Roof Flashing Details
Overview: Essential Knowledge
Residential roofing is typically made up of a multitude of materials andsurfaces whose primary task is to maintain a barrier between the interior and the weather.The most pervasive and difficult weather element to control is water. Roof flashing isusually the last line of defense in the battle against water penetration.
Flashing forms the intersections and terminations of roofing systems and surfaces, tothwart water penetration. The most common locations for roof flashing are at valleys,chimneys, roof penetrations, eaves, rakes, skylights, ridges, and at roof-to-wallintersections.
Flashing must be configured to resist the three mechanisms of water penetration:gravity, surface tension, and wind pressure. To achieve this, flashing can be lappedshingle style, soldered or sealed to function as a continuous surface, or can beconfigured with a non-continuous profile to defeat water surface tension.
Flashing materials must be durable, low in maintenance requirements, weather resistant,able to accommodate movement and be compatible with adjacent materials. Common modes offailure include exposure to salt air, excessive heat, acid rain, heavy snows, and scouringwinds.
Traditional materials and methods of installing flashing produce some of the longestlasting of building systems components. Those methods do, however, require experience andare time consuming.
Newer membrane materials and modern sealants are available that complement time-testedtechniques, but, regardless of the methods and materials employed, the basic principles ofroof flashing must still be adhered to, and the three water penetration mechanisms must beovercome.
Valley Details: A Visual Primer
A) OPEN VALLEY
The detail shows a typical open flashing for a shingle or slate roof. Two differentcants are illustrated. The cant strip can also be constructed as shown in Detail D. Theshingles or slate must lap the flashing at least 6 inches.
B) EQUAL SLOPES, UNEQUAL WATER FLOW
Where unequal water flow is expected, a baffle, 1 1 /2 inches high, should be installedas shown to prevent water of higher velocity from forcing its way past the opposite edgeof the valley flashing. The baffle can also be constructed as shown in Detail D.
C) CLOSED VALLEY
Intersecting roofs using a closed valley must have the same slopes so that the shinglebutts line up at the valley intersection. For roof pitches of 6" or more per foot theflashing extends at least 9" under the roof covering on each side. For roof pitchesless than 6 inches per foot the flashing extends at least 12 inches.
D) UNEQUAL SLOPES
This condition requires a baffle for the same reason as Detail B. it canbe constructed as shown in either detail. This detail also shows a different cant strip.Other methods of raising the shingles away from the copper are shown in Details A and B.
Techniques, Materials & Tools
Roof flashing materials can be classified into two primary groups: membrane and sheetmetal. Ice and water barriers and roll roofing are membranes.
The most typical sheet metal flashing materials are aluminum, copper, lead-coatedcopper, lead, stainless steel, galvanized steel, zinc, and Galvalume.
Both sheet metal and membrane flashing are available unformed or, for some particularapplications, in pre-formed configurations.
1. REPAIR EXISTING FLASHING
Small areas of loose, bent, split, corroded, or otherwise deficient flashing can oftenbe reinstalled, permanently patched with similar material, or can be replaced with newflashing. Asphaltic patching material is adequate for asphatic roofs, but is notrecommended for metal flashing because it will break down from ultraviolet (UV) exposureand movement of the metal.
It is also unsightly and an indication of poor maintenance. If large sections offlashing have deteriorated or have become loose or disengaged, it is time to remove theroofing material and install new membranes and flashing.
Repairs are cost-effective over small areas.
If large areas need replacement, or if serious leaks develop, postponing replacement may cause damage to the building’s structural elements or finishes.
2. INSTALL NEW COPPER OR LEAD-COATED COPPER FLASHING
Copper flashing is one of the most durable of roof flashing materials. It is also oneof the more expensive.
Typical applications include chimney flashing, valley flashing on tile, wood shake, orslate roofs, as base, step and counter flashing at roof to sidewall intersections, or asexposed or concealed ridge and hip flashing. Fabricating and installing copper flashing isa learned skill at one time reserved for experienced craftsmen. Although the material isrelatively soft and malleable, the techniques required to plan, cut, shape, fabricate, andinstall complicated shapes take practice and patience.
