CAPPING PARAPET WALLS
Also applicable to wing walls, plastered frame, fences, balcony railings, plastered modesty walls, stair railings, plastered beams and buttresses.
Improper installation of metal caps at tops of parapet walls and similar structural elements has been found to be a factor which contributes to intrusion of rain into buildings. Examples of problems which sometimes are introduced during installation of sheet metal coping, and which must be considered during investigation of intrusion of water into buildings, are:
a. Failure to properly install at least one and in many cases two layers of asphalt-saturated felt, weather-resistant building paper or mineral-surface roll roofing fully across tops of walls beneath metal coping, as cross flashing;
b. failure to provide sufficient width of cross flashing described above, so that adequate lap of flashing over lath paper, which is to be installed later, will shield the building against entrance of rain water;
c. the mistake of driving fasteners vertically downward through the top of the weather-barrier membrane described in sub-section (a), rather than horizontally through side flaps of flashing. The reason for prohibition of driving fasteners vertically downward through flashing or coping is to avoid penetration through the protective weather-resistant membrane. Penetration of fasteners may negate value of flashing and coping;
d. failure to lap adjoining sections of sheet metal coping sufficiently, and in a direction opposite the direction from which rain generally is driven by wind;
e. failure to solder junctures of coping at corners of parapet walls;
f. failure to lap adjoining sections of sheet metal coping at least four inches and preferably six inches;
g. failure to bend enough sheet metal down both sides of walls to form skirting or flanges. Insufficient depth of side flaps on metal cap may allow intrusion of wind-blown rain underneath flanges of coping, to enter the building system;
h. failure to fit coping snugly against vertical side walls of buildings; and
i. failure to install coping AFTER application of side wall treatment, such as clay bricks, cement plaster, wood or metal siding, except in special cases to be mentioned later.
Numerous problems may develop when water enters the top of a structure. One disadvantage is that, when water contacts wood framing members or sheathing within the system, wood must be expected to expand and impose great and unnecessary stress against exterior wall treatment. Also, wood rot frequently forms in enclosed spaces when moisture is in contact with wood for an extended period of time, or when wood cycles through periods of wet and dry condition. It is good practice to cover tops of parapet walls, wing walls and similar construction features with a sheet metal cap or wood plank, if the walls are finished in wood, cement plaster (stucco), bricks or concrete blocks. Specification of metal or wood coping is intended to insure permanent, trouble-free protection against ingress of rain into a building. Flanges on the sides of sheet metal coping should fit snugly against and extend downward on walls sufficient distance to prevent ingress of rain under coping when strong winds blow. Wind that drives rain against walls creates eddy currents which may force rain upward behind metal coping.
It is desirable that side wings of coping be formed with a slight outward flair, a projecting lip, at bottom edges, to assist in diverting dirt-laden water away from vertical side walls, and thus reduce staining.
Coping should not be installed on tops of parapet walls until the final covering of vertical walls has been applied. An exception to this statement is applicable when metal coping is to be applied on top of a railing or wall which abuts a vertical plastered wall located above the railing. In that case, a short section of coping, properly flashed and sealed underneath, should be installed against the vertical substrate which will be the base for the lath and plaster membrane. A continuous bead of sealant must be applied in lateral direction across the top of flashing which protects the wall or railing, near the outer end of the short length of coping, and at the inner, or abutting, end of coping, as well as parallel with sides of the flashing which was laid across the top of the wall beneath the coping, and against which future lath and plaster will be applied.
Two layers of cross flashing must be installed beneath the short length of metal cap which, because of the special need described above, has to be installed prior to application of plaster. Also, side wings of this cross flashing must extend at least three inches below the bottom of the flange of metal coping, to permit insertion of lathing paper behind flashing, shingle-fashion.
In a situation such as described above, where coping must be installed prior to application of plaster, the plasterer must trowel mortar upward to fill the void between the flange of metal coping and the base wall behind the coping.
Sheet metal coping or gravel stop, wood planks or other material should not be installed on tops of parapet walls, wing walls or other construction features which are to have cement plaster applied to vertical side walls of the feature, until wooden or metal plaster stop has been installed at the top of the exterior wall and the full thickness of plaster has been applied. If sheet metal coping is installed on top of a wall before the full thickness of plaster has been applied to side walls, plaster cannot be carried upward to the proper height on the wall. If incorrect sequence of installation of a cap on a wall does occur, and sheet metal coping or gravel stop is installed prior to placement of plaster, it is my opinion that the superintendent for the builder has erred.
