How to Install a Grate & Drain Trench in Front of a Garage Door

Grate and trench drains are ideal for rapidly removing storm water run-off to the sewers.

They have many uses in evacuating large amounts of run-off in a short time from architectural open spaces, and are ideally suited for use in front of a garage at the bottom of a sloping driveway.

1. Decide on the best method of construction for your budget and level of skill. Methods include the in-situ digging of a channel that is subsequently lined with concrete, and the use of concrete, metal or polymer trench liners.

2. Mark the course of the trench using an aerosol spray can or a heavy chalk line. The trench should be parallel to or across the anticipated direction of water flow. In situations where a great deal of surface water is anticipated, it is viable to install the trench at an angle greater than 90 degrees so that run-off will be disrupted equally all along its length. The lowest point should be farthest from the top of the slope.

3. Calculate the number of grates you will need by dividing the length of the finished drain by the length of each grate. Grates are available in numerous widths and styles; decide on grate type, then calculate how many will be required.

4. Calculate trench depth and width. Follow instructions on the liner packaging for calculating the optimum depth. These values are equally applicable to trenches lined with concrete in situ. Copy down the formulae from a trench liner, then work out how deep the trench must be. Determine trench width by the type of grate to be used; for example, using a 4-inch grate determines that the support lip at the top of the trench must be slightly less than 4 inches wide. Observe the grate manufacturer’s specific instructions on the packaging as to exact width and lip required.

5. Dig out the trench; depending on its size, use an adz, a pick and shovel or a ditch witch. If excavating through a preexisting hard surface such as concrete or asphalt, first cut the edges using a concrete saw. Dig the trench so that it slopes minimally toward the end where it will drain into the wastewater or holding system. If using a prefabricated liner, ensure the trench is deep enough that, after installing the bedding, the grate to be installed above the liner will be flush with grade, meaning the surface of the floor or ground.

6. Install the connection to the sub-surface drain at the lowest end of the trench first. Cut a "T" connection into the building’s preexisting sewer line or leave the end open if other contractors are to subsequently lay that line. Even if using prefabricated trench liner, this connection should ideally be bedded into concrete to prevent future movement and subsequent separation from the liner.

7. Lay in the wooden forms if constructing the trench liner in situ. Ensure the forms are exactly the width mandated by the grate manufacturer. Before pouring the concrete, install rebar to reinforce the structure against shifting and the expansion/contraction cycles caused by extremes in temperature. Use an angle grinder to cut the rebar to length and wire the separate pieces into a mesh. Pour the concrete, trowel off to shape, then brush the surface in the direction of water flow.

8. Lay in the prefabricated liner if using this method rather than in situ construction. Follow the liner manufacturer’s instructions concerning bedding-in; typically, the liner should be bedded into builder’s sharp sand or fine gravel. Pour some of the bedding material into the base of the trench, install the liners -- working backward from the connector at the lowest point, and ensuring each is properly connected to the next -- then pour more bedding around the sides.

9. Install the grates. Post advisories on the garage door that the grate should not be driven over for a prescribed period of days; this allows adequate time for fresh concrete to cure before subjecting the installation to use. Curing time is determined by environmental conditions; a full week is typical for ordinary -- not fast-setting -- concrete in a normal climate.