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Looking for answers from people building foundations near (not in) the 100 year floodplain. Does this trigger a floodplain build permitting process? Am I basically pigeonholed into a monolithic pour to avoid this? No one working on my foundation (excavators, concrete company etc) seems to know the answer or they think I should just "not worry about it".

My question is 100% a FEMA rules / what is allowed question. Not at ALL interested in whether I "should" or not. To dissuade your fears: This is a field in Montana near a creek that hasn't flooded the site in the last 100 years. I have some control over the flow in the creek. There are hundreds of houses around me, (many lower) which have never flooded (in 100 years). Even if this is all a terrible idea, I simply think it's worth the risk.

I DO care about the county or FEMA saying I did something wrong / making me change it. And, I DO care about a mortgage company forcing flood insurance (showing I am somehow in a floodplain) - pretty sure that won't happen however.

A picture says a thousand words:

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Floor joists and the entire home and all electrical, 100% of everything is above the flood plain elevation.

I must: Apply for a permit for any modification or even moving a rock inside the 100 year floodplain. Does this count since I'm digging beneath it? The county did not specify and there no building permits in this county. They were adamant on proving my site is outside the 100 year floodplain (unless flood plain permitted).

maplemale
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    Given "My question is 100% a FEMA rules / what is allowed question" then ask FEMA for clarification. – Solar Mike Jun 30 '20 at 17:34
  • @SolarMike I wouldn't ask FEMA the answer to 1 + 1 even if their rule book said " answer = 3". Like most enforcement agencies, they generally aren't familiar with their own rules until they want to enforce them after the fact through some auditing process. – maplemale Jun 30 '20 at 17:36
  • Since you have decided that you must listen to the county then perhaps you could choose the correct answer for the folks that took the time to advise you and answer your previous question. https://diy.stackexchange.com/questions/195069/flood-plain-craziness-do-i-really-have-to-listen-to-the-county-here/195484#195484 **MAYBE they will take the time and effort to advise you again**. – Alaska Man Jun 30 '20 at 17:38
  • @AlaskaMan Im not sure what you mean. I answered that question to the best of my ability, because no one really knew the answer. This is a followup question that is specific to someone building completely outside the 100 year floodplain. In my mind, this is completely different. Does it seem a bit too much the same to you? Never decided I "must listen to the county". Quite the opposite. I'd rather figure out the actual rules as the county seems to er on the side of not fully understanding them / caution. Each question should stand on its own, should it not? – maplemale Jun 30 '20 at 17:42
  • @maplemale My point is , if your answer is correct then **select** it as correct and do not just leave the question hanging in the system as a question that has not had a **correct** answer chosen. People will be more apt to try and help you if participate correctly, behave nicely and say **please a thank you**. – Alaska Man Jun 30 '20 at 17:46
  • @AlaskaMan Ah... sorry about that. I neglected to do that. Thanks for pointing it out! Still, this question stands on its own and is different than the one you are referencing. – maplemale Jun 30 '20 at 17:48
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    @maplemale "*Still, this question stands on its own and is different than the one you are referencing.*" Agreed, i was not suggesting otherwise. – Alaska Man Jun 30 '20 at 17:50
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    Are you sure this property is actually in a flood plain when measured via altitude? The USGS just arbitrarily drew lines called "flood plain" for a specific horizontal distance from a body of water that could flood. This is a very poor approximation but it was cheap and easy. For example my property was originally designated flood plain but it is, in fact, 44' above the 100 year flood line. All it takes is a survey to show that it's not actually flood plain. – jwh20 Jun 30 '20 at 18:05
  • @jwh20 Good points and good question! The location is definitely NOT in the floodplain. It's just outside and we did that on purpose with a BFE (base flood elevation) survey. I'm just afraid that the laws here are common sense. ie. If you are outside the flood plain, but you dig deep enough (below it) you are back in it? – maplemale Jun 30 '20 at 18:07
  • What is the elevation of the current ground on the site? – Jim Stewart Jun 30 '20 at 18:13
  • @JimStewart It is 3196.8ft above sea level. The BFE at the site is 3195.2 above sea level. The "floodway" 1/2 mile away is 3189 (high level of the creek). Interesting side fact: There is a 30 year old cement well pit at grade (will remove this) which has stains on the wall, 7ft deep. We are in a high water year right now and there are floods in the area as we speak. So it's interesting to watch a bad year unfold. Water level is at 7ft deep / bottom of well pit right now. – maplemale Jun 30 '20 at 18:25
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    Our crawlspace floor (soil) is below the 8' base flood plain elevation, probably at 5'. But the first floor is above that 8' BFE. So no flood insurance required. – SteveSh Jul 01 '20 at 00:17
  • @SteveSh Thanks! After all of the comments and answers stating this, I got more brave and double checked with the county. To my surprise they said: "The rules don't make any sense sometimes." And then they literally said this: "Go ahead and build a 50ft deep parking garage starting 1" above the base flood elevation, you're good to go". LOL – maplemale Jul 01 '20 at 15:35

