Sod is a quick solution for establishing a green lawn, consisting of mature grass and the soil layer, or turf, in which it is grown. Many homeowners consider laying this instant lawn directly over their existing turf. However, the definitive answer to whether you can successfully lay sod over existing grass is generally no, as this shortcut almost always leads to establishment failure. The new sod requires direct contact with the prepared, underlying soil to thrive, a condition that the old grass prevents.
Why Laying Sod Over Existing Grass Fails
The primary requirement for successful turf installation is a process called “knitting,” where the roots of the new sod quickly penetrate the native soil below. When existing grass remains, it creates a physical barrier that prevents root-to-soil contact. The dense layer of old grass and accumulated thatch acts like a sponge, separating the roots of the new turf from the ground they need to colonize.
This separation impedes the ability of the new grass to establish an independent root system. The roots of the transplanted sod, which are typically shallow, cannot push through the thick, interwoven structure of the old lawn. Consequently, the new turf is forced to survive solely on the thin layer of soil it was harvested with, quickly depleting its limited water and nutrient reserves. The living grass underneath immediately begins competing with the new sod for any available moisture and nutrients, starving the new installation before it can take hold.
Immediate Consequences of Skipping Preparation
Attempting to install sod over an old lawn leads to negative outcomes shortly after installation. As the old layer of grass and thatch begins to decay underneath the new sod, it decomposes unevenly, which creates an unstable surface. This decomposition process causes the new lawn to develop noticeable lumps and depressions, resulting in a bumpy yard.
Putting new turf over existing vegetation also compromises drainage, leading to water management problems. The dense, layered structure underneath can prevent water from percolating efficiently into the subsoil, causing water to pool on the surface or run off quickly. This excess surface moisture, combined with the decomposing organic matter, creates an environment for fungal diseases and rot, which can rapidly kill the new sod. Decomposition of the underlying organic material can also release gases that are toxic to the new grass roots, further hindering establishment.
The Proper Ground Preparation Process
A successful sod installation begins with the complete removal of the existing lawn, which can be accomplished chemically using a non-selective herbicide or mechanically using a sod cutter. Once the old turf is removed, the site must be cleared of all debris, including rocks, sticks, and old roots, to ensure a smooth, uniform base. The soil should then be tested to determine its pH and nutrient composition, as this analysis guides the next steps for soil improvement.
Following debris removal, the ground needs to be tilled to a depth of four to six inches to loosen the soil and promote better water and root movement. This is the ideal time to incorporate soil amendments, such as compost or topsoil, to improve structure, or lime and sulfur to adjust the pH level based on the soil test results. A starter fertilizer should also be tilled into the top few inches of the soil to provide immediate nutrients for root development.
Proper grading is then performed to ensure surface uniformity and correct drainage, which is achieved by raking the soil to create a smooth slope away from structures. The final grade should be approximately one to one-and-a-half inches below adjacent paved surfaces to account for the thickness of the new sod. The prepared area should be lightly rolled with a lawn roller to firm the surface and prevent excessive settling after the sod is laid. Finally, the soil should be lightly moistened a day or two before installation to prepare the bed for the new turf.