Copper can be fabricated into rigid, continuous custom shapes such as chimney cricketsor other special configurations. For step flashing roof to sidewall applications, copperis available in standard 5 by 7-inches pieces. The Copper Development Association (CDA)and individual manufacturers have excellent manuals of recommended flashing details. TheAssociation maintains that acid rain and the acid from red cedar shingles are not aproblem when water is not allowed to stand and when cant strips are used to raise theshingles off the copper surface (see Fig. 1 for recommended details).
The CDA recommends a minimum of 16 oz. plain or lead coated copper be used for valleyflashings and 20 oz. when slate or tile is used for the roof material. Lead-coated copperis considered the premiere flashing material in northern and maritime climates because itcombines copper’s durability with lead’s resistance to acid rain andcharacteristic of not staining adjacent materials.
With proper installation, can be one of the most durable of all exterior building components. Soldering joints and intersections is relatively easy, allowing for the formation of permanent, three-dimensional, continuous shapes. Copper is a relatively pliant metal, is easy to form and work, and ages to a familiar protective green patina. At least one manufacturer, Revere Copper Products, Inc. makes a pre-patinated copper flashing for those unwilling to wait.
A relatively costly material. Labor costs are higher than other flashing materials. Unless roofs are properly detailed with overhangs and drip edges, rain water run-off from copper flashing can stain adjacent materials.
3. INSTALL NEW ALUMINUM FLASHING
Aluminum is a versatile and durable material appropriate for many roof flashingapplications including chimney flashing, valley flashing, step flashing, base flashing,and counter flashing. Aluminum comes in rolls, sheets, and pre-formed shapes for specificapplications such as drip edges.
Aluminum comes in standard (mill) finish and factory paint finishes with white andbronze the most common. Aluminum may be left exposed to the elements with or without anapplied finish, but anodized or painted material will last substantially longer.
Coil stock 24 inches wide is available in about 30 standard colors for aluminum sidingand trim cover applications. Mill or paint finished aluminum readily accepts field appliedpaint giving flexibility to desired finishes. Available in thickness from .016 to .032inch. Thicker material will last longer in salty, acidic, or polluted environments.
Unlike copper, field soldering of aluminum is not practical. In order to createwatertight seams, joints must be lapped, mechanically fastened, and caulked when necessarywith high performance sealants. Fasten aluminum flashing to the framing with aluminumnails to avoid galvanic action between dissimilar materials (such as the aluminum flashingand steel nails), which can cause deterioration.
Aluminum is in the mid-cost range of metal flashing materials. A soft, workable material that forms easily and holds its shape well. Preformed aluminum drip edges and other related components speed installation and assure effective results.
Uncoated aluminum should not be placed in direct contact with concrete, mortar, or other cement-containing materials including fiber-cement siding and trim. The alkalinity of those materials will corrode bare aluminum. Creating water-tight joints and laps requires experience, skill, and high quality sealants.
4. INSTALL NEW GALVANIZED STEEL FLASHING
Galvanized steel flashing is an economical material made by coating sheet steel with alayer of zinc alloy, either through electroplating or hot dipping.
Typical gauges are 32 ga. (.010 inch) for roll product and 28 ga. (.015 inch) forpre-formed shapes. Roll flashing is available in widths up to 48 inches, usually in50-foot lengths. Galvanized nails should be used to attach galvanized sheet steel to theframe structure, as dissimilar metal fasteners can cause corrosion.
Contact with green lumber and treated lumber should be avoided, because the chemicalsin treated lumber can react with the steel, and green lumber has high moisture levelswhich can lead to rusting. Attachments must be mechanical, such as crimping, or throughthe use of sealants, or both. Choose only high performing exterior grade sealants. Typicalapplications include valley flashing, base flashing, counter flashing, and chimneyflashing. Common pre-formed shapes include "J" flashing for roof to sidewallintersections, drip edges, 90-degree bent base flashing, and shingle base flashing. Theseare normally available in 10-foot lengths.
