How to Build Driveway Culverts
Driveway culverts send water under or through a driveway and direct any runoff into a drain or storm sewer, preventing standing water on and around a driveway. Usually, these structures are purely functional and add nothing to the property. For less than the cost of a 16-inch metal culvert pipe, you can create a culvert system that is not only functional, but also adds a very nice look to the driveway.
This is a pipeless culvert system, but drainage pipe may be used if erosion is a problem. The pipeless culvert system involves creating low trenches, or culverts, in the driveway. They are deep enough to direct water, but not so deep or wide that they are noticeable when you drive over them.
Step 1: Drive Layout
Determine the length of the driveway. Measure from the high point on one side of the dip to the high point on the other side. Cut 4 support rails from 6x4 beams. Use one of these beams to lay out the actual support rail locations: place the 2 outside rails 6 inches inside the width of the driveway, and the 2 inner rails 2 feet inside each of the outer rails.
With the trenching shovel, notch the ground at each end of the rails so that they fit 4 inches below the high points on each side, standing edgewise. Save the soil you remove.
Step 2: Cross Rails
Cut 10 foot 4x4 cross rails. You will need 2 rails for each foot of driveway length. To lay the cross rails out, place 2 scrap blocks of 4x4, one on each outside rail, flush with the end. Place a cross rail next to the block, and mark both the support rail and the cross rail so that the pieces can be lined up easily later.
Move the blocks over the cross rail, and lay another cross rail next them. Repeat this process across the drive, effectively spacing the cross rails 4 inches apart. Be sure to mark the both the rails each time so you can position them later.
Step 3: Driveway Assembly
Drill a vertical hole all the way through both each cross rail and every support rail it intersects. Place 1½-inch washers over the shafts of your bolts, and insert the bolts downward through each hole. Place another washer over the bolt and apply the nut. Repeat this step for every intersection of cross and support rails.
Step 4: Culvert Grates (Optional)
Instead of using a culvert pipe, cut 2x2 stakes of differing lengths and drive them into the ground on the outside edge of the outer support rails. Attach the top ends of the stakes flush with the top of the support rail. Separate the stakes using one stake as a spacer, in the same fashion that the cross rails were spaced. The result is a wooden grate which filters debris and allows water to flow through.
If desired, a plastic drainage pipe can be installed between the grates, and then the area around the pipe filled to level with the top of the support rails.