FLASHING
Flashing a structure involves installation of some type of sheet material on or within a building, to provide protection against intrusion of rain or other source of water. A lateral benefit of flashing is additional seal against penetration of unwanted air from outside, or loss of heated or chilled air from within a structure. Flashing material may consist of:
- sheet metal such as copper, coated steel or lead;
- unperforated asphalt-saturated felt;
- heavy gauge weather-resistant building paper;
- mineral-surfaced roll roofing;
- plastic-reinforced paper;
- or other material which will deflect water throughout the service life of the building.
The Western Conference of Lathing and Plastering Institutes has adopted the following definition of flashing material, although flashing material is not limited to this particular classification, as I have noted above.
"Material for flashing shall be barrier coated reinforced flashing material and shall provide four (4) hours minimum protection from water penetration when tested in accordance with ASTM D-779. Flashing material shall carry continuous identification. Sealant shall be butyl to comply with Federal Specification TT-S-1657."
It is beneficial if material used for flashing resist curling, shrinking or undue wrinkling when exposed to sunlight, rain and wind, prior to being covered with building paper, lath and plaster; redwood siding; roofing or other covering.
No one should misconstrue application of building paper or felt over exterior walls as an act of flashing. These are separate functions, serving different purposes, and both installations must be accomplished to protect buildings against intrusion of rain or other source of water.
Flashing is installed at:
- valleys, hips, rakes and eaves of roofs;
- around all plumbing features such as vent stacks which project through roofs;
- around all mechanical equipment, and supports for the same, that rest on a roof surface;
- around supports for solar systems installed on roofs;
- at the juncture of parapet walls with a roof; - across the top of all parapet walls, underneath the exterior decorative and protective treatment of plaster, wood or other covering;
- at the juncture of walls of a clerestory with a lower roof;
Note: This admonition refers to flashing a clerestory which is to be installed by a roofer, as well as flashing behind exterior wall covering, which is installed by the builder.
- at the juncture of dormers with a roof;
- at the juncture of a chimney with roof and walls;
Note: Protective building paper or felt should be in place on a wall prior to start of construction of a chimney.
- at the juncture of the top of a wall with the roof. The juncture of exterior wall cladding of wood, plaster, masonry veneer or metal with the underside of a roof overhang must be sealed, flashed and counter-flashed in such a manner as to prevent intrusion of wind-driven rain;
- at any other location about a roof where protection against intrusion of water is needed or desirable;
- behind implantations attached to the top or sides of exterior walls;
- completely surrounding attic vents, and vents for clothes dryers;
- at the entry line of beams, ledges, pipes or other fixtures which penetrate walls;
- surrounding all four sides of windows exposed to weather;
Note: Some builders are unaware of the need to flash across the header of windows, or immediately below the sill.
- beside the jambs and across the header of all doors exposed to weather;
Note: This admonition applies to patio doors and regular doors in homes as well as equipment, service and personnel doors installed in commercial and industrial buildings.
- across the top of all modesty walls of balconies, installed underneath the exterior decorative and protective covering of plaster, wood, masonry, metal or other material;
- around electrical and plumbing services penetrations that pierce exterior walls;
Note: If, for some persuasive reason, flashing cannot be installed around these types of service penetration, a continuous and generous bead of caulking must be buttered around the penetration, before weather-barrier building paper or saturated felt is applied, to affect good seal. Sealant must be placed on the inward side of exterior wall covering, prior to placement of the cladding.
- at the juncture of the inner edge of a balcony deck with the wall of a building;
Note: Additionally, decks of balconies must be waterproofed and adequate provision must be designed and built into the balcony to permit drainage of water from the deck. This would include proper slope of the deck toward drainage exits; as well as installation of scuppers which have been placed and sealed properly, and are of sufficient size to accommodate maximum expected flow of water.