Lowest first cost of all metal flashing. A rigid material able to be formed into permanent three-dimensional shapes. Will accept paint in order to disguise the raw silver sheen at exposed applications. However, the thin oil coating on the steel must be removed before painting and primers designed for galvanized steel should be used. Preformed shapes and the 10-foot lengths make for quick, effective installation.
In its raw, unpainted form galvanized steel is the least durable of the metal flashing materials. In harsh environments, corrosion may occur in less than 15 years. Galvanized steel is a stiff relatively non-malleable material and is somewhat difficult to work with and form. Cannot be field soldered when forming custom fabrications as the required acid wash and heating process will damage the coating, exposing raw steel. Not cost-effective when used with long lasting roofing materials such as slate and tile.
5. INSTALL NEW GALVALUME SHEET METAL FLASHING
Galvalume is a product similar to galvanized steel developed by Bethlehem Steelapproximately 25 years ago. Where galvanized steel has a hot-dip coating of zinc,Galvalume™ is hot-dipped with an alloy consisting of 55 percent aluminum and 45percent zinc by volume.
By weight, aluminum makes up 80 percent of the coating. Galvalume™ is available inrolls 24 to 48 inches in width. Mechanical fasteners and sealants are typically requiredfor field fabrications. Choose only high performing, exterior grade sealants.
An economical material similar in cost to galvanized steel, Galvalume™ is considerably more durable, with approximately twice the service life of galvanized. Galvalume™ readily accepts paint finishes, further increasing durability. A rigid material that can be formed into permanent three-dimensional shapes.
Galvalume™ should not be allowed to contact concrete foundations, masonry chimneys, or cement board siding, because these materials hold moisture which can lead to deterioration of the flashings. Contact with green lumber, treated lumber, copper, or lead should also be avoided, because of reasons mentioned in paragraph no. 4 above, and because of the contact of dissimilar metals which can lead to destructive galvanic action. Generally available in coil form only. Preformed shapes such as drip edges etc. not generally available. Stiffness and rigidity of the material makes site braking and fabrication difficult; shop prefabrication is more practical. Galvalume™ should not be field soldered as the required acid wash and heat process will damage the coating, exposing raw steel.
6. INSTALL NEW STAINLESS STEEL FLASHING
Stainless steel is a very durable flashing material particularly suited for harsh,corrosive, environments.
It is available in rolls of 18 and 24-inch widths as well as sheet stock. Typicalgauges are 18 and 24 ga. Typical applications include valley flashing, base flashing, andcounter flashing. Stainless steel is generally not available in pre-formed shapes.Material costs for stainless steel sheet stock is roughly comparable with copper, makingit a premium priced material. Stainless steel nails are recommended for installation.
Of the most common roof flashing materials, stainless steel is the most durable and least affected by environmental corrosives such as acid rain or salt spray. Strong and rigid, stainless steel can be fabricated into complex shapes. It is a monolithic material which, when cut, maintains its corrosion resistance at the exposed edge. Stainless steel is not affected by contact with masonry mortar or concrete and can be field soldered using a special solder after acid etching of the surfaces to be adhered.
Although material costs for stainless steel flashing are comparable to copper, labor and installation are usually higher. Stainless steel is a very stiff, rigid material and is difficult, and often impractical, to bend and shape using a typical roofer’s brake. Shop prefabrication is often required.
7. INSTALL RHEINZINK FLASHING
Rheinzink is a metal flashing and roofing material comprising 99.99 percent high-gradezinc with 1 percent copper and 1 percent titanium alloys.
Zinc has been used as flashing in Europe since the early 1800s; Rheinzink wasintroduced into the U.S. in 1992. It handles and performs much like copper, but develops anatural blue-grey, grey-green color through weathering, or it can be obtainedpre-patinated. Available in sheet or roll form in .027 inch (24 ga.) and .031 inch (22ga.) thickness for flashing use.