It is advisable that some protective covering, such as sheet plastic, asphalt-treated building paper or other suitable material be placed temporarily across the top of each wall to shed curing water which would be sprayed from a hose, or to deflect rain water, until the final cap has been placed. The protective covering should be placed in such a manner that it can be lifted by plasterers while they apply their material to side walls, to be draped over the outside walls again after application of each coat of plaster. The purpose of draping the tops of parapet walls in that manner is to prevent ingress of rain water, or curing water sprayed from a hose, into the system, which would adversely affect plywood sheathing, gypsum board backing or wood framing.
Proper installation of coping involves application of one and preferably two layers of weather-barrier membrane such as asphalt-saturated felt, mineral surface roll roofing or weather-resistant building paper across tops of parapet walls, railings or similar situations, as cross flashing. Flashing membranes must be free of perforations or tears, and must extend continuously across the top and down both sides a minimum distance of six -inches. Fasteners such as nails, staples or screws must not be driven vertically downward through the top of flashing, but must be driven horizontally through side flaps. Side wings of cross flashing must permit insertion of lathing paper shingle-fashion underneath the flap, for a vertical distance of at least two inches, free of obstructions.
Exposure of cross flashing to sunlight, vandalism, weather and mistreatment by other trades must be avoided. Proper scheduling of the various trades by the builder's superintendent will protect cross flashing as well as lath paper from excessive exposure to possible damage or deterioration, and thus will help insure integrity of the building.
In addition to protection of flashing from penetration, fasteners must not be driven vertically downward through the top of sheet metal coping! That incorrect practice may allow intrusion of water into the interior of the wall system. Fasteners must be driven horizontally through side flanges of metal coping.
The depth to which flanges of metal cap should extend downward on walls may vary with conditions, such as exposure of a wall to wind, strength of winds in the area, amount of rainfall and other factors, but must be a minimum of two and one-half inches, and frequently should be more. In some situations, minimum depth of side flanges of metal cap should be four or five inches.
Additional assurance against penetration of rain water, forced upward behind side wings of coping by wind-driven rain, is provided by a bead of sealant or strip of flexible foam plastic inserted into the space between the flange of coping and the base wall behind. This seal has the added advantage of blocking wind-blown rain from the space between flanges of coping and the back wall, where contact with water on metal may cause rust to form on the underneath side of metal coping. Rust-bearing water would stain exterior walls, if allowed to drip down the sides. The bead of caulking should be extruded just before installation of the metal cap, and wings of coping should be pressed against the bead of sealant to form an effective seal at the interface.
It is good procedure that sheet metal coping be installed with overlap of adjoining sections designed to shed water, rather than scoop wind-blown rain into the system. This protection is accomplished by lapping metal coping in the direction that rain normally is blown by wind in the vicinity of the building. This factor should be considered and carried out by the builder in the same manner that hip and ridge roofing shingles are installed on the ridge of a roof, to deflect rain in the same direction that prevailing winds blow the water.
AH coping installed at tops of walls, whether made of sheet metal, cement plaster, wood or masonry, must slope inward toward the roof, rather than be level, or even worse, slope outward toward exterior walls. Coping must slope inward to deflect soiled rain water, residing on top of the parapet, modesty wall or other feature, so that water may flow away from exterior walls where dirt may stain visible surfaces, and flow inward toward the roof, to exit via scuppers and downspouts, to ground level. Flat coping promotes ponding of water.
Situations exist where it is desirable that cement plaster which protects and beautifies walls should be continuous over the top of parapet walls, railings or fences. This can be done successfully. Typical examples of this type of construction are flat-roofed buildings designed in Spanish-American or Mediterranean style. In these cases, it is desirable, because of esthetic considerations, that the same material continue over the tops of walls. The following information is intended as guidance in specifying and installing this type of construction. The most important function is that tops of parapet walls be wrapped with two layers of unperforated, good quality, weather-resistant and tear-resistant flashing. This is extremely important, because proper application of layers of good building paper or roll roofing across the crest of parapets is the final and critical barrier to entrance of water. This article stressed development of a sure seal against entry of water, because, if water contacts wood members within the stair railing, parapet wall, wing wall or other feature, the wood will swell with great force and rupture the plaster. If rupturing should occur, the water problem would be compounded, and eventually the condition that frequently but incorrectly is called "dry rot" may be expected within the wall system. The prevention, of course, is to seal the top of the parapet properly at the time of construction. Double-wrapping tops of walls with two layers of un-perforated roll roofing or good quality building paper serves well in this respect.
