Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
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 DORMER TYPES - PHOTO GUIDE
ROOF INSPECTION SAFETY & LIMITS
ROOF JOB PROBLEMS, RESOLVING
ROOF LEAK DIAGNOSIS & REPAIR
ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
ROOFING TILE SHAPES & PROFILES
ROOFING UNDERLAYMENT BEST PRACTICES
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
ZINC METAL ROOFING
Requirements & recommendations for use of underlayment on asphalt shingle roofs & on low slope roofs. 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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
[Click to enlarge any image]
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
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.
According to 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). 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.)
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
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 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.
-- Adapted with permission from Best Practices Guide to Residential Construction.
Roofing Felt Underlayment Installation Position
Reader Question: is it ok for roofing felt to run at right angles to the roof eaves?
2/10/2014 - Jenn Bullington said:
I live in Albany, New York and recently had a new roof put on my house. It has come to my attention (after the fact) that the roofer installed the tar paper/underlayment material vertically rather than horizontally. Is this against code? Is this something that should be addressed with my roofer and should I pursue action before a problem occurs? I'm worried. Thanks!
Roofing underlayment is specified in manufacturer's installation instructions and thus is indirectly require by building codes (stating that the manufacturer's installation instructions should be followed). Quoting from UNDERLAYMENT REQUIREMENTS on ROOFS - home
But it is worth noting that some building codes such as the Canadian CBC do not require full roof underlayment below asphalt shingles. So at least in those cases the felt positioning question does not arise.
I'm not sure building codes describe underlayment positioning, but indirectly that installation probably is in violation of the best practices guides & probably the roofing manufacturer's instructions in that at least all the refs we have show placement parallel to eaves and with upper courses lower edges overlapping lower course top edges.
And in my OPINION, because the felt is punctured thousands of times by the roofing nails that fasten down roof shingles, its ability to protect a home from leaks after the shingles have been installed is reduced significantly anyway.
In fact a worse mistake than running felt at right angles to the eaves is probably running felt lapped "wrong way" with upper course lower edges underneath rather than on top of the upper edge of the courses below. If some dope makes that mistake and if there is ever a shingle blow-off and water runs underneath the remaining shingles, a horizontal felt edge lap mistake will do a great job of sending nearly ALL of the leaked water underneath the felt. Still nothing much may happen until that water hits an open butt joint between two sections of roof sheathing. Then it may come pouring in at that opening.
Watch out: What you should do before any arguing with your roofing contractor is give the manufacturer of your roofing shingles a call, speak to the warranty department, and ask if the position of roofing felt as we've discussed affects the shingle warranty. It might. I suspect that roof covering manufacturers are thrilled when a product failure claim can be blamed on improper installation - which is in my opinion often the case. Put another way, if the roofer was inexperienced or uninformed enough to do something funny with the felt, one wonders what other mistakes were made: improper nailing for example.
Roof Underlayment Requirements for Wood Shingles, Shakes, or Clay Tiles
Continue reading at STANDARDS for ROOFING or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs) about using roofing felt or undelayments
Questions & answers or comments about the requirement for and installation of underlayment on asphalt roof shingles and similar materials. .
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References