Foundations & Drainage: Calculators, Diagrams & Guides
2 calculators · 6 diagrams · 2 guides
Water is patient. It finds the low spot, saturates the soil behind the wall, and pushes — and most failed retaining walls and wet basements trace back to drainage that was skipped or undersized. The calculators in this hub quantify the fix: French drain trench volume, perforated pipe length, washed gravel tonnage, filter-fabric square footage, and the block counts, base depth, and geogrid layers a segmental retaining wall needs to stand up straight for decades.
The diagrams make the buried parts visible: a proper French drain cross-section with fabric wrapping the stone (not the pipe alone), minimum slope to daylight, retaining-wall batter and drainage stone zones, and where the geogrid actually goes in the backfill. These are the details inspectors look for and water exploits when they are missing — and the ones worth studying before the trench is open.
The math follows the codes and manufacturer specs that govern this work — IRC R405.1 for foundation drainage, the NCMA design tables for segmental walls, and pipe-flow basics for sizing runs. Everything is free with no signup, and each calculator links back to its full step-by-step guide when you want the reasoning and the build sequence behind the numbers, not just the takeoff quantities for the truck.
Foundations & Drainage calculators
- French Drain CalculatorFree French drain calculator estimates gravel (yd³ & tons), pipe length, and filter fabric. Get the right trench size, slope, and #57 stone amount.
- Retaining Wall CalculatorEstimate blocks, caps, base & drainage stone, geogrid, rebar, timber, or gabion baskets for any retaining wall. Per NCMA & IRC. Free, no signup.
Guides & references
Foundations & Drainage · 6 diagrams
- Foundations & Drainage
Retaining wall cross-section — base, buried course, batter, drainage gravel, and pipe
Most of a retaining wall is hidden: a compacted stone base with the first course buried, blocks that step back (batter), a drainage-gravel chimney with filter fabric, and a perforated pipe at the base that daylights the water out.
- Foundations & Drainage
Why retaining walls fail — hydrostatic pressure without drainage
Trapped water is the #1 reason retaining walls fail: it builds hydrostatic pressure that tips the wall. Back the wall with gravel, filter fabric, and a perforated pipe so the water drains out and never loads the wall.
- Foundations & Drainage
Retaining wall geogrid reinforcement and when an engineer is required
Above ~3–4 ft a wall needs geogrid: layers between courses reach back into the backfill (≥ 0.6 × H) to tie the soil into one reinforced mass. Get a permit + engineer when the wall is over 48″, carries a surcharge, has a slope above, or is tiered (IRC R404.4).
- Foundations & Drainage
French drain trench cross-section — bedding stone, perforated pipe holes-down, stone cover, and filter-fabric wrap
Inside a French-drain trench (IRC R405.1): ≥2″ of washed #57 stone bedding, a perforated pipe laid holes-DOWN, ≥6″ of stone cover, the stone column wrapped in filter fabric, and the trench run ~12″ beyond the footing. Holes face down so water enters as soon as the water table rises.
- Foundations & Drainage
French drain slope — 1% (1/8 inch per foot) minimum fall to daylight
A French drain runs on gravity: pitch the pipe at least 1% — that's ⅛″ of drop per foot, or 1 ft per 100 ft — continuously to a lower outlet (daylight, dry well, or pop-up). Too flat and it stagnates; over ~5% and fast flow scours the gravel.
- Foundations & Drainage
French drain filter fabric — burrito wrap vs sock vs none
Filter fabric keeps soil fines out of the open-graded #57 stone. A burrito wrap protects the whole stone column (the pro default); a sock wraps only the pipe and clogs faster; no fabric silts up. Order ~20% extra for laps and the top fold (overlap factor 1.20).
Frequently Asked Questions
How much gravel do I need for a French drain?
Figure the trench volume (length × width × depth), subtract the pipe, and convert to tons at roughly 1.4 tons per cubic yard for washed drainage stone. A typical 18-inch-deep, 12-inch-wide trench uses about half a ton per 10 feet. The French drain calculator handles the conversion and adds the fabric takeoff.
What slope does a French drain need?
The working minimum is 1 percent — one inch of fall per 8 to 10 feet of run — so water moves rather than sits in the pipe. Steeper is fine; flatter risks silting. Always verify you have somewhere legal and downhill for the water to go before you dig, because the outlet elevation controls the whole design.
When does a retaining wall need geogrid reinforcement?
Most segmental block systems are gravity-stable to about 3 or 4 feet depending on the block and soil. Above that — or below it with a slope or driveway surcharge behind the wall — the manufacturer tables call for geogrid layers extending back into the soil. Many jurisdictions also require an engineer’s stamp above 4 feet, so check before you build tall.
Does the fabric go around the pipe or around the gravel?
Around the gravel. The filter fabric’s job is to keep soil fines out of the stone bed while water passes through; wrapping only the pipe (or using cheap pre-socked pipe alone in silty soil) lets the stone clog from the outside in. The trench cross-section diagram in this hub shows the correct burrito-style wrap.