The answer to 'when to replace underlayment' is that most experts recommend replacing your roof underlayment every 10-15 years, or whenever you replace your roof. Underlayment is the layer installed between the roof deck and the shingles, and it plays a crucial role in protecting your home from water damage and leaks. Over time, underlayment can become worn, cracked, or deteriorated, reducing its effectiveness.
Regularly inspecting and replacing your underlayment as needed is an important part of maintaining your roof and ensuring the long-term protection of your home. If you notice any signs of wear or damage, such as cracks, tears, or curling, it's a good idea to have a professional roofing contractor evaluate your underlayment and determine if it needs to be replaced.