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ROOFING INSPECTION & REPAIR
AMERICAN CEMWOOD ROOFING
BEST ROOFING PRACTICES
BUILT UP ROOFS
CATHEDRAL CEILING VENTILATION
CERTIFICATIONS for ROOFING CONTRACTORS
CHIMNEY FLASHING Mistakes & Leaks
COLD WEATHER ROOF TROUBLE
DECKS, ROOFTOP CONSTRUCTION
EPDM, RUBBER, PVC ROOFING
EXTRACTIVE BLEEDING on SHINGLES
FIRE RETARDANT PLYWOOD
FLASHING on BUILDINGS
FLAT ROOF MOISTURE & CONDENSATION
Green House or Solarium Roof Leaks
HEAT TAPES & CABLES on Roofs for Ice Dams
ROOF ICE DAM LEAKS
MASONITE WOODRUF FIBERBOARD ROOFING
NOISE CONTROL for ROOFS
PLASTIC ROOFING TYPES
PVC, EPDM, RUBBER ROOFING
ROOF ARCHITECTURAL STYLES - PHOTO GUIDE
ROOF CLEANING RECOMMENDATIONS
ROOF COLOR RECOMMENDATIONS
ROOF INSPECTION SAFETY & LIMITS
ROOF LEAK DIAGNOSIS & REPAIR
ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
SADDLE CONSTRUCTION at CHIMNEYS
SNOW GUARDS & SNOW BRAKES
STANDARDS for ROOFING
STRESS SKIN INSULATED PANELS
TEST LABS - ROOF SHINGLE
TREES & SHRUBS, TRIM OFF BUILDING
TRUSSES, Floor & Roof
UNDERLAYMENT REQUIREMENTS on ROOFS
VENTILATION in BUILDINGS
WALK-ON ROOF SURFACES
WARRANTIES for ROOF SHINGLES
WORKMANSHIP & ROOF DAMAGE
Roofing underlayment - roofing felt requirements: this article series discusses roofing felt: the requirements for use of an underlayment, such as roofing felt, tarpaper, or other underlayment products beneath asphalt shingles and other roofing materials. We also discuss the moisture permeability of roofing underlayments in hot humid climates.
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Our photo (page top) shows felt underlayment in good condition as a worn out roof was being stripped. It looks as if the installer of asphalt roof shingles (still visible at below right but now worn out) may have placed roofing felt on top of a still older wood-shingle roof.
At left is a peel-and-stick mineral surfaced roofing underlayment shown during installation of a metal standing seam roof. Photo courtesy Galow Homes.
As stated in Best Practices Guide to Residential Construction:
Our OPINION is that on new asphalt shingle, tile, fiber cement, slate, and many wood roofs, most roofing contractors apply an underlayment membrane of roofing felt ("tar paper"), or fiber-reinforced roofing felt. A special underlayment may be recommended by the manufacturers of specific roofing product and hybrid products.
Our photo (left) shows new felt underlayment in place during an asphalt shingle roof installation in New York.
While the requirement for felt underlayment beneath asphalt roof shingles seems to be a topic of almost timeless argument subject to much arm-waving and little reading of manufacturer's instructions and warranties, various sources recommend or require installation of a felt underlayment over the roof deck before asphalt roof shingles are installed.
Booth & Roberts reported at length on the uses of underlayments on asphalt shingle roofs, citing (quoting):
The authors point out that
NRCA, Building Code, & Manufacturers Recommend or Require Felt Underlayments on Shingle Roofs
Booth & Roberts and other sources also report that underlayment is required or recommended below shingles. [Reference numbers are to references in the cited document.]
Five Best-Practices Reasons for Using Roofing Felt Underlayment
Best Practices Guide to Residential Construction lists five good reasons to install roofing underlayment:
The roof deck should be sound and level before laying the
underlayment. Fifteen-pound or heavier felt underlayment
Underlayment On Standard Slopes where Asphalt Roof Shingles are Installed
As stated in Best Practices Guide to Residential Construction:
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 at left).
Secure each shingle 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.
Underlayment Application of Asphalt Shingles on Low Slope Roofs
Continuing from from Best Practices Guide to Residential Construction:
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.
Eaves Flashing Recommendations for Asphalt Shingle Roofs
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,” page 97 in the printed text Best Practices Guide to Residential Construction).
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 at left).
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.
Adapted/paraphrased with permission from Best Practices Guide to Residential Construction.
Inadequacy of Roofing Felt as a Vapor Barrier for Asphalt Shingles in Hot Humid Climates?
A Building Sciences Corporation report also elaborates the usefulness of placing a vapor barrier on the roof deck below shingles in hot humid climates. BSC points out that: [some paraphrasing -DF]
[OPINION-DF: from exterior roof inspections at all times of day and seasons, we have not observed this time-related morning roof shingle buckling in the Northeastern U.S. nor in Florida, nor the Southwest, though the authors report the phenomenon. It is possible that the authors are not quite correct that daily buckling and relaxing of roof shingles can be ignored on a vented roof as harmless, since certainly the product is expected to remain flat, and flexing daily might reduce its anticipated wear life.]
[OPINION-DF: we argue at ROOF VENTILATION SPECIFICATIONS that un-vented roofs are not a best building method in any climate.]
[QUESTION-DF: we note that the test chamber constructed by BSC was itself in an enclosed, air-conditioned space, and that the underside of the test chamber roof was at least in part exposed to the air conditioning. It seems possible that the reduced humidity and lower temperatures on the "interior-side" of the test roof may have contributed to moisture behaviors that vary from what occurs in the field. Attics and under-roof spaces such as in an un-vented "hot roof" cathedral ceiling are certainly not exposed to cool dry conditioned air. BSC may have addressed this concern but we did not find it in the referenced article.
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