We got a question from Ryan in Crest Hill, Illinois, and apparently, he has a crawl space where the floor of the crawl space is below the grade of the outside. The outside dirt is higher, and the crawl space dirt is lower. He was wanting to know, does he need a sump pump?
Water Will Travel the Path of Least Resistance
Great question Ryan, remember water will travel the path of least resistance. That being said, in many areas with clay soil, the foundation wall may be easier to travel through than the soil itself. Especially, if you already have a water intrusion problem and you have fixed it from the outside only. The water already formed a path inside so extending downspouts or installing a french drain in the yard may not be enough to correct the issue.
Water Will Enter Low Crawl Spaces
If the crawl space floor is lower than the finished grade outside, it will move into the crawl space eventually. We recommend to our customers if your crawl space soil is lower than the outside grade, install a sump pump and interior french drain. According to HUD.gov:
” B. Interior vs. Exterior Ground Level.
The interior ground level must be above the
outside finish grade unless:1. Adequate gravity drainage to a positive out fall is provided, or
2. The permeability of the soil and the
location of the water table is such
that water will not collect in the
crawl space, or 3. Drain tile and automatic sump
pump system are provided.“
Hydrostatic Pressure Applies Great Force
If you have water outside the crawl space, hydrostatic pressure will also be forcing the water into the crawl space. The deeper the wall of the crawl space and the more dirt on the outside of the crawl space, the greater the hydrostatic pressure. The greater the hydrostatic pressure, the more force is applied and water is pushed its way into the crawl space.
Now if you had the opposite, the crawl space dirt is higher, then you wouldn’t have to worry about it. In this case, water that builds up inside the crawl space would naturally want to flow to the lower outside area. Unfortunately, that is not how most homes with crawl spaces are built.
Install a Sump Pump and French Drain
Yes, it is my recommendation, if your crawl space floor is lower than outside grade with or without standing water in your crawl space, whether it’s building code or not, you should put in a sump pump with a pit and discharge the water outside the crawl space. We would also recommend a full perimeter trench or french drain if you see that you’re taking on water in several different areas because it makes it easier for that water to get to the sump pump.
Keep in mind that a sump pump will move about 4800 gallons per hour, but it is only as effective if the water is making its way to the pump. The clearer the path that you give the water to get from the other side of the crawl space to the pump, the faster it’s going to move that water out of the crawl space.
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What Next?
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Do I need a drainage mat underneath my crawl space vapor barrier?
Is it absolutely necessary? No. Is it the best way to complete your vapor barrier as a finished product? YES!
Here in Wilmington, many of our customers love to store items in their crawl space. A drainage mat will provide invaluable strength and puncture resistance to the vapor barrier. With a drainage mat in place (and some inexpensive gym mat on top), our customers find they can store a ton of items in there!
More importantly in our coastal area, we find that drainage mats are an excellent addition to a water management system, allowing the system to function more efficiently. End-state: a drainage mat (underlayment) is the way to go to complete an encapsulation system! Call us today!
-Andrew Davis, Owner, Crawl Space Ninja Wilmington, NC
4 thoughts on “Crawl Space Floor is Lower than Outside Ground Level”
Does an interior perimeter French drain inside the crawl space need to be dug deeper than the side wall of the crawl space?
It should be level with the interior dirt of the crawl space. https://youtu.be/J3hWdj4iD3I?feature=shared
Sorry, my question was not properly worded. Please allow me to explain.
We have multiple low spots in our crawl space with gravel over vapor barrier. Every time we get a heavy rain we have multiple puddles in these low spaces. After looking through your videos, I believe it’s hydrostatic pressure causing the crawl space flooding in our home where the crawl space floor is 3-4’ below grade. Despite drainage boxes and grading the exterior, we still get crawlspace flooding.
The contractor we reached out to wants to do a full perimeter French drain plus encapsulation. I don’t see how this would help unless the trench is dug quite deep, so that the bottom of the trench is below the level of the bottom of the concrete crawl space wall, to intercept this hydrostatic force coming from the exterior.
Hi Jim, if you have low sections of crawl space pooling water, an exterior drain will do nothing because it will hit the low sections before reaching the drain. Would it be possible to build up the low sections to match the rest of the crawl space floor and connect the low sections that you raised with the exterior drainage system to ensure the water is vacated? Hope that helps.