The first or lower layer may be 15 pound asphalt-saturated felt. Grade B building paper or mineral-surface roll roofing. The protective layer to be installed over the first application may be mineral-surface roll roofing, saturated felt or Grade D building paper. The upper layer is to protect the lower sheet against ill-effects of sunlight, abrasion, weather and other damage, in addition to deflection of intruded water.
Two layers of building paper must be laid across the top of the wall and must extend down both sides a distance of at least six inches. IT IS MANDATORY THAT NO FASTENERS OF ANY KIND BE DRIVEN THROUGH BUILDING PAPER OR ROOFING ANYWHERE ON TOP OF A PARAPET WALL, RAILING OR SIMILAR FEATURE. All fasteners must be driven into sides of the parapet wall, at or below a line two inches below the top of the wall. If either layer of protective paper is torn or perforated, that layer must be rejected or covered with a replacement layer of unbroken building paper or roofing. Sections of building paper laid across the top of walls are to be overlapped weatherboard fashion along the wall. Sheets should overlap at least six inches or more in longitudinal direction, along the top of the wall.
Sequence of application follows. Weather resistive building paper and stucco netting or expanded metal lath shall be applied to vertical side walls. Paperback stucco mesh or paperback expanded metal lath may be used instead of separate components on the vertical wall, but not over the crest of the wall without first applying a separate un-perforated layer of good weather-resistive building paper or roofing fully across the crest of the parapet. After the last, or uppermost section of lath and paper backing has been applied to a vertical wall, two layers of cross flashing are to be laid across the crest of the parapet and downward over the top edge of the vertically-applied layer of paper backing attached to the wall below, shingle-fashion.
The uppermost edge of stucco mesh or metal lath on the vertical wall is not to be fastened until the building paper that covers the crest has been lapped over paper on side walls. The stucco mesh or lath is to be secured near, but not at the top of the wall, which will also secure the two layers of roofing or building paper that cover the balance of the parapet. A strip of self-furring stucco netting should be laid across the top of the parapet and attached by fasteners driven ONLY THROUGH OVERHANGING SIDES, AT LEAST TWO INCHES BELOW THE TOP OF THE PARAPET. The system is to be applied as paper-over-paper and wire mesh-over-wire mesh. It is suggested that paperback stucco mesh not be applied across the crest of the parapet. Instead, separate application of building paper or roofing felt and stucco mesh is preferred.
Additional assurance against intrusion of water at the plastered crest of parapet walls is obtained by forming an arch or ridge in the plaster which covers the top of the wall, or by creating a slope in a plane surface, with the slope directed inward toward the roof, to direct rainfall and dirty water onto the roof, toward scuppers and downspouts. Design or construction of level tops of parapet walls, plastered balcony railings, beams and tops of wing walls should be prohibited.
The texture on side walls may be deep, such as a coarse dash, heavy skip-trowel or other finish. Deep or sharply-defined texture should not be continued across the crest of walls. Instead, texture should be modified to avoid excessive entrapment of rain water in pockets of texture at the top. Plaster applied to tops of wing walls, parapet walls, modesty walls and similar features should not be troweled smooth. Plaster applied to tops of walls should be one inch thick.
The addition of a fortifying agent such as acrylic resin to cement plaster laid across the crest of parapet walls is beneficial. The benefit obtained is additional resistance to cracking plus improved resistance to absorption of water.
If a fortifying agent such as acrylic resin is not incorporated in plaster applied to the crest, it is advisable to treat the plaster on top of walls with a good sealing compound such as acrylic resin or sodium silicate. The sealant may be applied as soon as plaster has developed initial set, before dirt can collect on top of the wall.
Sheet metal capping should not be installed across the top of parapet walls immediately underneath portland cement plaster. Multiple layers of good quality roll roofing or moisture-resistant building paper serve well as protection against ingress of water.
