i noticed on my roof that along every roof joist the shingles are cracked. what could cause this?

Question

before i re roof my house i want to solve the issue of the shingles cracking along every roof joist. the shingles are 20 years old. the deck boards are 1/2 inch plywood and are staggard when laid. i have soffetts and roof ridge vent. why would have cracking?

Answer 1

if all you have for a roof sheathing is 20 year old 1/2" plywood you need more than shingles.......like a new roof of at least 3/4 inch....the cracking in the shingles is from the old ply failing, sagging, moving around; nails pulling through, butt joints of two sheet edges on the rafters moving, etc etc etc.