WEEP SCREED INSTALLATION
Weep screeds are metal devices installed at the foundation plate line of exterior plaster walls if the walls are of stud framework. The screeds are intended to provide relief for the exit of rain water which might possibly intrude into a building at or near the roof, at vent pipes which pierce the roof, at a chimney, at or around windows or doors, or at other possible locations about a structure. Weep screeds serve the desirable purpose of allowing continuing downward passage of intruded water from the roof or elsewhere, to exit at the base of the plaster wall and drain away.
In addition to metal devices manufactured specifically to serve as weep screeds, plaster stop has typically been employed as weep screed in the Pacific Northwest for some years, as well as on a number of installations in California. Plaster stop has served well in that capacity.
Weep screed, per se, is manufactured with and without holes punched in the protruding lip below the vertical attachment flange. The author does not believe that holes punched in the angular protrusion are necessary to achieve good venting at the base of the wall.
Installation of weep screeds at foundation plate lines of framed, plastered walls is required by the Uniform Building Code. That requirement was first published in the 1973 Uniform Building Code. That -1973 edition stated: "A weep screed shall be provided at the foundation plate line on all exterior stud walls. The screed shall be of a type which will allow trapped water to drain to the exterior of the building." The requirement that weep screeds be installed at the foundation plate line of stud walls has been continued in successive editions of the UBC, and, on several occasions, the printed statement has been refined. The requirement is always incorporated in Chapter 47 of the UBC.
The 1982 edition of the UBC, in Section 4706(e), contains the following brief requirement in Chapter 47. "A weep screed shall be provided at or below the foundation plate line on all exterior stud walls. The screed shall be placed a minimum of 4 inches above grade and shall be of a type which will allow trapped water, to drain to the exterior of the building. The weather-resistive barrier and exterior lath shall cover and terminate on the attachment flange of the screed.
The 1985 edition of the UBC states, in Section 4706 (e) of Chapter 47, that: "A minimum 0.021-inch (No. 26 gauge) corrosion-resistant weep screed with a minimum vertical flange of 3's inches shall be provided at or below the foundation plate line on all exterior stud walls. The screed shall be placed a minimum of 4 inches above grade and shall be of a type which will allow trapped water to drain to the exterior of the building. The weather-resistive barrier and exterior lath shall cover and terminate on the attachment flange of the screed."
Subsequent to publication of the 1985 edition of the UBC, a proposal was offered to the I C B 0, which suggested consideration of several changes in Section 4706(e). One suggested change was that the decimal designation of 26 gauge be omitted, and the second suggestion was that the 3-1/2 inch height of the vertical flange be changed to 1-3/4 inches.
Concrete brick pavers, natural stone, ceramic or quarry tiles or other construction materials are not to be placed, subsequent to installation of lath and plaster on vertical walls, in a manner or at an elevation that would block the weep screed from providing drainage from behind the wall system. The architect, builder or superintendent for the builder should establish elevations for placement of weep screeds at locations which may have been specified by the architect other than those attached to the foundation plate line. In addition, the architect should specify the upper elevation of concrete, pavers or tiles, or decking material, prior to installation of lath and related trim sections. That information must be communicated to the lathing contractor prior to start of application of lath.
Note that the building code does not suggest installation of a weep screen on other than frame walls. The code does not require or suggest installation at elevations higher than the foundation plate line of a building. An architect or designer may elect to specify that a weep screed be installed at the base of a vertical stud wall which terminates at a balcony deck, at a flat or sloped roof which adjoins the wall, or elsewhere. If so, certain factors should be considered, and the height above the deck surface or roof, on an elevated wall, at which a weep screed is to be installed must be specified by the architect or the builder, and supervised by the builder. Occasionally, a plastering contractor may elect to voluntarily install a weep screed at elevations higher than the foundation plate line. If that is done, he should discuss the matter with the architect and/or builder, to have the proper elevation established by the designer or general contractor.
Unless instructed otherwise, a lather probably would attach the weep screed to the toe plate or bottom plate of the elevated framed wall above the deck or roof section, just as is done at the foundation plate line.
It may be a part of the design of the building that a lightweight concrete slab or other type of built-up decking material should be placed on a balcony deck or walkway, adjacent to the plaster walls, after the walls have been finished. If so, the elevation of the weep screed should be established by the architect or builder at a height that will cause a gap of perhaps one-eighth inch to one inch between the bottom of the weep screed and the top of the lightweight concrete slab or other built-up decking material. Because of possible lack of communication among the architect or designer, the builder, and several subcontractors, a built-up deck may prevent formal functioning of installed weep screeds.