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The flood plain issues I had to deal with is more about the openings in the foundation to allow water to exit. As others have said have a Survey, my current home was in the flood plain until the survey , huge insurance jump because the lender required flood plain insurance until the survey. I am planning on an additional 3 car garage the county is the one that issue the permits, not fema.

If the county asks show them the structure is not in the flood plain, when you get your permit build to your plot plan and you will be fine. I have a friend that built a house on poles to put the structure above the flood plain his footings for the poles were well below the 8’ or more “level identified as the 100 year level” the county only required engineering stamps on the plans, just like roof trusses require in most locations. He has never had to have flood insurance.

Don’t bring up issues that you don’t want to deal with because as you said they go to the ultra safe side because they don’t want to be wrong. After living through several 500+ year floods he started enclosing the lower section I have not talked to him but last time I was in Corvallis the entire lower 1/2 was enclosed.

Monolithic pours are not the only way to go but you are not in the flood plane so get your permit. Make sure you plan for drainage like a French drain under the low side with a heavy footing above that for insurance that it will drain this doesn’t have to be on the plans. So submit your plot plan to the county and pay the fee. If they ask , show the survey. When approved start building and enjoy your home.

Ed Beal
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  • Thanks very much for this answer! Corvallis? As in Ravalli County MT? Nice - same county i'm building in. Actually, this county requires zero build permits. It's state permits only: Electric / plumbing – maplemale Jun 30 '20 at 19:59
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    Corvallis in Oregon , wow you are lucky we have permits for everything here, well, septic, plot plan, house , electrical if the property crosses a creak that’s has fish just add another 40-80k for the required bridge, but it’s a nice area. – Ed Beal Jun 30 '20 at 20:06
  • Ed Beal what about surface swales or vertical drains to underground piping on the high side to intercept surface water moving downhill and piling up against the foundation? – Jim Stewart Jul 01 '20 at 00:40
  • The biggest problem I have seen is water that gets inside a perimeter foundation having exit points that allow the drainage. All the water weight has damaged foundation walls and when the seepage turns into a flow the dirt is pulled out under the foundation creating a failure point, I saw a bunch of foundation failures in the 70’s Northern California and a couple of damaged ones over many years in Oregon. – Ed Beal Jul 01 '20 at 13:12
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    @JimStewart I'm 90% on the flat. I used to have a house in Newport, OR not far from Ed Beal. It was easy there because it was hilly and draining strategy was obvious. Over here, the highest spot on the whole 56 acres is where i'm building and it's only a couple of feet higher than the rest. Not sure how to do drainage properly here and will have to do some reading / get some advice. Most builders here simply don't do it unless you're on a hill to keep runoff away. – maplemale Jul 01 '20 at 15:20
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    @Ed Beal - Yes. Down here on the Delmarva Peninsula and along the coast, if you're building in the flood zone (simplistically, 8' above the mean high tide mark), you're required to install flood vents so that if any flood waters get in, it can escape without exerting undue outward pressure on the walls. – SteveSh Jul 01 '20 at 23:48
  • Thanks Steve this is what I was talking about on the premier foundation drainage. In Guerneville Ca. They have really bad floods when I was first starting out my dads company fixed quite a few homes that the premier foundation wall or stem wall had washed out underneath and some that collapsed out from the pressure the repairs had to have “vents” some of these homes were +30’ above the flood plane and They did not expect this kind of flood again those got compacted drains under the foundation. The lower ones had large openings I think 4 were required but it has been 50 years.+ – Ed Beal Jul 02 '20 at 13:39