How do you lay roof underlayment?
Short answer. Lay roof underlayment in horizontal rows starting at the eave, each row lapping over the one below so water always runs across a seam, never into it. On roofs 4 in 12 and steeper, the IRC calls for one layer with at least 2-inch head laps and 4-inch end laps offset by 6 feet, while slopes from 2 in 12 up to 4 in 12 need two layers.
Order of operations
- Eave drip edge.
- Self-adhered leak barrier in valleys, and at eaves or around penetrations where used.
- Field underlayment, starting at the eave and working up.
- Rake drip edge on top of the underlayment.
Laps and offsets
- Head lap: how far each row overlaps the row below. The code minimum is 2 inches, but most synthetic brands print lap lines calling for more, often 4 inches or so. Follow the roll.
- End lap: where two pieces meet within a row. At least 4 inches, and offset from end laps in the row below by about 6 feet so seams never stack.
- Valleys: field underlayment laps over the valley leak barrier.
- Walls: turn the sheet up the wall a few inches behind where flashing will go.
- Ridge: lap over the ridge from one side to the other, or stop short where a ridge vent slot will be cut.
Low slope roofs
For slopes from 2 in 12 up to 4 in 12, the IRC calls for two layers: a 19-inch strip along the eave, then full 36-inch sheets each lapping the one below by 19 inches. That doubles the coverage where water moves slowly.
Practical tips
Keep the sheet flat and wrinkle free, since wrinkles telegraph through shingles. Synthetic is slick when dusty or damp, so walk carefully and wear soft-soled shoes. Check the product's UV exposure limit and get shingles on well within it.
See synthetic underlayment vs felt and ice and water shield in Georgia.