Sometimes there's little choice in the placement of your lawn but to build it over a layer of soil that's unsuited for the growing of a manicured landscape. When you're in an area that experiences drainage problems where you're placing your new lawn, a layer of gravel beneath the grass can be of some help. Once placed, the gravel absorbs and then directs the moisture beneath your lawn, drawing the liquid away from structures and grass to keep your new lawn from becoming a swamp due to a lack of drainage.
Clear the ground cover over the proposed lawn area using a spade to cut through and remove sod. Prune any bush limbs away and cut trees to a stump. Pry the bush and tree remnants from the soil to clear them away.
Excavate the lawn area to a depth sufficient to place and slope the gravel. You'll need a hole at least 6 inches deep to provide room for the gravel layer as well as the topsoil for the lawn. Excavate the soil by hand or with a machine if you're qualified to operate construction machinery.
Run a vibratory plate compactor along the base of the excavation to compress the soil before adding the gravel.
Fill the excavation with a 3-inch layer of 1/4-inch crusher fine gravel subbase. This subbase consists of various sized gravel and stones along with stone dust that compacts well and creates a drainage layer beneath your soil.
Spread the subbase with a wide contractor rake, sloping the gravel steadily away from structures towards the edge of the lawn so that the water drains away from any buildings adjacent to the lawn.
Compact the sloped gravel with a vibratory plate compactor. Turn on the compactor and run it over the gravel from the high point downward, compressing the material so that it does not shift after placement of the topsoil. Use two or three passes with the compactor to compress the gravel as much as possible.