Easily worked into complex shapes. Can be soldered. Manufacturer claims a lifespan of 100 years+ if properly detailed. Self-healing patina. Can be pre-patinated. Competitive in price with copper and less expensive than lead-coated copper. Inert material, will not leach chemicals and is recyclable. Will not stain adjacent materials. Extensive specification and detailing information available.
Underside of the metal requires ventilation to allow protective patination to develop. If the underside is allowed to stay damp, white rust and corrosion can reduce service life severely. Requires proper detailing - the underside must be protected by bituminous sheet material against: alkaline influence (e.g., fresh concrete or mortar); acid reacting antifreeze agents; harmful influence of wood preservative. Contact with copper should be avoided.
8. INSTALL NEW LEAD FLASHING
Lead is one of the oldest flashing materials. It is durable and malleable, making it afavorite for use as cap flashings, in complex intersections, and with materials that havecomplicated profiles such as clay and concrete tiles.
Available in rolls from 6 to 20 inches wide, typically in 2.5 pounds/square foot.(0.0391") and 3 pound/square foot (0.0468 inch) weights.
The use of lead has fallen off as a flashing material because of the concern over leadpoisoning. Whereas lead can be used safely with appropriate handling techniques,precautions should be taken to avoid inhaling lead dust or fumes and to avoidhand-to-mouth transfer when eating or smoking. More detailed suggestions are availablefrom the Lead Industries Association (LIA) (see Further Reading).
Inert material relatively unaffected by salt and acid rain. Easily formed into complex shapes. Easily cut and soldered, long service life.
A relatively soft material that can tear, especially at right angle cuts. Can fatigue when fastened on all sides. Must be used with caution, requires special handling procedures.
9. INSTALL NEW ROLL ROOF FLASHING
Mineral surfaced roll roofing is an economical flashing material for some roof valleyinstallations.
When installed in conjunction with asphalt roofing shingles, either three-tab orlaminated, roll roofing valley flashing can be a quick, relatively durable installationwith the ability to remain functional for 15 to 25 years depending on location, solarorientation, and the quality of the material and installation. The recommendedinstallation involves two layers.
The base layer, a minimum of 18 inches wide, is applied mineral surface down, and thesurface layer, 36 inches minimum width, is installed mineral surface up (Fig. 2). Thematerial comes in roll widths of 18 and 36 inches, with 36 inches being most common.Mineral surface roll roofing is available in colors to approximate asphalt shingle colorsallowing for a more continuous appearance.
The valleys may be installed continuously with no need for laps and seams as expansionand contraction length wise is not a large factor.
Recommended fasteners are hot dipped galvanized or aluminum roofing nails with minimum12 gauge shanks and 3 /8 inch heads. The roll roof valley flashing should be considered apart of the roofing system and should be replaced when routine re-roofing is required.
An economical material which offers quick, simple installation, with the ability to last the life of the asphalt single roof system. No special skills or tools are required for installation.
Mineral surface roll roofing is appropriate for use in valleys only and with asphalt roofing products. Roll roofing will tend to shrink somewhat in width causing the material to lift from the base of the valley, leaving a void beneath it, which is susceptible to puncture (Fig. 3). Valley flashing of this type should never be walked on, and other impacts should be protected against. Roll roofing is not self sealing and is increasingly being replaced by ice and water barriers in colder climates.
FIGURE 2: ROLL ROOF VALLEYFLASHING
FIGURE 3: VALLEY CONSTRUCTED OFROLL ROOFING
10. INSTALL NEW ICE AND WATER BARRIER MEMBRANE
Ice and water barriers are relative newcomers to the world of flashing materials.
Initially intended to combat ice damming at the eaves of sloped roofs and inhibitdamage from wind-driven rain, they are now also used in conjunction with more traditionalflashing materials as an additional line of defense against water entry at chimneys,valleys, skylights, and at other roof penetrations.