- at the juncture of balcony railings with an exterior wall, or where planks which form the top of modesty or wing walls, or other structural, protective or decorative members, penetrate exterior wall covering;
Note: It is critical that a generous amount of caulking compound be buttered entirely and continuously around any such penetrating beam or other member, just within the alignment, and prior to installation of, the future exterior wall covering. The purpose of this treatment is to prevent wind-blown rain from intruding into the wall system at this point. Flashing and a complete bead of sealant should be placed behind exterior wall covering around each such penetrating section of wood or metal.
In summary, flashing must completely surround all openings in exterior walls in accordance with a statement in the Uniform Building Code, quoted below:
"Section 1707 (b) FLASHING ANO COUNTERFLASHING: Exterior openings exposed to the weather shall be flashed in such a manner as to make them weatherproof."
The Uniform Building Code also states, in Section 1707 (c):
"WATERPROOFING WEATHER-EXPOSED AREAS:
Balconies, landings, exterior stairways and similar surfaces exposed to the weather and sealed underneath shall be waterproofed." SEALANTS
Architects, builders and installers of windows, doors and vents should appreciate the need for a continuous bead of sealant in conjunction with fully and properly installed flashing, in some instances, surrounding penetrations of walls, such as windows, doors, vents and at beam or railing penetrations. Typical examples relate to installation of windows and sliding glass doors; two fairly common locations of intrusion of water, especially wind-blown rain, into buildings. These fixtures need full and proper lashing around the entire perimeter of the penetration, plus a continuous bead of sealant completely around the opening, behind the nailing fin of the window or sliding glass door, with special emphasis at the two upper corners of window and door frames.
If the design or construction of corners of windows and sliding glass door frames is not adequate for the purpose of blocking passage of water into the wall system, the builder or window subcontractor should embed the top two corners in a full bed of caulking compound.
Use of sealant in conjunction with flashing, and sometimes without flashing, is an important factor in preventing intrusion of water into buildings, and most builders use caulking beneficially. Several types of sealants are available, and some are listed below.
Silicone (clear and pigmented)
Butyl Rubber
Thiokol
Styrene Butadiene
It would be prudent for a potential user to seek advice from a technically qualified person, such as someone associated with a manufacturer of sealants, concerning choice and application of caulking compound for a specific purpose and set of conditions.
Additionally, two continuous beads of sealant must be laid on the plate underneath door sills, the full length of the sill, and the sill pressed firmly against this caulking, to prevent intrusion of water below sliding glass and regular doors.
A description of application of flashing and sealant during the course of installation of entrant fixtures follows.
APPLICATION OF FLASHING
A suitable, but not the only method of flashing around windows, patio doors and other penetrations of walls is provided below.
Asphalt-saturated felt or weather-resistant building paper is attached by the builder or window subcontractor to the framed opening, where windows or other fixtures are to be inserted, in such a manner as to completely encompass the edges of the framed opening. Felt or building paper is to be installed shingle-fashion, with upper sheets lapping over lower sheets. A continuous bead of caulking compound is to be applied over the felt or building paper around the entire perimeter of the opening, directly underneath the future location of the nailing flange. The nailing flange of the window, sliding glass door or vent is pressed into the bead of sealant while the caulking compound is still fresh and free of surface film, to achieve absolute seal behind the window or other penetration.
After the window has been set in the opening, counter-flashing is installed to completely surround the window in the following manner. I emphasize the phrase "to completely surround" because some builders and carpenters do not believe that windows need to be flashed across the bottom, below the sill, while others have the misconception that there is no need to install flashing across the header. Almost all seem to agree that flashing should be placed beside the jambs of windows, doors and vents.
Builders should frame openings sufficiently large to permit windows, patio doors and other fixtures and entrants to be inserted, plumbed and leveled without forcing or springing the frame of the window or door while, at the same time, allowing sufficient overlap of a nailing flange over framing to accomplish complete seal. A suitable width of nailing flange for metal frame windows and doors is one and one-half inches, but some units do not have this preferred width. Mitered corners of metal frames of windows and doors must be welded fully and strongly enough to resist future opening, if sprung out of shape by mishandling, otherwise the unit could allow passage of water. Workmen should avoid throwing window and door frames off delivery trucks, or other damage to units.
