J-Wiley: Best Practices for Roof Underlayment Installation & Detailing InspectAPedia® -
Guide to best roofing practices, excerpts from Best Practices Guide to Residential Construction, Steven Bliss
Is roofing felt needed under asphalt shingle roofs?
Roof shingle warranty requirements for roof shingle underlayment
NRCA recommendations for roofing felt underlayment
Permeability of felt underlayments in hot humid climates
What are the benefits from using roofing underlayment or felt?
Questions & answers about the requirement for and installation of underlayment on asphalt roof shingles and similar materials.
This article explains the benefits of and best practices use of roofing felt or other roofing underlayment products beneath asphalt shingles and other roofing materials. We discuss underlayment or felt installation details for standard slope roofs, low slope roofs where shingles will be installed, and low slope roofs in freezing climates.
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Best Practices Guide to Uses of Roofing Underlayment
Our page top photograph shows 15# roofing felt underlayment being installed in preparation for placing a new roof on an older home in Poughkeepsie, NY. This article includes excerpts or adaptations from BEST ROOFING PRACTICES, adapted from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.
Roofing underlayment or felt protects the roof deck from rain before the roofing is
installed.
Roofing underlayment or felt provides an extra weather barrier in case of blowoffs
or water penetration through the roofing or
flashings.
Roofing underlayment or felt protects the roofing from any resins that bleed out of
the sheathing.
Roofing underlayment or felt helps prevent unevenness in the roof sheathing from
telegraphing through the shingles.
Roofing underlayment or felt is usually required for the UL fire rating to apply
(since shingles are usually tested with underlayment).
The roof deck should be sound and level before laying the
underlayment. Fifteen-pound or heavier felt underlayment
is required by code in some areas. Whether or not it is
required, underlayment is cheap insurance against problems.
Note that building codes or roof product manufacturers also require using roofing felt in many circumstances including to protect the shingle warranty - see ROOFING FELT UNDERLAYMENT REQUIREMENTS for details.
Guide to Roofing Underlayment or Felt Installation for Standard Roof Slopes
On roofs with a slope of 4:12 or
greater, use a single layer of 15 lb. asphalt-saturated felt,
starting at the eaves and lapping upper courses over lower
by a minimum of 2 inches.
Vertical joints should lap a minimum
of 4 inches and be offset by at least 6 feet in successive
rows (see Figure 2-2). On roofs with a slope of 4:12 or greater, use a single layer of
minimum No. 15 asphalt-saturated felt, starting at the eaves
and lapping upper courses over lower. Run the felt 6 inches
over ridges and hips from each direction, and 6 inches up
any adjoining walls.
Secure each course along seams and edges with
enough corrosion-resistant nails to hold it in place until
the roofing is installed. In high-wind areas, apply fasteners
a maximum of 36 inches on-center along overlaps.
For best protection against leaks, run felt 6 inches
over ridges and hips, from each direction, and 6 inches
up any adjoining walls.
Valleys should be lined with a
full width of roofing felt (or bituminous membrane)
pushed tight into the valley so there is no slack. Side
courses of underlayment should run over the valley lining
and extend 6 inches past its edge. (see Best Practices Guide to Residential Construction “Valley Flashing”
page 59.)
Guide to Roofing Underlayment Installation on Low Slope Roofs with Asphalt Shingles
Asphalt shingles can be used on roofs with
a slope of 2:12 to 4:12 if double-coverage underlayment is
used. Start with a 19-inch strip of 15 lb. asphalt-saturated
felt along the eaves, and lap succeeding courses by
19 inches as shown in Figure 2-3.
Wherever there is a possibility of ice or snow buildup
or the backup of water from leaves or pine needles, install
a self-adhering bituminous membrane along the eaves that
extends up the roof to a point at least 36 inches inside the
interior wall line. An alternative approach, not widely used
anymore, is to seal all laps in the lower courses of roofing
felt with lap cement or asphalt plastic cement.
In areas with extensive snowfall or windblown rain,
the best protection against leakage is to cover the entire
low-slope roof area with a bituminous membrane, as
shown in Figure 2-4.
Vertical end laps should be at least 3 inches and horizontal
laps 6 inches. If the roof changes to a steeper slope,
for example, where a shed dormer joins the main roof, extend
the membrane 12 to 18 inches up the main roof slope.
Bituminous membranes are self-healing around nail holes,
and because they bond fully to the sheathing, any leaks
that occur cannot spread. As a safeguard against expensive
callbacks, many roofers now apply membrane to the entire
surface of any roof with a slope of 4:12 or less.
Guide to Eaves Flashing by Roofing Underlayment Installation on Low Slope Roofs in Freezing Climates
For low slope roofs in cold climates where ice dams and ice dam leaks are likely, Bliss recommends a double layer of No. 15 felt underlayment as shown in Figure 2-3 above.
With slopes from 2:12 to 4:12, use a double layer of No. 15 underlayment as shown.
Where water may back up from ice or debris from trees, protect the lower portion of
the roof with a bituminous eaves flashing or fully cemented felt, as shown.
OPINION-DF: use a self-sealing self-adhering bituminous membrane such as Ice and Water Shield in these locations to provide maximum leak protection. These products seal around nails used to secure the membrane in place. Mr. Bliss makes this recommendation in text that follows and in Figure 2-4 (left).
The best defense against ice dams in
cold climates is a so-called “cold roof,” consisting of high
levels of ceiling insulation separated from the roof surface
by a free-flowing vent space (see “Preventing Ice Dams on Roofs,”
or see page 97 in the printed text Best Practices Guide).
Where a cold roof cannot be achieved due to
complex roof shapes, unvented roofs, or retrofit constraints,
ice dams may form during severe winters, in some cases,
causing pooled water to wet wall cavities and interior
finishes.
Where adequate insulation and ventilation cannot be
assured, self-adhering bituminous eaves flashing should
be installed. The membrane should go from the lower edge
of the roof to a point at least 24 inches inside the interior
wall line (Figure 2-5).
Where two lengths of eaves flashing meet at a valley,
run each across the valley, starting with the length from
the roof with the lower slope or lesser height. The valley
flashing should later lap over the eaves flashing.
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"Unvented Roofs, Hot-
Humid Climates, and
Asphalt Roofing Shingles
Research Report - 0306
Feb-2003
Building Science Corporation", Building Science Corporation, 30 Forest Street,
Somerville, MA 02143
Quoting from the article abstract:
"When constructing unvented roofs with asphalt shingles in hot-humid climates, a vapor barrier must be
installed between the asphalt shingles and the roof deck." - Web Search 5/21/2010
NRCA Roofing & Waterproofing Manual, 5th Edition, National Roofing Contractors Association, 10255 W. Higgins Road
Suite 600
Rosemont, IL 60018-5607
Telephone: (847) 299-9070 Toll Free: (866) ASK-NRCA (275-6722), e-mail: info@nrca.net. - http://www.nrca.net/rp/pubstore/details.aspx?id=243&c=9
Asphalt Shingle Roofing [Inspection & Installation Guide], Coon Rapids Minnesota, Coon Rapids City Hall
11155 Robinson Drive
Coon Rapids, MN 55433
763-755-2880, - Web search 07/12/2010
City Website - http://www.ci.coon-rapids.mn.us/ Contact the City of Coon Rapids at 763-755-2880
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