How to Install Grass on a Rooftop Deck

Calculate the size of your rooftop deck with a measuring tape. Multiply the length by the width to determine how many square feet you need to cover with an underlayment system and grass.

Spread the underlayment system on the deck. The system contains insulation, a waterproofing barrier, drainage and filter fabric.

Measure and cut the waterproofing material to size. Some green roofing systems come with a single-ply sheet membrane and others have more than three layers. Spread it out over the section of roof that will grow the grass.

Unroll the geosynthetic filter fabric and lay it down in parallel rows along the roof. The filter will prevent fine particles of soil from clogging the drainage layer.

Add an insulation layer if you want to keep the water stored in the system from stealing cool air in the summer and heat in the winter. The insulation can range from a drainage board to cellular glass. Follow the manufacturer's instructions.

Put a shaped plastic membrane drain on the roof. The system is comprised of continuous sheets of polymers that are shaped to form pegs or dimples. Drainage occurs on one or both sides of the membrane. Spread the sheets, cutting some to size if necessary.

Spread 4 inches of garden soil on top of the underlayment system with a shovel. Grass roots are shallow, so a few inches is sufficient. Dump the soil from one edge of the deck to the other.

Rake the soil until it is level. Moisten the soil with the hose.

Spread a half-pound of nitrogen fertilizer per 1,000 square feet of planting area. Rake it into the top of the soil.

Fill a hand spreader with grass seed and fine sawdust. Combine one part seed with four parts fine sawdust. Every type of seed has a recommended planting rate. Follow those recommendations on the seed bag to determine how much seed to use per square foot.

Walk back and forth over the deck, spreading seed. Work in rows to ensure even coverage.

Run the metal rake over the grass seed to cover it with 1/4 to 1/8 inch of soil. The soil will speed up the germination process and keep the seed from blowing away in the wind.

Moisten the seeded area twice a day until seeds germinate. Don't saturate the soil but don't let it dry out, either. Continue to water at least once a week. Given the increased air flow on the roof, the grass roots are more prone to drying and need more frequent watering than grass planted on the ground.