Yard Drainage Grading Calculator: Fall Away From the Wall
Work out the fall you need away from the foundation, the soil to add, and how much roof water each downspout delivers.
Most wet basements start outside. Ground that slopes toward the house, or is simply flat, delivers roof and surface water straight to the foundation. This calculator compares the fall you have with the fall you want, estimates the soil needed, and shows how many gallons each downspout carries per inch of rain.
When this tool helps most
Use it first, before any interior work. Regrading and extending downspouts is the cheapest change with the largest effect on most wet basements.
How to use this tool
- Measure how far out from the wall you can reasonably regrade.
- Enter the fall you want over that distance, using guidance for your area.
- Measure the current drop over the same distance, using a negative number if it slopes back toward the house.
- Enter the wall length and the number of downspouts on it.
- Enter the roof area draining to each downspout.
Worked example
Ten feet of regrading with a 6 inch target and only 1 inch of current fall needs 5 more inches of fall, which works out to a 5 percent slope and roughly 2.3 cubic yards of soil across a 30 foot wall. Each downspout serving 500 square feet of roof delivers about 312 gallons per inch of rain, so two downspouts put 623 gallons against that wall in a single inch of rainfall.
Understanding your results
The gallons figure is usually the one that changes minds. Hundreds of gallons arriving beside a foundation wall during one ordinary storm explains a great deal about a wet basement, and extending a downspout is inexpensive.
Common mistakes to avoid
- Piling soil up against the siding, which causes rot and pest problems instead.
- Sloping only the topsoil while the compacted layer underneath still runs toward the house.
- Extending a downspout two feet and considering the job done.
- Treating the rule of thumb as a code requirement without checking locally.
How to get the most accurate result
- Measure your own basement or crawl space with a tape measure instead of using typical sizes.
- Use the numbers printed on your product labels, manuals and data sheets.
- Change one input at a time to see which assumption matters most.
- Print or save the result and bring it to your installer or contractor.
Next steps
If surface work is not enough, the French Drain Material Calculator sizes a subsurface drain.
Frequently asked questions
A widely used rule of thumb is about 6 inches of fall over the first 10 feet. It is guidance rather than a universal code requirement, so check what applies where you live. Far enough that the water does not simply soak back to the foundation, and downhill where the grade allows. The volume figure in this tool shows why a short extension is rarely enough. No. Keep soil well below siding, weep holes and any sill plate. Build the slope outward rather than upward.How much slope do I need away from the house?
Where should a downspout discharge?
Can I just add soil against the wall?
This tool is for planning and education only. It does not store your entries. Structural judgement belongs to a licensed structural engineer, radon mitigation design to a certified mitigator, and mold over roughly 10 square feet to a remediation professional. New circuits are a job for a licensed electrician. Read our disclaimer.