Plumbing under frost protected shallow foundations
Frost-protected shallow foundations (FPSF) are designed to avoid deep footings below the frost line by utilizing insulation to raise the frost depth around the building. This allows for shallower foundations, potentially saving on excavation and concrete costs. The attached image illustrates the frost line being raised due to this construction method and a similar illustration is provided in ASCE (American Society of Civil Engineers) 32, “Design and Construction of Frost-Protected Shallow Foundations”.
Section P2603.5, Freezing, of the 2018 IRC requires the water, soil and waste pipe to be protected from freezing. The frost depth in my area is 42 inches but the utilization of a frost-protected shallow foundation raises this depth to the bottom of this foundation’s insulation. Therefore, can one install plumbing under a frost-protected shallow foundation less than the typical frost depth (e.g., 42 inches) and without insulation due to this type of foundation? I do know the water service is required to be 6 inches below the frost depth… so it would need to be 6″ below the elevation of the insulation.
Note. I am an inspector so I can only require code compliance. I may recommend but cannot mandate something not written in a IRC. I try very hard to keep personal opinions to myself.
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This may be obvious but the ground only freezes because of cold air. It has to fight against heat from the house and heat from the ground. In most cases frost will move away from the house as it goes deeper into the ground, so I see no reason to worry about placing plumbing anywhere you want inside the building perimeter, at least in climate zones 6 and lower.
Unfortunately I don't know of a code section in the IRC or UPC that directly addresses this.
"so it would need to be 6″ below the elevation of the insulation"
I'm not sure how exactly you arrived at this interpretation. It is literally impossible to comply with this. How would the drain get from the house to 6" below the elevation of the foundation?
Trevor,
I think it is based on a misunderstanding. On unprotected foundations the frost depth descends from grade, so it makes sense to run the services 6" below the prescribed depth. With FPSFs the entire area underneath is above freezing, so going deeper serves no purpose.
Tony,
The only complication I can think of is where the services exit the protection of the FPSF. There they would need to either descend to the frost depth, or be protected by insulation until they did.
A bit off topic, but the main drain going from under my house to the septic tank is probably only a foot below grade (frost depth is at least 3 feet, iirc). I guess that's a violation. Would putting 3" of insulation above the line rectify that?
Waste drains don't have to be below the frost line, at least according to the IRC and UPC, only water lines. I still spec them to be below the frost line.
The drain in my last house was completely exposed for 30 feet under the house. . Never froze[4 ft frost depth]
I would think the only issues would be where the insulation ends the freeze sensitive utilities need to be below frost depth, which means I would think that means they will be 4 foot down no matter what
I renovated a house on piers here in Maine several years ago, which had about 20' of exposed drain pipe. It froze several times while we were there, over a winter while my clients stayed elsewhere. It would probably fair better with heavier use because the warm drainwater would help. It led to a macerating pump buried outdoors so it didn't get heat from the septic tank.
A friend of mine here in Maine has about 40' of exposed pipe leading to a septic tank. It freezes solid every winter, probably due to the low pitch and that she lives alone so it doesn't get a lot of use. She's off-grid so she can't use heat tape except when she runs a generator to thaw the pipe. (She built directly on bedrock and can't blast so she's stuck with it.)
20 years ago I lived on Nantucket over one of the coldest winters on record. (The harbor froze, the Coast Guard icebreaker got stuck in the ice so they had to send a second icebreaker to get the first one out.) Winters there are usually mild and in-town housing is dense and hilly, so there are a lot of indoor macerating pumps. Many, many waterlines and wastewater lines froze solid, leading to giant messes. And there weren't enough plumbers and contractors to get to all of the projects in a short timeframe.
Those are some of the reasons I strongly recommend that if you're in a cold climate, you run your drain lines below the frost line, or as close as you can get.
On the other hand, my drain line is only 18" below grade, due to bedrock, but both my wife and I work from home and cook a lot so it gets pretty heavy use and hasn't frozen in the ten years we've been here.
THe water line in that house was zero inches below ground. HEat cable and insulation above, but not below since it sat on the ledge. About 4 inches of foam around it where it entered the house.
There was a good pitch to the drain line , very steep for one section that no doubt helped
It did lead to a septic tank[and pump and recirculating sand filter] so no doubt heat there.
In retrospect having brand new fixtures with zero leakage also helped.
One wonders if an old drafty barn of a place where one leaves faucets dripping on the coldest days may contribute to freezing a drain solid
Thank you for the great comments. I agree that the 2018 IRC is silent on this issue of sewer and water lines under a frost protected shallow foundation.
To respond to the comments, Section P2603.5 requires the water service to be not less than 12 inches deep and not less than 6 inches below frost line. That specific dimension was not for "waste" or other drains.
Building sewers are also applicable to this section that it is required to be protected from freezing. I agree that building sewers are less prone to freezing than water service pipes because they carry warm wastewater and have intermittent flow, which helps raise the surrounding soil temperature. Yet, building sewers connected to public sewer mains in my jurisdiction are required to be below frost depth per our sewer district standards. We have pressure sewer systems and we have had building sewers freeze even below 48 inches deep. The only exception in the 2018 IRC is building sewers connected to private sewage disposal systems. The depth of such sewers is determined by the local jurisdiction.
You can also insulate sewer and water lines to protect it from freezing, especially in restrictive soils. We have shale banks in my area so that tends to be utilized . Section P2603.5 of the 2018 IRC does allow protection from freezing by insulation or heat or both.
Again, thank you for all the great comments. I was trying to ascertain if someone knew of a code section I am unaware of that discussed plumbing pipes under a frost protected shallow foundation as well as the frost line raises at this type of foundation.
Be well and safe.