How to Build a French Drain
Why You Should Build a French Drain
The simplest of French drains were historically used to drain wet field soils and areas where water pooled, but also to provide drainage and improve soil stability around buildings built upon posts and pilings. In a modern application of an old concept French drains are reasonably easy to construct, and can be constructed with no visible swale or ditch. French drains are less expensive to install where traditional weeping tile and other solutions may be difficult or unattractive.
French drain simplicity
To construct a French drain is not difficult. In it’s simplest form it merely is a steep-sided trench appropriately dug and filled with coarse gravel which will allow water to drain away from the chosen site. Historical agricultural French drains were the simplest of trenches filled with brush, brick rubble, stones or gravel, virtually any materials that would allow water to drain, enter the ditch, and flow away. Another common typical use of French drains includes a similar construction designed to collect and hold water until it soaks into the soil. That use is also known as a “dry well”, but most French drains are for the purpose of area drainage.
Modern Adaptations of French Drains
Modern construction methods suggest the use of little but coarse gravel to within a few inches of the finished surface topped with coarse sand. Grass can be grown on the surface, or sod can be installed. No visible trench or ditch is necessary.
In another modern design, a perforated pipe such as the inexpensive and commonly available flexible plastic weeping tile may be installed close to the bottom of the trench bedded on a layer of gravel. The use of pipe is unnecessary with the exception of locations where the slope to the discharge point might be steep. Protection from scouring and erosion of the trench walls can be achieved by enclosing water flow. A word of explanation is warranted, a steep grade in a French drain is highly undesirable. Water flowing through the gravel may scour the sidewalls of the drain, washing soil from walls. Where a steep slope is involved, it is essential to install a fabric filter on the sidewalls of the structure, and consider installation of a drainage tile in the bottom of the trench to prevent this potential damage.
The slope of the bottom of the trench is critical, requiring a suitable discharge point to a lower elevation for disposal of the water. Adequate drainage can be established with as little as 0.5% drop, which is only 6″ over a distance of 100 feet, and a 2% slope , 2 ft. of drop over 100 ft of distance) is considered to be ideal.
Here is how to build a French drain for your lawn, a field, or around a building:
1. Stake the direction and layout of the French drain, carefully considering the elevations required and the water disposal point.
2. Dig the trench, carefully setting the grade of the bottom of the trench, which will determine not only the direction of the flow of the water, but the rate of flow.
3. Trenching for a French drain can be 12 to 18″ inches deep but only 6 to 8 inches wide. It can be much larger and deeper depending upon the soil conditions and requirements of the project, but for surface drainage in normal circumstances a trench 12-18″ deep and 6″ wide will suffice. Trenching can be done manually or by using a mechanical continuous chain excavator or other suitable equipment if available.
If obstacles such as large boulders are encountered, simply reroute the drain around the obstacle, carefully maintaining the grade of the bottom of the trench.
4. Verify and correct the bottom grade of the trench very carefully to ensure a minimum of 1% slope if possible, which equals one foot of elevation drop for each 100 feet of length.
If you are setting the elevation using a 4 ft. carpenter’s level, use 1/2″ for 4 feet, which approximates a 1% slope.
5. If a pipe is to be installed, add about 2″ of gravel to the bottom of the trench, carefully following the required grade. Lay 4″ perforated weeping tile with the holes facing down to prevent the tile from filling with dirt. Water will enter the pipe by flowing up through the holes in the bottom of the pipe. Note that if the drainage pipe has extra holes in the top and sides, install a filter fabric over the pipe prior to covering the pipe with gravel.
6. Fill the trench to within 3″ of the top with coarse gravel, preferably uniformly sized 1/2 to 1″ cobbles or similar crushed rock which resists compaction. Do not use gravel that has fines, soil, sand or dirt in it, which will eventually compact, preventing the free flow of water. For better performance, consider lining the trench completely with filter fabric prior to filling.
7. Fill the trench to within 2 or 3″ of the surface with the chosen free-draining cobbles, gravel or crushed rock.
8. A soil filter fabric on top of the gravel is optional at this stage, but is recommended. Installation of a soil filter fabric or mat prevents infiltration of fines and will result in a longer drain life.
9. Fill the top of the trench with free-draining coarse sand. Avoid using fine sand or coarse sand containing soil.
10. Correct the grade as necessary, and plant grass seed or establish sod over the trench.
Now that you have successfully built a French drain, perhaps you can build a small terrace or fill some holes with that pile of dirt that is left over. Get creative!