The elevation of threshold sills in doorways will determine the ultimate height of built-up balcony deck covering, because it is desirable that low spots on a deck, with resultant ponding of water, should not occur in front of doors. Also, of course, the level of the exterior deck covering should be lower than the level of the interior finished floor, so that rain water does not flow inward at a possible void in sealant placed below the sill.
It is necessary that a waterproof deck membrane be extended vertically upward on walls beside a balcony deck, to an elevation which will allow safe overlap of building paper on the wall, downward over the top of the deck membrane. The height to which the waterproof deck membrane is extended upward on an adjacent vertical wall may limit the elevation at which the weep screed may be placed. The reason for that possible limitation is that the weep screed, as well as the building paper weather-barrier installed behind plaster on the adjacent wall, must adequately overlap the top of the vertical segment of the waterproof deck membrane a safe distance. If the waterproof deck membrane has not been extended upward a sufficient distance on the wall, the weep screed must, of necessity, be placed lower on the wall than is desirable.
Another limiting factor which I have observed is subsequent placement of ceramic tiles, quarry tiles or brick pavers installed vertically or horizontally at the base of vertical walls adjacent to balcony decks. Installation of tiles at the base of balcony walls may adversely affect the venting capability of weep screeds. That possibility must be considered beforehand by the architect or designer, and the builder, to establish the optimum height at which a weep screed should be placed. Such matters can be determined and decided upon during the pre-job conference.
A related blocking problem frequently occurs at ground level, on construction projects. Sometimes builders or their concrete subcontractors place concrete against a vertical plaster wall at an elevation higher than the weep screed. Builders or other subcontractors may install paving stones, quarry tiles or brick pavers against the base of an adjacent plaster wall, at an elevation higher than the weep screed. Note that the building code requires that the weep screed be installed at the foundation plate line. If concrete walks, patios, driveways,steps or other construction elements are placed at a level that inhibits the venting action of the weep screed, that error of commission is beyond the control of the plastering contractor. The builder and the concrete contractor should recognize the need for, and the function served by the weep screed, and avoid blockage of that venting device.
It is advisable that a strip of asphalt-saturated felt, R-15 building paper, or other good building paper, at least 8 inches to 9 inches in width, be placed over the top vertical flange of the weep screed or plaster stop, to rest upon the outward protrusion of the metal trim, prior to placement of lathing paper.
REMEDIAL TREATMENT OF A BLOCKED WEEP SCREED
If a builder, concrete contractor, masonry contractor, or a home owner, has blocked the weep screed installed at the base of a wall, the problem may be remedied. Also, an individual may desire, for any reason, that an additional opening be formed at the base of a plaster wall, where that wall joins a horizontal or sloped surface, such as at:
- the juncture of a wall with a balcony deck;
- where a wall joins a flat or sloped roof surface;
- at the top of tiles which had been installed vertically or horizontally against the base of a plaster wall;
- immediately above and parallel with the top of a concrete slab, concrete stairs, a brick-paved surface; or
- a concrete balcony deck.
To make that type of opening, a saw cut may be made in the plaster membrane, at some convenient and appropriate level above the base. That distance could be as shallow as 1/8 inch to one or several inches above the surface of the slab, balcony deck or roof. The saw cut should NOT cut through the entire thickness of the plaster membrane. The purpose of that admonition is to protect the weather-barrier of lathing paper, and the up-turned waterproof deck membrane, located behind the plaster, from unnecessary damage. The weather barrier on the wall and deck membrane should not be violated.
A saw cut which extends approximately one-half to two-thirds through the thickness of the plaster membrane should be adequate. After the masonry saw blade has cut partially through the thickness of the plaster wall, an edged, wedge-shaped device should be driven carefully into the saw kerf, to fracture the remaining thickness of the plaster wall. The weather-barrier behind the plaster is not to be pierced. Shattered pieces of plaster should be removed from below the horizontal cut, and all debris cleaned from the void.
Subsequent to cleaning debris from the newly-created void, the exposed barrier membrane behind the plaster could be examined by means of a mirror to determine if the weather-barrier membrane had been cut in any place. If a cut is found in the barrier, the perforation should be brushed with asphalt emulsion or cut-back, or other good sealant, to effectively seal the penetration.
In addition, I recommend that the entire newly-exposed surface of deck membrane, flashing, and/or lathing paper be coated with asphalt to protect the surface from the adverse effect of sunlight and weather. Otherwise, that newly-exposed surface may be subject to degradation upon exposure to sun and weather.