Some manufactures also recommend full surface coverage below the roof shingles at lowslope applications. Ice and water barriers can be installed under all standard sloped roofmaterials. The materials are easy to work with even for the first time installer. Ice andwater barriers are generally self adhering and applied directly to roof decking after theremoval of the release paper.
Manufacturers claim that they self seal around nail and fastener penetrations. The mostcommon material compositions are modified bitumen with a granular surface, modifiedbitumen with a polyethylene surface, and rubberized asphalt with a polyethylene surface.Some membranes are available in roll form, for miscellaneous flashing uses.
Relatively inexpensive protection against water penetration from ice damming. Can provide an additional measure of protection against wind-blown rain in high-wind areas.
Some types of ice and water barriers should not be installed in hot climates, which can cause heat degradation, or under dark roofs such as slate, because high summer roof temperatures (up to 180F) can cause membrane to bleed. Ice and water barriers can conceal poor detailing and workmanship which may cause later problems. Can degrade when exposed to ultraviolet light.
Take It Further: Reading List & Supplier Info
- Architectural Sheet Metal Manual, Sheet Metal and Air Conditioning Contractor’s National Association (SMACNA).
- Copper in Architecture, Copper Development Association, Inc.
- Copper and Common Sense, Revere Copper Products, Inc., P.O. Box 300, Rome, NY 13442–0300.
- Flashings and Weatherings, Lead Industries Association, www.leadinfo.com.
- Gutters, Flashing and Roof Details video, Copper Development Association, Inc.
- Old House Journal - Guide to Restoration, Old House Journal, 1992.
- Roofing and Cladding; and Working Safely With Sheet Lead -- Lead Industries Association, www.leadinfo.com.
- Roofing and Flashing Problems, Canada Mortgage and Housing Corporation, rev. 1995.
- Steep Roofing Manual, National Roofing Contractors Association, 1996.
- SSINA Designer Handbook and Standard Practices for: Stainless Steel Roofing, Fascias and Copings, Specialty Steel Industry of NA, February 1995.
- Troubleshooting Guide to Residential Construction, Journal of Light Construction, 1997.
- Alcoa Building Products, Box 3900, Peoria, IL 61612; 800–962–6973, www.alcoahomes.com (aluminum building products).
- Bethlehem Steel Corporation; Bethlehem, PA 18016; 800–352–5700, www.bethsteel.com/divisions/sparrow/ spgalval.html (information on galvanized and Galvalume™ products).
- H. Bixon & Co., P.O. Box 1198, New Haven, CT 06505; 203–777–7445 (supplier of sheet lead).
- Heckman Industries, 405 Spruce Street, Mill Valley, CA 94941; 800–841–0066 (SBS rubberized asphalt in roll form).
- Revere Copper Products, Inc., P.O. Box 300, Rome, NY 13442–0300; 800–950–1776, www.reverecopper.com (pre-patinated copper and other copper products).
- Rheinzink Canada Ltd., 4560 Dawson Street, Burnaby, British Columbia, Canada V5C 4C; 604–291–6171; firstname.lastname@example.org (zinc flashing, roofing, siding and gutter systems).
- Specialty Steel Industry of North America (SSINA), 3050 K Street, N.W., Washington, DC 20007; 800?982?0355; www.ssina.com (information on stainless steel).
- Tamlyn and Sons; 10406 Cash Road, Stafford, TX 77477; 800–334–1676; www.tamlyn.com (roofing accessories).
- W.R. Grace & Co., 62 Whittemore Ave., Cambridge, MA 02140; 800–444–6459; www.gcp-grace.com (ice and water barriers.
This story is excerpted from The Rehab Guide:Roofs , one in a series of guidebooks produced by the U.S. Department of Housing andUrban Development (HUD) to keep the design and construction industry abreast ofinnovations and state-of-the-art materials and practices in home rehabilitation.
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