It is good practice that corneraid be installed as reinforcement in vertical and horizontal corners of cement plaster, where appropriate. If corneraid is installed at upper horizontal edges of plastered parapet walls, wing walls, horizontal beams or other exposed plastered construction elements such as those named under the title of this technical article; nails, staples or screws should not be driven vertically downward into the top of the wall or beam, through building paper or roofing felt, to attach corneraid. If corneraid is installed at horizontal alignments, instead of stripping corners, fasteners must be driven horizontally through sides of corneraid and through the side flap of roofing felt or building paper membrane, to avoid perforation of the protective lamination which shields tops of walls and beams. The following procedure is suggested.
Corneraid will be installed at the top of and parallel with upper edges of walls or beams, on both sides. The "wall" legs (portion that will be attached in horizontal alignment to and be parallel with vertical side walls) of corneraid are to be nailed, stapled or screwed to the wall. After corneraid has been installed along both top edges of walls or beams, a hog ring or tie wire is to be employed to firmly join the two center legs, which lie on top of the wall or beam. Hog rings or tie wires must be spaced closely enough to prevent upward bowing of corneraid. The manufacturer of CornerAid produces corner reinforcement called "TIE-ON" CornerAid, which has been designed especially for application where tie wires or hog rings are to be used. The described procedure allows both the corneraid and weather-barrier flashing to deliver their full potential, in terms of benefit to ultimate quality.
Corneraid may be wire-tied to the top edge of vertically-applied stucco netting or metal lath, in addition to fasteners driven horizontally through sides.
Corner wires must be completely embedded in plaster to prevent future rust stains.
I discourage installation of corner bead on exterior portland cement plaster jobs, because of potential problems of delamination of plaster from flanges of beads, and oxidation of metal, which is followed by rust stains on exterior walls.
where one parapet wall butts against the side of another parapet, on a roof, the possibility of movement within the structure should be considered, and provision made for protection against water intrusion through a crack which might develop at that juncture.
If fasteners have been driven vertically downward through sheet metal coping and flashing, the cap should be replaced, in my opinion.
If it has been determined that rain enters a building at tops of parapet walls, wing walls, balcony railings or similar construction features, remedial work must be done to resolve the problem. The following suggestions are offered for consideration.
The existing metal cap must be removed in preparation for protection of walls against intrusion of rain water. If no cross flashing had been installed beneath the coping, to shield the building from rain, it is mandatory that flashing be installed in accordance with procedure described earlier in this article. Flashing may consist of roll roofing, treated building paper or mineral-surface roofing.
If original flashing was not installed properly, was perforated by fasteners driven through the top, torn or suffered deterioration, it must be overlaid with new, good quality flashing. One or two layers of weather-barrier must be placed over the top and sides of existing flashing. The weather-shield must be of sufficient width to overlap sidewall protection, shingle-fashion, at least two inches. New flashing is to be secured by fasteners driven only through sides, or bonded with asphalt emulsion.
A continuous bead of caulking should be applied behind the bottom edge of side flaps of replacement flashing, over existing protection, and the wings of new flashing pressed into the bead of caulking compound to affect good seal. he value of the suggested bead of sealant xtruded at the base of flaps is that caulking provides assurance to compensate for limited overlap of replacement flashing. Existing in-place plaster, brick veneer, wood or other surface treatment limits the depth to which overlapping new flashing can cover existing building paper on side walls.
An alternative procedure to application of the described continuous horizontal bead of sealant is application of asphalt emulsion over the juncture of new flashing to the existing shield. Asphalt emulsion should be brushed over the horizontal juncture in a thick lamination, sufficiently wide to cover both edges and block future entrance of wind-blown rain.
If nails were driven downward through tops of metal cap, it is my opinion that the coping should be replaced and the wall shielded with new flashing and new, un-perforated metal cap. New metal coping is to be attached with fasteners driven only through side flanges.
If wood or metal railings set on top of modesty walls of balconies were not flashed and sealed fully and effectively by carpenters at the juncture of railings with abutted vertical surfaces at the time of installation, and before plaster was applied to vertical exterior walls at and above the railings, rain water must be expected to enter buildings by being blown along the top of the railing and into the gap at the juncture of railing with vertical wall.
Repair of this unsatisfactory situation should involve, at minimum, careful cleaning of the gap between railing and vertical wall. which the railing abuts. Careful cleaning is necessary to achieve good seal of this opening against intrusion of water. Opposing faces of wood or metal railing and plaster must be scraped clean of accumulated soiling, otherwise caulking may not be effective. After the opening has been cleaned and vacuumed carefully, flexible foam plastic stripping should be packed against the rear of the gap. Top quality, weather-resistant and shrink-resistant caulking compound must be forced into the opening, to be pressed firmly against the strip of flexible foam plastic and to completely fill the gap from railing upward to overhanging wall membrane. The bead of sealant should be painted after placement, and periodically during the life of the building.
