.
Michael,
I'm having trouble getting an opinion from anyone. I had designed the house on a slab. But the darn Geotech engineer was so excited about the piers...you see them a lot in the desert...but mostly 'cause folks are navigating a rocky pad...Kurt Chirstie and Dave McAdam have done some incredible work with Blue Sky Systems...with a pier and metal floor system that takes care of all the seismic concerns...but it radical overkill where we are.
I want to make sure the house and the deck settle and shift at relatively the same rate. So I guess I was thinking piers would be easier...and but I'm not sure what happens when I price the concrete out against the beams...But as far as the green bit goes...the high fly ash may be enough to keep me sleeping...I guess the real question is can you steer me toward a post that talks about continuously insulating a raised floor?
this is super helpful...thanks!
The concrete-free slab...that's packed/leveled rock and sand between the footing walls, correct? That might be a solid choice...with a beam around the perimeter and one running along the one bearing wall midships, that might be a really good choice...
Sabotcat, here is an article I wrote about a concrete-free slab that I did: https://www.greenbuildingadvisor.com/article/minimizing-concrete-in-a-slab-on-grade-home. There are links to related articles as well.
Here's one I wrote that includes a basic detail for building on piers: https://www.finehomebuilding.com/2020/04/07/the-evolution-of-a-pretty-good-house. Certainly far from comprehensive. And here's one that shows a project I'm working on now: https://www.greenbuildingadvisor.com/article/all-about-helical-piles.
Holy katz!
thanks for these...I really appreciate the follow up. As an old man, I'm going to print these out before I read them!
I really appreciate the help.
Sabotcat, I'm not sure if I addressed your airtightness question in the articles I shared. I put plywood or OSB under the floor framing, taping the seams and connections to beams to create a continuous air control layer.
Did the engineer specifically state that the disturbed soil is compacted well enough to support a slab without going down to rock?
Jon,
that was his sense. He thought 4 foot piers could hold us without going down the bedrock...which is only six feet down. Now it's got me wondering if helical posts would hold in the sandy soil, I can't believe they would. but who knows?
I just have to worry about insulating the floor...they basement was going to have insulation below it and around it.
If it's just the difference between 4' and 6', I'd probably go with the "surer thing" of 6'. Perhaps allowing fewer posts and so similar in cost.
When we get to four feet...we hit bedrock in about half the locations. Other places we get down to six feet before we hit rock. I think it's just a matter of deciding if we're going to sink into the soil with a larger diameter...or drill down to bedrock...
or, of course, pour some beams. or a conventional slab.
Half your foundation sitting on bedrock and half supported by disturbed soil? Hard to think of a better way to get differential movement.
Jon,
I'm replying up here because there's no reply on the last comment. Sorry if I wasn't clear. Nobody wants to split the piers between compression and friction. It's either one of the other...and the soils engineer seems to think we can just go friction piers all the way around.
But I'm with you... this thread began because no body I could find out here talks at all about airtightness strategies on piers...the viability of the piers will be completed by a guy with a calculator. As far as caveats on cost, energy efficiency and continuous insulation...I'll keep poking around for answers.
onward!
Mark
I think Michael's points are good. What height do you want? Do you value a concrete floor? I'd give limited weight to the environmental impact of more concrete - a $500 donation to the right environmental cause is likely to be more more beneficial.
While building on the ground automatically provides some insulation and air sealing, you can always make an elevated, vented underneath design better insulated and tighter.
jon,
true all around. I've printed out Michael's articles...and am going to let the structural engineer inform this more than my own sense of what's going to save the world. Also, connecting to the deck will add a piece that may push us one way or the other. Thanks for sticking with me on this.
mark
Hi,
From your other posts - I think you are in the CA desert. Seismic issues may be harder with a pier foundation. A perimeter stem wall with a raised wood floor would be easy & normal. Ask your engineer if piers would work.