The answer to 'why is my deck thin grass' is that there are a few common causes for thin, sparse grass on a deck. One of the main reasons is lack of sunlight - decks often create shaded areas that prevent grass from getting the full sun it needs to thrive. Additionally, the soil under a deck is typically compacted and less nutrient-rich, making it difficult for grass to grow. Foot traffic on the deck can also wear down and damage the grass over time.
To fix thin deck grass, you may need to overseed the area with a shade-tolerant grass variety, aerate the soil to improve drainage and nutrient absorption, and apply a balanced fertilizer. However, if the underlying issues like poor soil quality or excessive shade persist, it may be better to consider alternative ground cover options like mulch, pavers, or a low-maintenance groundcover plant instead of trying to maintain grass.