What happens if you don't put tar paper under shingles?

If you do not put tar paper, more accurately called roofing underlayment or felt, under shingles, you expose your roof and home to significant risks and almost certainly void manufacturer warranties. Underlayment serves as a critical secondary water barrier between the shingles and the wood deck. Shingles are not completely waterproof, as wind-driven rain, ice dams, and capillary action can force water underneath them. Without underlayment, any water that gets past the shingles will directly soak into the plywood or OSB decking, leading to wood rot, mold growth, and eventual structural decay. In cold climates, without ice and water shield in valleys and eaves, ice dams can cause major interior leaks. Additionally, underlayment provides a temporary weather barrier during installation and protects the deck from sap and unevenness. Building codes in almost all jurisdictions require underlayment meeting ASTM standards, typically 15 or 30 pound felt or synthetic underlayment. Skipping it can cause your roof to fail inspection, void your Atlas, GAF, or Owens Corning warranty, and give your insurance company grounds to deny a claim. Synthetic underlayment also provides a non-slip surface for roofers. Never install shingles directly over bare wood.