On many projects, I have observed that the effectiveness of installed weep screeds has been nullified through blockage by some material placed or installed subsequent to completion of application of lath and plaster. That deprivation of effectiveness of installed weep screeds could have been caused (and avoided) by the architect, the general contractor or his superintendent, a concrete contractor, a masonry contractor, a paving contractor who placed asphalt or portland cement concrete pavement against the base of a wall above the level of the weep screed, or other trades. It is my opinion that the person given responsibility for the entire project, such as an architect, a builder, or his project manager, should determine the elevation at which weep screeds are to be placed, and communicate that information to all other concerned parties such as deck contractors, lathers, concrete contractors, paving crews, roofers and other involved persons.
Alignment of the weep screed must comply with Section 4706(e) of the applicable edition of the Uniform Building Code. On too many projects, I have observed that asphalt paving, a concrete driveway, or a concrete walk or patio slab has been placed against the base of a plaster wall where a weep screed had been installed, but was placed at a finished elevation higher than the screed. That material placed at a later time blocked the weep screed and reduced its effectiveness as a line of exit for possible intruded water.
Another problem which I encountered sometimes, which forms a dam against the screed, is installation of brick pavers, ceramic or quarry tiles, or natural stone laid upon a horizontal substrate against the base of a wall, above the level of the weep screed. I have also encountered vertically installed quarry tiles, thin brick pavers, and ceramic tiles at the base of plaster walls, over the outside of previously installed weep screeds. Another problem of that general nature is the construction of concrete steps and stair landings against the base of previously plastered walls, to block installed weep screed.
Observation of the problems described above has caused me to realize that some person with broad knowledge of the construction project, and with responsibility for the various sub-trades on the roject, MUST EXERT CONTROL TO SCHEDULE ALL SUB-TRADES IN LOGICAL AND PROPER SEQUENCE, AND TO DEFINE THE ELEVATIONS OF VARIOUS CONSTRUCTION MATERIALS WHICH MAY BE PLACED ADJACENT TO OTHER MATERIALS ON A PROJECT.
REMEDIAL TREATMENT FOR MISSING WEEP SCREED
On a project on which weep screed has not been installed where required by the Uniform Building Code or by project drawings prepared by the architect, the following remedial procedure is recommended.
A saw kerf could be cut partially through the plaster membrane, a short distance above the base of the plaster wall. A diamond-tipped circular saw blade serves well to accomplish that action. Subsequent to cutting the partial-depth saw kerf, a wedge-shaped tool (chisel, pry bar or other) should be driven into the saw kerf, to fracture the remaining thickness of the plaster. That combination of partial-depth saw kerf plus fracture will provide adequate exit for possible intruded water. That is all the remedial work that is necessary to compensate for omission of weep screed.
I do not recommend breaking away plaster from the base of a wall, to install a weep screed after construction. That action would be over-correction, and the final result might not present a satisfactory appearance.
Note that no remediation is needed if water is not intruding into the building system.
August 1991 (Supersedes April 1988 and June 1989 editions)
Weep screeds should be installed at the base of exterior plaster stud walls, in accordance with the prescription contained in Section 4706 (e) of the Uniform Building Code. That stipulation is applicable whether or not the plaster membrane has been overlaid with any of the following materials:
- latex paint
- thin brick veneer
- ceramic tiles
- elastomeric coating or thicker elastomeric finish coat, or any other topping.
The intent of that stipulation is to provide an exit for possible intruded water which might have gained entry behind the plaster membrane at or near the roof, at attic vents, through or immediately around windows or doors, or at any other location. Additional overlay or coating of the plaster membrane does not preclude a provision for exit of water from the base of a wall at a foundation plate line, as is accomplished by installation of a weep screed. The admonition recorded above is applicable even though the base of the wall, at the foundation plate line, is vented in some other manner to allow exit of moisture, because of Section 4706 (e) of the Uniform Building Code. However, from a practical aspect, the writer considers that the intent of the I C B 0 has been served suitably if the base of the wall has been constructed in a manner that does allow exit of possible moisture from behind the membrane.
Some persons who have investigated structures have mistakenly reported that lack of a weep screed at the base of a vertical plaster wall, where that wall abutts a flat or sloped roof, should have had a weep screed installed. Those persons should be aware that Section 4706(e) of the Uniform Building Code, or any other portion of the code, does not require or suggest installation of a weep screed at the juncture of a wall with a roof.
Additionally, it has not been the custom of the design or construction industries that a weep screed be installed at that juncture. It should also be noted that a weep screed is not necessary at that location, for the following reasons. The base of a plastered wall, where it abutts a flat or sloped roof, is not sealed. Rather the juncture is sufficiently open that any possible intruded water could escape from behind the vertical membrane. There is no concrete at the roof line to which cement plaster could bond and create a water dam, as is the case at the foundation plate line. Also, during the course of future re-roofing, existing roofing may be removed and replaced, or new roofing may be installed over existing roofing, against the base of a plastered wall, without ill-effect.