Caulking at all locations should be checked every few years for possible shrinkage of sealant, loss of bond to adjoining surfaces, or deterioration of caulking.
After exposure to sunlight, heat and weather, asphalt or resinous sealant, applied over tops of fasteners driven vertically downward through the top of coping installed on a parapet wall, and over junctures of sections of metal or wood coping, will dry, shrink, crack and lose value as a sealant. It is, therefore, necessary that the owner of a building remove deteriorated caulking, clean the base and then apply new sealant to those locations. The writer does not recommend that fasteners be driven downward through metal caps. He does, however, recognize that some workmen will do this, and that the resulting problem must receive lifetime sealing. It is well to spray aluminum paint over asphalt or resinous sealant which has been applied to coping, to protect the caulking for an extended period of time against adverse effect of sunlight, heat and weather.
Adherence to careful, workmanlike construction procedures during installation of coping on parapet walls and similar structural features is critical to protection of buildings against intrusion of water, particularly wind-blown rain. It has been found, on many construction projects, that failure to adhere to workmanlike procedure has caused intrusion of water into buildings. Good practice includes but is not limited to:
- installation of good quality cross flashing, properly installed;
- sufficient overlap of adjoining sheets of flashing;
- protection of flashing against damage;
- freedom from vertical fastening of flashing or coping;
- placement of caulking compound in locations where needed;
- snug fit of side flanges of metal coping to vertical wall surfaces;
- adequate protective overlap of adjacent sections of sheet metal coping;
- soldering of junctures of metal cap at inside and outside cornerss of parapet walls; and
- all other critical factors.
MODESTY WALLS AND STAIR RAILINGS
Situations exist on many modesty walls, stair railings, balcony railings and projecting beams which require special protection to prevent ingress of rain water, or water from melted snow or ice. The juncture of a horizontal or sloped construction feature with a vertical wall surface must receive particular attention from the architect and builder to avoid intrusion of water into the wall system at that junction. Preventative measures must be accomplished by the builder prior to installation of the final covering of the railing.
To prevent entrance of water at the described juncture, a galvanized steel or aluminum saddle with soldered or otherwise permanently sealed joints should be installed where the end of the rail, modesty wall or other feature meets a vertical wall. That is necessary whether the top of the rail is a wooden plank, cement plaster, quarry or ceramic tile, stone or other material. A section of mineral-surfaced roofing, heavy roofing felt, or several layers of asphalt-treated building paper may be applied in lieu of the metal saddle, to affect continuity of the rail with the vertical wall. Joinings in mineral-surfaced roofing, roofing felt or asphalt-treated building paper should be sealed against leakage by means of continuous beads of asphalt emulsion or asphalt cut-back. Bituthene or similar heavy-gauge flexible, waterproof sheeting may be used instead of roofing or saturated felt. The saddle must seal the juncture adequately against, intrusion of water. The saddle should have flanges which overlap the top and sides of the horizontal or sloped railing, and be sealed to flanges which lie flat against the vertical wall surfaces, as well as being sealed at the inside bend of the ninety-degree or other angle of the corner formed by the junction of the rail with the wall.
All of the above should have been accomplished prior to the time that a carpenter is directed to install a wood cap, or a plasterer to apply a plastered cap, or other material be installed at the top of the rail or wall.
Under no circumstances should flashing be applied to the horizontal or sloped wooden top plate of the railing or beam end, with lack of continuity, at the vertical wall surface. Instead, there must be a continuous saddle of metal, Bituthene, asphalt-saturated felt, mineral-surfaced roofing or other suitable material, properly sealed, installed at that location in a manner which will seal against intrusion of water.
The side flanges of that metal, Bituthene, asphalt-saturated felt, mineral-surfaced roofing or other material which forms the protective saddle must overlap vertically installed building paper on the side walls below the top of the railing or wall. The upper flanges of the saddle, installed against the vertical wall above the rail, must be overlapped by building paper which is applied to the upper portion of the wall at a later time. All lapping of components is to be done shingle-fashion.
It is my opinion that the builder should install the described metal, Bituthene, roll roofing or asphalt-saturated felt saddle when the railing or wall is framed by his carpenters, or shortly before the lathing crew arrives on the project.