Sliding glass doors introduce a special problem of intrusion of water, especially wind-blown rain, because many doors are shipped disassembled and put together on the job site. Assembly may be done by people who have not received or have not heeded instructions from the manufacturer, or who have failed to assemble or to install the unit correctly.
Windows are to be set into framed openings by placing the top of the window frame against the framing header. A modicum of looseness should exist at jambs and sill. A window frame is not to be forced, racked or strained in order that it be installed in the opening, if a window or sliding glass door is dropped prior to installation, the unit must be examined carefully for possible damage which might invite intrusion of water.
SILL FLASHING
An eight or nine inch wide strip of flashing is laid across the nailing flange at the bottom or sill of the window, with the upper edge of the strip of flashing placed against the underneath side of the outward extrusion at the base of the window frame. The two ends of this horizontal bottom strip should extend eight or nine inches outward past the jambs of the window, on both sides. Only the top edge of this horizontal strip of flashing is to be secured permanently. The bottom edge and both ends should be tacked to prevent damage from curling and possible tearing in wind and sun, until building paper is installed later, at which time building paper will be inserted upward behind this bottom section of sill flashing, in accordance with standard procedure of weatherboard application. After insertion of felt or building paper behind the sill flashing, the entire area of applied flashing and building paper is to be stapled securely. In the case of a low-set window, it is good practice for the builder or window installer to apply building paper the full height from the bottom of the plate line to the bottom of the window sill, behind sill flashing, at the time the window is flashed.
JAMB FLASHING
The two vertical sections of flashing must be applied next. They should be cut sufficiently long so as to extend the width of the flashing (approximately eight or nine inches) above the top of the window and an equal distance below the sill, and they are to be installed over the horizontal strip of sill flashing.
HEADER FLASHING
The top horizontal section of flashing is to be applied last, and is to be cut sufficiently long so as to extend to the outer edge of both vertical strips of side flashing. The bottom edge of header flashing should be sealed against the full height of the outer face of the nailing flange with a continuous bead of caulking.
Building paper to be applied later will overlie the jamb and header flashing; therefore, the inner and outer edges of these three sections of flashing should be secured sufficiently well that future loss of water-shielding capability does not occur.
The continuous bead of sealant which is to be applied behind the nailing flange of windows and doors is not to be construed as a substitute for flashing. Some members of the Architectural Aluminum Manufacturers Association believe that windows and sliding glass doors should be caulked on the interior and exterior face of the nailing flange. I agree.
Flashing must be installed by the builder in such a manner that the lather can install building paper or the attached paper of paperback lath underneath sill flashing and on top of flashing installed at the sides and top of windows, sliding glass doors and other fixtures.
Flashing also must be installed downward over the top flange of weep screeds, horizontal control joints, Z-metal and other trim, to have the lower edge of flashing rest upon the horizontal projection of each piece of trim.
At least two layers of substantial building paper, asphalt-saturated felt or other unperforated sheet material, or one layer of valley tin, should be laid across the top of parapet walls, beneath coping, whether the coping is sheet metal, cement plaster, clay tiles, wood, masonry or other material. Under no circumstances should a fastener be driven vertically downward through this flashing surmounting parapet walls, to perforate the flashing and reduce its value. Fasteners must be driven horizontally through the skirting of the flashing which extends down each side of the parapet wall.
REPLACEMENT OF WINDOWS OR DOORS
Replacement of windows, patio doors or other fixtures which penetrate the exterior wall of a building, or insertion of new units where none existed in a wall, requires that flashing be inserted into the wall system behind the wood, brick veneer, cement plaster, metal or other exterior wall covering.
In the case of windows, flashing must be placed below the sill, upward beside both jambs and across the header, just as must be done in new construction. Many builders and window subcontractors prefer to install lightweight coated sheet steel or sheet copper for replacement work, rather than roofing felt, because the stiffness of metal facilitates insertion of flashing behind existing wall cladding.
Insertion of flashing between construction layers of existing walls must be done weatherboard fashion, not only in relation to other sections of newly-installed flashing, but in relation to existing building paper or other construction elements.
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