How To Do a Roof Underlay

Installing a roofing underlayment protects and extends the lifespan of your roof, saving you work and money over the long term. If you think of your roof as a whole system made up of layers, roofing underlayment sandwiches between the wooden sheathing and your choice of roofing material. The sandwiched underlayment provides an additional water-shedding layer in case of a future leak in the exposed roofing material. Roofing underlayment also provides insulation and cushions the underside of your roof, which helps to prevent distortion in roofing material such as metal panels.

Corrosion-resistant roofing nails
Roofing staples (optional)
Staple gun (optional)

Instructions

- 1
  Align the underlayment with the rake or side edge of the roof and the drip edge on the eave or lowest edge of the roof. Fasten the underlayment with corrosion-resistant roofing nails at 12- to 36-inch intervals, working across the roof from side to side and parallel with the eave edge.
- 2
  Overlap the top edge of the installed course by 2 inches with the next underlayment length. Fasten the roofing underlayment through the center of the overlap at the same interval distance as you did in Step 1.
- 3
  Continue to install rows of underlayment, working up the roof in parallel overlapping courses. Overlap side edges of underlay by 4 inches, and fasten through the overlap to prevent the wind from lifting the material. Stagger side overlaps from course to course by a minimum of 6 feet to reduce joint vulnerability across the roof.
- 4
  Extend the underlayment 6 inches over each side of roof transitions, such as hips on downward slopes and at the ridge line on the top of the roof. Fasten the exposed lap over a roof transition to prevent wind uplift.

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