The answer to 'why is my deck thin grass' is that there are several common causes for thin, sparse grass around a deck. This can be due to the deck blocking sunlight, soil compaction from foot traffic, or the deck's construction inhibiting proper drainage and air circulation. Symptoms include bare patches, slow growth, and a generally thin, unhealthy appearance of the grass.
To address this issue, you can try aerating the soil, improving drainage, and overseeding the affected areas. However, if the problem persists or the deck is older, you may need to consult a professional landscaper to evaluate the situation and recommend more extensive solutions, such as deck repairs or replacement.