Roof Underlayment for Asphalt Shingle Roofs
Home Builder’s Guide to Coastal Construction
FEMA 499/June 2005 
Technical Fact Sheet No. 19

Purpose: To provide recommended practices for use of roofing underlayment as an 
enhanced secondary water barrier in coastal environments

Note: The underlayment options illustrated here are for asphalt shingle roofs. 
See FEMA publication 55, Coastal Construction Manual, for guidance concerning 
underlayment for other types of roofs.

Key Issues
--Verify proper attachment of roof sheathing before installing underlayment
--Lapping and fastening of underlayment and roof edge flashing
--Selection of underlayment material type

[Textbox]
Note: This fact sheet provides general guidelines and recommended enhancements 
for improving upon typical practice. It is advisable to consult local building 
requirements for type and installation of underlayment, particularly if specific 
enhanced underlayment practices are required locally.
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Sheathing Installation Options
The following three options are listed in order of decreasing resistance to 
long-term weather exposure following the loss of the roof covering. Option 1 
provides the greatest reliability for long-term exposure; it is advocated in 
heavily populated areas where the design wind speed is equal to or greater than 
120 mph (3-second peak gust). Option 3 provides limited protection and is 
advocated only in areas with a modest population density and a design wind speed 
less than or equal to 110 mph (3-second peak gust).

Installation Sequence – Option 1 (footnote 1)
1.	Before the roof covering is installed, have the deck inspected to verify 
that it is nailed as specified on the drawings.
2.	Install self-adhering modified bitumen tape (4 inches wide, minimum) over 
sheathing joints; seal around deck penetrations with roof tape.
3.	Broom clean deck before taping, roll tape with roller.
4.	Apply a single layer of ASTM D 226 Type II (#30) or ASTM D 4869 Type IV 
felt.
5.	Secure felt with low-profile, capped-head nails or thin metal disks 
(“tincaps”) attached with roofing nails.
6.	Fasten at approximately 6 inches on center along the laps and at 
approximately 12 inches on center along two rows in the field of the sheet 
between the side laps.
7.	Apply a single layer of self-adhering modified bitumen complying with ASTM 
D 1970 over the #30 felt throughout the roof area.
8.	Seal the self-adhering sheet to the deck penetrations with roof tape or 
asphalt roof cement.

Installation Sequence – Option 2 (footnote 1)
1.	Before the roof covering is installed, have the deck inspected to verify 
that it is nailed as specified on the drawings.
2.	Install self-adhering modified bitumen tape (4 inches wide, minimum) over 
sheathing joints; seal around deck penetrations with roof tape.
3.	Broom clean deck before taping, roll tape with roller.
4.	Apply two layers of ASTM D 226 Type I (#15) or ASTM D 4869 Type II felt 
with offset side laps.
5.	Secure felt with low-profile, capped-head nails or thin metal disks 
(“tincaps”) attached with roofing nails.
6.	Fasten at approximately 6 inches on center along the laps and at 
approximately 12 inches on center along a row in the field of the sheet between 
the side laps.

Installation Sequence – Option 3 (footnote 1, 2)
1.	Before the roof covering is installed, have the deck inspected to verify 
that it is nailed as specified on the drawings.
2.	Install self-adhering modified bitumen tape (4 inches wide, minimum) over 
sheathing joints; seal around deck penetrations with roof tape.
3.	Broom clean deck before taping, roll tape with roller.
4.	Apply a single layer of ASTM D 226 Type I (#15) or ASTM D 4869 Type II 
felt.
5.	Tack underlayment to hold in place before applying shingles.


Footnote 1: Note: If the building is within 3,000 feet of saltwater, stainless 
steel or hot-dip galvanized fasteners are recommended for the underlayment 
attachment.
Footnote 2: Note: (1) If the roof slope is less than 4:12, tape and seal the 
deck at penetrations and follow the recommendations given in The NRCA Roofing 
and Waterproofing Manual, by the National Roofing Contractors Association. (2) 
With this option, the underlayment has limited blowoff resistance. Water 
infiltration resistance is provided by the taped and sealed sheathing panels. 
This option is intended for use where temporary or permanent repairs are likely 
to be made within several days after the roof covering is blown off.

General Notes
--Weave underlayment across valleys.
--Double-lap underlayment across ridges (unless there is a continuous ridge 
vent).
--Lap underlayment with minimum 6-inch leg “turned up” at wall intersections; 
lap wall weather barrier over turned-up roof underlayment.

Additional Resources
National Roofing Contractors Association (NRCA). The NRCA Roofing and 
Waterproofing Manual. (www.NRCA.net)

[End of Fact Sheet No. 19]