How to Install Roofing Underlayment

What You'll Need
A hammer
A tape measure
Protective eye wear
Work boots

Roofing underlayment should be installed on decking material that has no old or damaged underlayment or shingles. It is an vital part of the roofing process because it prevents water and air from damaging the roof. The underlayment should meet specific standards to be effective. This job will probably take about eight hours to complete. Check the weather to be sure you will have a day of good weather to work.

Step 1 – Safety

Put on your protective eye wear and work boots before you begin. Be careful moving up and down the ladder. Be cognizant of the edges of the roof while you are working. Have someone with you while you work or make sure someone is aware that you are working on the roof.

Step 2 - Preparing the Roof

Sweep any debris off of the roof.  Be sure to remove any old underlayment or nails as well.

Step 3 – Preparing the Underlayment

Open the roll of underlayment.  Mark a chalk line 35 inches above the roof edge to align the underlayment. Place it on the outside roof edge. Position the roll so that 2 inches of the underlayment are able to hang over outside roof edge decking. Use your hammer and nails to secure the underlayment next to the outside roof edge. Drive the nail with hammer at the top corner of the underlayment. This will enable the underlayment to pivot while you are installing it.

Step 4 – Rolling the Underlayment

Install the underlayment along the roof decking. If necessary, adjust how the material ls aligned to keep it hanging over the decking material. Smooth out the underlay to prevent any bumps.

Step 5 – Nailing the Underlayment

Use the hammer and nails to secure the underlay material as it is rolled and smoothed out along the roof. Drive the nails along the bottom edge of the overlayment. Be sure to overlap each line of overlayment as you work your way up the roof top. Be sure there are no spaces for air movement between the underlayment and the roof decking.

Step 6 – Placing Underlayment on Varying Roof Surfaces

If your roof has varying surfaces, the underlayment should be rolled one foot beyond the valley between them. The nails used to secure the underlayment should be secured at least 1 foot from the valley as well. Be particularly careful to make sure there is no space for air movement or water leakage between the underlayment and the decking at these valley areas.

Step 7 – Finishing the Roof Underlayment

Continue the underlay process until the whole roof is complete. Install the overlayment over the roof ridge so that it overlaps by eight inches on both sides. Be sure there are no bumps or creases in the material. In places where the roof is attached to the wall, position the materials four inches up the wall. If possible, slide the overlayment underneath the wall covering.