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Building Pad Elevation
There’s a house being built along the south half of my block. I walk by the lot every day when I walk my dog. The form boards for the building pad are in place. The building code for my area requires that the finished floor elevation must be a minimum of eighteen inches above the crown of the road. I remember when fill was being brought in. It looked like three different types of soil due to the different colors. A good portion of the lot was very wet after the hurricanes last year. I took a picture of the lot survey that’s in the permit box, but I can’t find the picture right now. The house is being built along the very south side of the lot because a good part of the lot is labeled as marshland, I think.
I never thought of it before, but doesn’t a structure sink into a building pad? The finished floor elevation might eventually be several inches less than what’s required. If I was putting a house on that lot, I’d have it built a foot higher than what is required. I hope the homeowners know the history of water intrusion into the area. I say probably not, and they’ll have to add drainage structures to their property in the future.
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15 Replies
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If a structure sinks several inches into a building pad it means the pad was either constructed of unstable soils or not properly compacted.
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No, buildings should not sink into the ground. That’s a bad thing. It can tear a structure apart, I’ve been in some where that’s happened and it’s not pretty.
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Probably cut it down to bedrock and hauled in an “engineered fill” and compacted it good. Probably was a geotech on site during compaction to check the soil type, the moisture, and take samples for lab analysis and certification by a PE. Done right, settling should be minimal.
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The building code for my area requires that the finished floor elevation must be a minimum of eighteen inches above the crown of the road.
Is there a logical reason for this? I can’t immediately think of one.
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Flooding.
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There are aspects that are much more important regarding flooding than the road. It would be possible to have a flood proof house lower than this, and a house taking on water above this height, depending on the rest of the terrain.
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I couldn’t find any specific building code for Orange County, but I did find something for Palm Beach County.
Boynton Beach, Florida
Sec. 3. Minimum Finished Floor Elevation (Non-Flood Hazard Area).
The finished floor elevation of all habitable space of new construction not located in a flood hazard area shall be elevated a minimum of twelve (12) inches above the average crown height of the adjacent roadway. The finished floor elevation of all habitable space of building additions and/or substantial improvements not located in a flood hazard area shall be no lower than the existing finished floor elevation.
(Ord. 13-013, passed 6-4-13; Am. Ord. 13-020, passed 7-2-13; Am. Ord. 17-029, passed 9-19-17; Am. Ord. 21-026, passed 11-16-21)Sec. 4. Minimum Lowest Floor in Areas of Indeterminate Drainage (Zone AO).
In areas of indeterminate drainage subject to sheet flow (Zone AO), lowest floors shall be at or above the elevation specified in the Florida Building Code or at least 2 feet above the average crown of road, whichever is higher.
(Ord. 10-025, passed 12-7-10; Am. Ord. 13-013, passed 6-4-13; Am. Ord. 13-020, passed 7-2-13; Am. Ord. 17-029, passed 9-19-17; Am. Ord. 21-026, passed 11-16-21)As you can see, the above ordinances passed 11-16-21. My minimum of eighteen inches above the crown of the road could be outdated for my county. The logical reason for these ordinances must be to prevent flooding.
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Yeah but I think the point is parking lots and paved roads are considered impermeable surfaces and tend to collect water during heavy rain, so the cheapest and most efficient way to protect adjacent houses is to simply raise them above the road.
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“No, buildings should not sink into the ground.”
I used to think that too……
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I guess in Florida terrain is a hypothetical concept.
A lot like Florida, for the lack of relief, here in Houston they create terrain through typical lot drainage
design. (see exhibit 2)
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Either muck or sugarsand on top of a cavernous limestone aquifer.
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Probably cut it down to bedrock and hauled in an “engineered fill”
I got a chuckle out of that, even though I know it’s feasible in many areas. But where I live the bedrock is beneath as much as 3,000 feet of alluvial soil, so for all practical purposes it doesn’t exist.
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The 18″ rule is standard code. I’m guessing that’s the rule across the country.
Think of it this way, an engineer once told me the three first rules of engineering:
1. The most important rule, “Where does the water go”
2. The second most important rule, “Where does the water go”
3. Third most important rule, “Where does the water go”
Everything flows from those three rules (see what I did there?)
Roads shouldn’t end up being dikes or dams.
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@jim-frame that’s the way it is here in a lot of places. There is no actual “bedrock” at least not what is usually considered bedrock.
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