How to Install Underlayment

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Felt paper, also called building paper, is installed on roof decks as insurance in case leaks develop in shingles or flashing. It’s sold in several weights, but heavier 30# paper is a good choice for use under shingles, and may be required by code.

 

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Overview

Felt paper, also called building paper, is installed on roof decks as insurance in case leaks develop in shingles or flashing. It’s sold in several weights, but heavier 30# paper is a good choice for use under shingles, and may be required by code.
In cold climates, codes often require an additional underlayment called “ice and water shield” or “ice guard” that’s used instead of standard felt paper for the first one or two courses of underlayment, which is what we’re showing here. In cold climates, apply as many courses of ice and water shield as it takes to cover 24 inches past the roof overhang. An adhesive membrane, the ice guard bonds with the roof sheathing to create a barrier against water backing up from ice dams.
If you apply the felt paper straight, you can use the lines on the paper as references when installing the roofing materials. This will help keep your rows of shingles running in a straight line.
For optimum roof protection, apply ice and water shield in valleys, along the eaves, and along the rake edges of the roof. Apply 30# felt paper over the remainder of the roof.

What You'll Need

Tools:

Chalk line
Hammer stapler
Flat pry bar
Utility knife
Tape measure
Caulk gun
30# felt paper
Ice and water shield
Staples
Roofing cement

Materials:

Chalk line
Hammer stapler
Flat pry bar
Utility knife
Tape measure
Caulk gun
30# felt paper
Ice and water shield
Staples
Roofing cement

 

Step 1

How to Install Underlayment

Snap a chalk line 35 5⁄8" up from the eaves, so the first course of the 36"-wide membrane will overhang the eaves by 3⁄8". Install a course of ice and water shield, using the chalk line as a reference, and peeling back the protective backing as you unroll it.


Step 2

How to Install Underlayment

Measuring up from the eaves, make a mark 32" above the top of the last row of underlayment, and snap another chalk line. Roll out the next course of felt paper (or ice guard, if required) along the chalk line, overlapping the first course by 4". Tip: Drive staples every 6 to 12" along the edges of felt paper, and one staple per sq. ft. in the field


Step 3

How to Install Underlayment

At valleys, roll felt paper across from both sides, overlapping the ends by 36". Install felt paper up to the ridge—ruled side up—snapping horizontal lines every two or three rows to check alignment. Overlap horizontal seams by 4", vertical seams by 12", and hips and ridges by 6". Trim the courses flush with the rake edges.


Step 4

How to Install Underlayment

Apply felt paper up to an obstruction, then resume laying the course on the opposite side (make sure to maintain the line). Cut a patch that overlaps the felt paper by 12" on all sides. Make a crosshatch cutout for the obstruction. Position the patch over the obstruction, staple it in place, then caulk the seams with roofing cement.


Step 5

How to Install Underlayment

At the bottom of dormers and sidewalls, tuck the felt paper under the siding where it intersects with the roof. Carefully pry up the siding and tuck at least 2" of paper under it. Also tuck the paper under counter flashing or siding on chimneys and skylights. Leave the siding or counter flashing or siding unfastened until after you install the step flashing.


 
 

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