Most homeowners that purchase trench drain are looking to resolve an existing drainage problem. From my observation, driveway drainage problems are the most common, so let’s talk about how to install a trench drain in an existing driveway. The problem can vary from being merely a low spot in the drive that causes the pooling of water to the unfortunate deluge of water that enters the garage each time there is a heavy rain.
In this situation, the homeowner was getting water into his garage with every hard rain. Further investigation suggested that water was running through a seam at the asphalt-garage floor interface and filtering to the foundation which caused dampness at the base of his basement walls and an over active sump pump. In addition to having a driveway that gently sloped toward the house, his house was at the low end of the street, which made his site more prone to collecting street run-off. To make matters worse, he had downspouts from the roof on either side of the garage door that supplied more water to the affected area.
To remedy his problem, the customer decided to install a trench drain abutting his garage floor to accept the water from his downspouts. The water was to be discharged toward his back yard, which sloped to a creek. The home owner selected a pre-sloped 6″ wide polymer concrete trench drain (Polycast 600) with cast iron grates and steel channel protectors. The drain channels of this system were pre-sloped (not neutral) so water would flow the moment it entered the drain.
Step One – Excavation
Prior to any work, the contractors hung a sheet of plastic to protect the garage door and siding from slurry and concrete splashes. They cut a line in the asphalt 20 inches in front of the garage door using a concrete saw. Then they cut the asphalt strip into smaller squares for easy handling during removal. The diamond blade on this saw had a 6″ cutting depth, which made it able to cut through the 4 inches of asphalt and another 2 inches of the gravel below.
The installers manually removed both the sectioned asphalt and underlying gravel. In this case, gravel and asphalt was put into the back of a pick-up truck and taken to a land fill for disposal. During this time, the downspouts were fitted with the 4″ PVC fittings necessary to divert the roof water into the trench and drainage pipe.
To assemble the channels, placement began at the lowest point (invert out) and progressed to the shallowest end. The contractors established a level line just below the surface of the asphalt and 14″ from the edge of the garage floor. They attached installation hardware to the first polymer channel section (4 foot length) and set it in place, suspended 4 inches above the excavated surface, with #4 rebar.
The top edge of the channel was adjusted to meet the level line by using installation chairs. Vertical adjustments were done by sliding the installation chair on the rebar. Horizontal alignments were made with the adjusting bolts on the installation chair.
After leveling the first channel, the contractors added installation hardware to the end of the second channel and attached in an “end to end” manner with the first channel. Again, they suspended the channel 4″ above the excavated surface and adjusted laterally and horizontally on the rebar supports to match the level line. The installers repeated the technique until the last channel section was put in place.
Once the channel was assembled, the contractors installed end caps (with knockouts) and attached down spouts and piping. They wrapped the grates in plastic sheeting both to protect them from concrete and to keep concrete out of the trench. Some people place a strip of plywood in the grate recess during concrete pouring. This is a good idea. It allows you to keep the grates clean and the trench free from concrete and not deal with the plastic sheet.
Step Three – Pouring Concrete
For this installation, the drain was centered in a 20″ wide excavated trench that was approximately 12″ deep. A minimum 4″ space was below the channel and 6″ on either side of the channel for concrete to fill. No reinforcing was used in the concrete (4000 psi mix). The concrete truck discharged the mix directly into the trench. Two men placed the concrete in the trench using hand tools. It would have been useful to have a pencil vibrator during the placement. Nevertheless, concrete pouring only took about one half hour.
Once the concrete was placed and the truck had left, the installers removed the grates to clean up the channels. They finished the cement with trowels and edging tools. Once finishing was complete and the concrete began to set, they locked the grates into place and tightened the bolts.
Once the concrete dried, the contractors tidied the installation area, replacing dirt areas affected by excavation, returning flower planters to their places, picking up rubbish and power washing the driveway for the homeowner.