Footers for Frost-Protected Shallow Foundation
Anybody have a link for clear up-to-date guidance on FPSF construction with a minimum of overbuilding, for a small and simple addition?
Hi, I’m a homeowner who has done some work on houses in the past, but am not a pro, and am looking to put a small extension on behind my 1925 home in St Paul, MN. I’m only planning to make it approximately 12′ wide by 16′ long, single story, and I’d like to set 16″ deep rows of CMUs on top of, and inside of, a couple layers of XPS myself, with 4′ wings to the exterior, put top plates on ’em, and run the joists across. This all seems a very simple way to be able to run plumbing and electric from the ba
I’d like to know how simply I can do this without calling in much help.
1) A redundancy question: Must there absolutely be a footer running the length of this addition below the foam and CMUs? Doesn’t compacted gravel, the psi capacity of XPS, and perimeter rows of concrete mean there’s sufficient footer for a single story to bear upon without another layer of one buried below the XPS?
If there must be an additional lower footer, where do I find requirements for its size and rebar schedule?
2) How deep below-to-above grade do these different layers need to begin and terminate at? As long as there’s sufficient clearance above grade to my threshold, and as long as the wing insulation is 12″+ below grade when I’m done, how shallow can I make this foundation? Can’t it, in fact, begin nearly at grade?
Thanks for your expert guidance.
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You're asking a lot of different design questions that really can't be adequately addressed on a forum like this. There's a reason that architects charge what they charge - there's a lot of knowledge that goes into drawing those pretty pictures. Since you don't have the knowledge of building codes or construction standards, your options are to do a LOT of reading and learning or to hire the knowledge. Depending on your local code administration, some jurisdictions allow simple additions like this to be designed by a homeowner, builder or architect. Regardless of who designs it, it still must meet local codes.
For FPSF design guidelines, you can start by Googling "Revised Builder's Guide to Frost Protected Shallow Foundations." You should be able to find a free .pdf download from HUD.
Hi, Peter, yes, I am indeed on the Doing A Lot of Reading program, thanks. And yes, as an owner-builder, I'm here asking specific questions in service of meeting code requirements, without overbuilding. I do, also, have some knowledge of building code and construction standards, which is why (a) I'm asking a fairly specific set of questions; and (b) I'm asking for answers that adhere to the most recent iteration of the building code, as many FPSF publications that show up in search results are fairly dated. I do see you're an engineer--would you care to answer in a more helpful and less-condescending way? If not, then thanks for your time, sort of.
Complication: I was trying to be helpful. I I cam across as snarky, I apologize. The Builder's Guide I posted is a reasonably current document that should be in conformance with current codes. Your local jurisdiction will still have to approve any use of its guidelines. This guide has done the engineering work for a certain subset of possible FPSF solutions. If you stay within the borders of the Guide, you should be pretty well off. If you want to step outside the borders of the Guide, you will need engineering advice. Not just simple advice from a free online forum, but actual engineering advice from someone familiar with local building requirements, soils and temperature profiles. Yes, it is possible to place foundations on top of EPS insulation. The foundation must be strong enough in all directions to stay together and spread the loads onto the EPS and soil. This includes wind, seismic and other loading. It would be difficult to do this with only CMU. A grade beam would probably be a better solution. The numbers attached to all of this require analysis.
Here's a start:
https://codes.iccsafe.org/content/MRC2015/chapter-3-building-planning
https://codes.iccsafe.org/content/MRC2015/chapter-4-foundations
Chris
I have worked with the St. Paul building department and they are very thorough and accommodating. My project was in Highland Park and the building inspector was Mike P. If he is still there you would do well to run your questions by him. He is one of the best inspectors I have worked with.
Thanks, all, this is helpful. I also took another look at the article in FHB Feb/Mar 21, and sure enough, there's no sub-insulation addl footer, unlike other diagrams I've been eyeing (where escapes me).
The most recent revised guide I'm finding online is dated Sept 2004, 17 years ago.
Thanks also, Doug, for the St Paul building inspector details--good news.
I'd love, frankly, with enough compacted gravel under foam, to have to deal with barely any concrete at all, and just set my floor joists on top of wood beams, but I suppose that dream is just too wild.
Be sure to size footings for the lesser of the soil and the properly derated PSI value of the foam. Some say around 1/3 the rated foam PSI value, so one might need 40 psi foam for a ~2000 PSF soil footing design. Is there a lower cost valid answer to this other than "hire an engineer"?
I'm attaching a blurry crop of the FHB article detail (by Josh Edmonds). It's interesting. We've got:
• A whole lot of compacted gravel;
• vertical, subslab, and wing EPS of sufficient density and thickness for conditions;
• Thickened-edge concrete slab.
Can't this be done concreteless, with floor joists strong-tied on sleeper beams, and glued-and-screwed subfloor over the top? Does perhaps the weight of the concrete (against uplift) and compressive strength make it nonnegotiable?
I have been having similar questions about shallow frost protected foundations and the need for any concrete at all. Did you find a solution? I've been looking for an answer about another idea I have that is foamed glass aggregate compacted as a footer at or near grade and building with lumber directly on top of that. It would be insulated, have drainage, and supposedly has the psi strength to support a footer according to some websites selling the material. This makes me thing it could act as a footer and have a beam laid directly on top of it.
ddreed,
Practically I suspect you are right. As it is well outside the prescriptive solutions in any building code, the complication would be getting an engineer to sign off on it. As it has both structural and water intrusion implications, that might be a challenge.