The answer to 'why is my underlayment cracked' is that the underlayment has likely become brittle and damaged over time due to exposure to UV rays, heat, and weather. This can cause the underlayment to become dry, brittle, and eventually crack or split. Cracked underlayment can lead to leaks and water damage, so it's important to address the issue promptly.
Common causes of cracked underlayment include age (most underlayments only last 10-20 years), poor installation, and exposure to extreme weather conditions like intense sunlight, heavy rain, or high winds. Symptoms may include visible cracks, curling or buckling of the underlayment, and signs of leaks or water damage in the attic or living spaces below.