Do you have to put down underlayment, and how do you put it down?
Short answer. Yes. Building codes and every major shingle manufacturer require underlayment over the deck before shingles go on. Put it down on a clean, dry deck starting at the eave, with leak barrier in valleys and around penetrations first, then roll the field underlayment in overlapping horizontal rows and shingle over it promptly.
Why it is required
The International Residential Code lists approved underlayment types for asphalt shingles, and shingle fire ratings and warranties assume an underlayment is in place. Shingles shed water, but wind-driven rain, ice and the occasional lifted tab push water underneath them. Underlayment is the second line of defense that keeps that water off the wood.
Before you put it down
- Fix the deck: replace soft or delaminated sheets and pull protruding nails.
- Make sure it is dry: Georgia mornings bring heavy dew. Laying underlayment over wet wood traps moisture.
- Sweep it clean: granules and debris under the sheet show as bumps and can puncture it.
- Install eave drip edge so the underlayment can lap over it.
Putting it down
- Leak barrier in valleys, around chimneys and skylights, and wherever else you are using it.
- Field underlayment from the eave up, each row lapping the one below by the printed amount.
- Cap fasteners in the printed pattern.
- Rake drip edge over the underlayment.
Timing
Every synthetic has a UV exposure limit, and felt wrinkles if it gets rained on and sits. Summer afternoon storms in Georgia build fast, so the safest plan is to put down only what you can shingle the same day.
See synthetic underlayment vs felt and what to expect on replacement day.