In time, every roof needs to be replaced, usually due to the long term effects of weathering. If a residential roof is more than 20 years old, it is a prime candidate for re-roofing. Ways to ensure your roof needs replacing include:
Look for things on the inside such as places where the roof deck is sagging, signs of water damage or leaking, dark spots and trails, and outside light showing through the roof.
On the exterior on the rood, pay attention to such things as damaged flashing, missing shingles, curling, blistering, buckling, and rotting and algae growth.
Visually inspect your roof for cracked torn, bald or missing shingles.
Scan the roof for loose material or wear around chimneys, vents, pipes or other penetrations.
Watch out for an excessive amount of shingle granules (they look like large grains of sand) in the gutters – this is a sign of advanced wear.
Check for signs of moisture, rot, or mould. Even if wet spots are not always directly under your faulty shingle; water can travel down to its lowest spot before it drips. Mould, fungi and bacteria can grow quickly – within 24-48 hours of a water-related problem
Examine the drainage, and make sure gutters and downspouts are securely attached. Also, ensure all drains are open and allow water to exit, and all gutters and downspouts are free of debris.
Check that all bath, kitchen, and dryer vents go entirely outside of your home, not just into the attic.