Never lay sod on top of an existing lawn. If you do so, your sod will have trouble taking root. A layer of grass, weeds, or hard soil beneath your sod will contribute to dying sod that is invaded by weeds. Instead, take proper steps to remove the old lawn with a sod cutter. Then, prepare the soil by tilling and leveling before you lay your sod.
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5 Disastrous Effects of Laying Sod on Top of Grass
Sod needs direct soil contact in order to take root and thrive. Unless you use a sod cutter to remove the existing lawn, your new, green lawn will falter and die. If you skip the steps for removing old grass before laying sod, your lawn will suffer the following setbacks.
Improper Sod Rooting
Sod that is laid on top of grass has nowhere to take root. Rather than good sod-to-soil contact, the roots will make contact with old grass blades and thatch. Even if your sod does attempt to send out roots of its own, the top layer of soil is packed with existing grass roots. This makes it hard for your sod to get a foothold. It won’t remain in place or be able to gather its own water and nutrients.
- Sod laid on top of existing grass will have an extremely hard time taking root.
- If your sod can’t develop roots, it won’t be able to take up water and soil nutrients.
- Sod that struggles to root can shift, curl, or be easily damaged by foot traffic.
The best way to avoid poor sod rooting is to use a sod cutter to remove all existing grass and weeds. Then, till the soil to provide an ideal surface for your sod to take root. You may even consider adding compost to give your sod a boost of organic matter fuel.
Dry, Dead Sod
This point goes hand-in-hand with failing to take root—sod that’s installed on top of an existing lawn is extremely prone to drying out and dying. Without the ability to establish a root system, sod can’t pull moisture from deeper soil layers. Instead, it is forced to depend on the thin layer of soil that is included with the sod. A hot day can bake sod dry, killing it quickly.
- Sod laid on top of grass dries out easily, which can kill your sod.
- The layer of grass beneath the sod prevents sod from developing deep roots to gather water from the soil.
- Dry sod is the number one killer of new lawns. Avoid this by removing old grass first.
Even in the best conditions, with properly prepared soil, water retention is a challenge for new sod. Laying sod on a bed of grass only increases this challenge and creates a larger risk that the sod will dry out. Even drought-prevention tricks like laying sod in winter, won’t stop sod from drying out and dying if it’s laid on top of your current lawn.
Weed Invasion
Sod laid on top of an existing lawn is extremely likely to be invaded by weeds. The dry, poorly rooted sod will shrink, creating gaps between pieces of sod. Here, weeds and remnants of your old lawn will sprout through, turning your beautiful lawn into a patchy, uneven mess.
- Sod on top of grass will often be invaded by weeds that grow between the pieces of sod.
- Weeds are more likely to invade sod on top of grass because your sod will dry out and shrink, leaving room for weeds to sprout.
Along with existing grass, it’s essential to remove weeds before installing sod. A weed that seems harmless now will spread all over your yard once you lay new sod over your lawn. As both the sod and old grass struggle, weeds will gain ground.
Slimy Grass
Anyone who’s ever laid sod on top of an existing lawn can tell you, your old lawn won’t quietly decompose. Instead, the grass under the sod will turn yellow and die, then develop a slimy texture that breeds fungus and grass disease. This diseased grass can infect your sod, killing your new lawn. Also, your sod will slide on top of the slimy grass, making walking across your lawn a hazard.
- Old grass under sod will not decompose—after the grass dies it will become slimy and host fungal diseases.
- Fungal diseases in the dead grass can infect and attack new sod.
The tough plant matter in grass and thatch—called lignin—resists decomposition even after the grass dies. Grass that isn’t properly removed won’t decompose quickly. Instead, it sticks around for months or years, becoming a dead layer that harbors disease.
Flooding
Laying sod over grass adds 1–1.5 inches (2.5–4 cm) of height to your lawn. This may not seem like much but it can cause some serious flooding issues. If adding sod makes your lawn higher than paved areas, such as driveways, patios, and walkways, water will flow from the grass to these hardscaped areas, flooding them.
- Sod on top of grass raises the height of your lawn.
- If your sod is higher than paved areas, water will flood onto your driveway, walkways, and toward foundations.
- Prevent flooding by using a sod cutter to remove old grass and topsoil before sod installation.
The flooding caused by new sod on top of old grass can also direct water towards foundations, which can cause flooding or undermine your home’s foundation. This is why it’s critical to use a sod cutter to remove grass along with the top 1–2 inches (2.5–5 cm) of topsoil before laying new sod. It ensures your lawn will remain at the appropriate height and resist flooding.
Can You Lay Sod Over Crabgrass?
As with lawn grass and other weeds, do not lay sod over crabgrass. Crabgrass is an annual grassy weed that is extremely resilient. It won’t be stopped by sod but will instead sprout through the cracks between pieces of sod.
- Do not lay sod over crabgrass.
- Crabgrass will grow between pieces of sod and invade your new lawn.
- Remove your old lawn—including crabgrass clusters—with a sod cutter before installing sod.
- Consider waiting 2 weeks for more crabgrass to sprout, then killing it before laying sod.
Instead of laying sod over crabgrass, remove the existing lawn and topsoil with a sod cutter. This is best followed up by rototilling to break up roots and loosen soil for easy sod rooting. Then, if you want to really eradicate crabgrass, water your lawn and wait 2 weeks. Kill any new crabgrass that sprouts before laying your new sod.
Can You Put Sod Down Over Existing Grass?
The top reasons you should never put sod down on top of existing grass are:
- Your sod will struggle to take root because the old grass blocks soil access.
- Without proper root formation, your sod is likely to dry out and die.
- Weeds and old grass will grow up through the seams in your new sod, causing an unsightly lawn.
- Old grass that dies under sod will become slimy and host fungal lawn diseases.
- An added layer of sod will raise the height of your yard, causing water to flood onto paved surfaces.
Properly installing sod requires several specialized steps to remove old grass and prepare the soil, but it gets great results. You can tackle the job yourself or you can hire a professional landscaper to get rid of your old grass before laying sod.
